NIST Weights and Measures 1 of 12 September 2019

Frequently Asked Questions

About

Commercial Hydrogen Vehicle Refueling

General Questions

1. Why is the U.S. continuing to develop hydrogen fuel cell vehicle technology?

U.S. industries and government are at the forefront of research and development of hydrogen

and fuel cell technology to power vehicles, homes, businesses, and remote equipment such as

cell phone towers. The goals of this effort were to lessen U.S. dependence on foreign oil and

advance an environmentally clean, safe, and reliable energy agenda. Hydrogen is one of many

alternative fuels being explored. When hydrogen is used to power a hydrogen fuel cell electric

vehicle (HFCEV), it offers one possible solution to meeting some of the growing demand in the

U.S.A. for a reliable supply of clean and sustainable energy.

Hydrogen is considered the ultimate clean vehicle fuel. The only emissions from a HFCEV are

heat and clean water, which makes hydrogen a fuel that can help to protect the environment 

and keep our air cleaner. Hydrogen gas is 14.4 times lighter than air and dissipates rapidly and 

harmlessly if released into the air. 

Hydrogen has higher energy content by weight than other fuels, but lower energy content by 

volume. This means that sufficient quantities of hydrogen stored in liquid or compressed 

gaseous form onboard a HFCEV can provide a comparable driving range to conventional 

gasoline-powered vehicles. Fuel cells are lighter in weight than batteries used in plug-in electric 

vehicles, also making hydrogen fuel cell vehicles lighter and more suited to long-range 

applications than battery electric vehicles. In addition, FCEVs can be fully refueled in under 

5 minutes at a hydrogen fueling station, compared to the 3 hours to more than 20 hours it takes 

to recharge battery electric vehicles. Charge time is dependent on the type of battery electric 

vehicle and the type of electric vehicle service equipment.

Hydrogen can be produced using renewable energy sources such as the wind or sun. Renewable 

energy technology is an attractive investment because the return on investment is enduring and 

is unbounded by a finite pool of natural resources that must first be located, then extracted and 

refined, and eventually depleted.

The most common methods used to produce hydrogen today rely on resources that are readily 

available in the United States; for example, electrolysis of water and steam reformation of 

natural gas. As such, hydrogen can be produced regionally in ways that meet the economic and 

social goals of each region. Factors to consider include the available feedstocks or energy 

resources used to produce the supply of hydrogen, the energy efficiency of those methods used 

to produce the hydrogen; the infrastructure in place to transport, distribute, deliver, and store

the hydrogen; and the efficiency of the energy conversion devices in converting the hydrogen 

into work, such as propelling a vehicle. Every advance in technology that reduces the cost or 
NIST Weights and Measures 2 of 12 September 2019

improves the efficiency of these components is a permanent cost-savings improvement. The

advances within the hydrogen and fuel cell industry today are part of a major effort to make

hydrogen a viable step towards American independence from foreign energy imports. 

2. What is different about driving a fuel cell electric vehicle (FCEV)?

Hydrogen fuel cell vehicles (HFCV) are designed to replicate the experience of operating a

gasoline powered vehicle. FCEVs are designed to have similar response times, speed, and

acceleration as vehicles with gasoline powered engines, and are smoother and quieter than

vehicles using internal combustion engines. However, the exhaust and refueling connection

differ from a gasoline powered vehicle. The exhaust from a HFCV is clean water and heat rather

than air pollutants and greenhouse gases. Because the hydrogen fuel cell powers an electric

drive motor to power the vehicle drive train, a HFCV is much quieter than a vehicle with an

internal combustion engine. HFCVs have a driving range of approximately 480 kilometers (km)

(300 miles) using a single tank of hydrogen fuel. Some HFCVs have achieved an impressive 800

km (500 mile) driving range, significantly more than the average battery electric vehicle.

3. How is hydrogen fuel made available today?

The predominant storage method for hydrogen fuel cell electric vehicles (HFCEVs) is gaseous

hydrogen storage. However, there are auto and dispenser manufacturers that have designed

equipment for liquid hydrogen fueling applications; although it is most likely that liquid

hydrogen storage may be used at a fueling station for delivering compressed gaseous hydrogen

to a vehicle. A compressed gas fueled HFCEV uses hydrogen that is stored onboard the vehicle

in a pressurized cylindrical tank. Due to low volumetric energy density noted above, hydrogen

gas must be compressed to high pressure to store enough fuel to achieve a 480 kilometer

(300 mile) or more driving range.

4. How can I purchase hydrogen for a fuel cell vehicle?

Dispensers for a hydrogen fueling station are very similar in appearance to gasoline dispensers.

You may even find hydrogen fuel dispensers in the same filling station with gasoline dispensers.

Hydrogen fuel dispensers sit on an island where vehicles can pull in and gain access to a hose

that connects to the vehicle to deliver the fuel. Hydrogen dispensers will have a customer

interface with a display and keypad controls that a driver can use to start and stop the delivery.

The process for fueling a fuel cell electric vehicle is quite similar to a gas pump and takes 

approximately the same amount of time – roughly three to five minutes. The driver pulls into 

the station and stops next to an available dispenser. The transaction is initiated by following 

instructions on the dispenser’s display screen. The driver takes the hose and attaches it to the 

vehicle fueling connection and secures the connection by rotating a lever on the nozzle. The 

driver activates a control to tell the dispenser to start the fueling process, and the dispenser 

does the rest. A typical passenger vehicle will accept between 1 kilogram to 6 kilograms (2.205 

pounds to 13.23 pounds) of hydrogen fuel, to achieve a fill up depends on how much hydrogen

is in the fuel tank at the beginning of the fueling process and the tank size. When the fill is 

complete, the dispenser automatically stops the delivery and signals the driver to disconnect 

and return the hose.

As this fact sheet is being published, work is underway in the U.S. hydrogen and weights and 

measures communities to make certain the necessary components in the hydrogen 

infrastructure are in place for the start of national commercial sales of hydrogen vehicle fuel. 

The compliance process for public gasoline fueling stations has been in place for over 100 years. 
NIST Weights and Measures 3 of 12 September 2019

Hydrogen fuel will be available for sale through refueling dispensers based on a recognized unit 

of measurement. This is already taking place in California. This allows consumers to make value 

comparisons when purchasing a hydrogen fuel cell vehicle and hydrogen fuel for their vehicle. 

As of July 2010, hydrogen vehicle fuel is to be sold by mass units of the kilogram (kg). 

5. Where does hydrogen fuel come from?

The most commonly used methods for hydrogen production include:

• Natural Gas Reformation. The majority of hydrogen produced today is for industrial 

purposes and is made by reforming natural gas. This same process is used to produce 

hydrogen for vehicle fuel. 

• Electrolysis is another process whereby electricity is used to split water molecules

(which can be supplied from a municipal water supply system) into oxygen and 

hydrogen fuel. The electrolyzer used to split the water may be located onsite at the 

fueling station and powered by some other source of energy. The electricity for 

powering the electrolyzer may be generated by a renewable energy technology using

wind, solar, geothermal, or hydroelectric power, thus, providing further reductions in 

greenhouse gas emissions.

• Hydrogen can also be produced from the gasification of coal or biomass

(sustainable/renewable plant and animal materials), photo-biological production 

(algae), or reforming certain liquid fuels. 

• Nuclear power has the potential to produce hydrogen from high temperature 

electrolysis. 

These are many of the known methods currently in use and under development to produce 

hydrogen. The list of alternative methods for hydrogen production continues to grow as 

researchers make new discoveries.

6. How much hydrogen do I need in order to travel as far as I can on a gallon of gas?

Since July 2010, gaseous hydrogen vehicle fuel is sold by mass units of the kilogram (kg). 

Coincidentally, the energy content of 1 kg (2.205 pounds [lb]) of hydrogen is slightly higher than

the energy content in 3.785 liters (1 gallon) of gasoline (which has a mass of approximately 

2.732 kg [6.025 lb]). To compare a fuel cell vehicle and a gasoline internal combustion engine 

vehicle in terms of distance travelled for a certain amount of fuel, it is important to consider 

that a fuel cell electric vehicle converts fuel energy into locomotive work two to three times 

more efficiently than a gasoline powered vehicle. Fuel cell electric vehicles achieve 40 % to 60 % 

energy efficiency, compared to gasoline powered vehicles that typically operate at 20 % 

efficiency. The quantity of compressed gaseous hydrogen fuel required to travel the same 

distance as a gallon of gasoline would weigh less but would occupy more space than the 

gasoline.

Comparing solely by mass, 1 kg of hydrogen contains roughly three times more energy than 1 kg 

of gasoline. In contrast, when the energy content of equal volumes of hydrogen gas (at 

70 megapascals [10 000 psi]) and gasoline are compared, hydrogen has four times less energy.

7. Is hydrogen safe as a vehicle fuel?

Equipment in use at hydrogen refueling stations is designed, constructed, and installed in 
NIST Weights and Measures 4 of 12 September 2019

accordance with safety codes and standards, and fuel cell electric vehicles (FCEVs) must meet 

the National Highway and Transportation Safety Administration’s highest safety standards. 

Extensive research has been conducted by government, industry, and academia to assess 

hydrogen’s properties and behavior as they relate to the safe storage, transportation, and 

delivery of hydrogen fuel. Therefore, the likelihood of a safety incident occurring while refueling 

a FCEV is not any greater than other refueling operations. Stations will be posted with safety 

information (such as no ignition sources) and attendants trained to respond to hazardous 

situations. 

Hydrogen is non-toxic. A hydrogen system designed to appropriate codes and standards can 

provide a comparable (or even higher) level of safety than gasoline systems. If hydrogen is 

released into the atmosphere, it does not pollute the air or ground water. Because hydrogen is 

14 times lighter than air, it very quickly dissipates and will not remain in high concentrations if 

released. A hydrogen system such as in a FCEV may, in fact, be designed to release hydrogen in a 

controlled manner, facilitating venting or even flaring of the hydrogen as the situation warrants. 

Hydrogen is colorless and odorless. Additive odorants (like the sulfur-containing butyl 

mercaptan used to facilitate the detection of natural gas) are not compatible with fuel cell 

technology. Instead, detection equipment is in use in fueling stations and is also available for 

use in operations where a hydrogen release might be entrapped in an enclosed space.

Hydrogen is stored in liquid or gaseous form, and delivered during fueling as a gas at high 

pressure. The high pressure is necessary to compress the hydrogen enough to enable sufficient 

amounts of hydrogen for suitable driving distances. You will see dispenser service pressure 

expressed in metric units of megapascal (MPa). Hydrogen fuel cell electric vehicles have storage 

pressures up to 70 MPa (10 000 psig). High strength carbon fiber reinforced tanks are designed 

and manufactured to contain these pressures safely. Earlier designs of fuel storage cylinders 

have been used successfully in compressed natural gas (CNG) vehicles for onboard storage of 

high pressure gaseous fuel for many years.

8. What other uses are there for hydrogen fuel cells?

Hydrogen fuel cells, along with powering passenger and fleet vehicles, can also be found in 

stationary and portable power generation applications. Hydrogen fuel cells are used in 

stationary applications as backup power generation systems to provide a reliable uninterrupted 

energy source for hospitals and data centers. Hydrogen fuel cells have also seen recent success 

in warehouse and distribution center operations where they are used to power forklifts

operated in enclosed spaces where engine exhaust emissions are undesirable, and where fleet 

management favors fewer vehicles with less down time. Hydrogen fuel cells, which have a 

higher energy density and lighter weight than batteries, are also being marketed in portable 

applications to power personal electronic devices such as cell phones, chargers, and laptops.

9. Where can I learn more about what is happening with hydrogen as a vehicle fuel?

The NIST Office of Weights and Measures maintains a hydrogen information website available 

at: https://www.nist.gov/pml/weights-and-measures/legal-metrology-devices/hydrogen-usnational-work-group. This resource is dedicated to providing information on the latest efforts to 

develop uniform, fair, and appropriate legal metrology requirements for commercial hydrogen 

measurement. The web page has links to other websites that provide additional information on
NIST Weights and Measures 5 of 12 September 2019

the development of related hydrogen topics such as basic safety, fueling station locations, fact 

sheets, and more. Other websites for more information are available at:

https://www.energy.gov/eere/fuelcells/fuel-cell-technologies-office

http://energy.gov/eere/fuelcells/hydrogen-storage; 

http://energy.gov/eere/fuelcells/hydrogen-delivery; 

http://www.fchea.org/factsheets/; 

http://www.h2usa.org/faq; and 

http://cafcp.org/faqs

Questions Related to Legal Metrology

1. What legal metrology standards exist for hydrogen fueling applications?

The National Institute of Standards and Technology (NIST) Handbook 44, “Specifications, 

Tolerances, and Other Technical Requirements for Weighing and Measuring Devices,” Section 

1.10 General Code and Section 3.39 Hydrogen Gas-Measuring Devices Code provide 

specifications, tolerances, and other technical requirements for hydrogen measuring devices. 

NIST Handbook 44 requirements form the basis for type evaluation under the National Type 

Evaluation Program and for routine inspection and test procedures for commercial measuring 

equipment. NIST Office of Weights and Measures (OWM), through a Department of Energy 

(DOE)/NIST sponsored U.S. National Work Group, has developed a draft Examination Procedure 

Outline (EPO), which is a detailed step-by-step inspection and test procedure (using a 

gravimetric method) for a hydrogen dispenser, based on NIST Handbook 44 device code 

requirements. Test procedures for other alternative test methods are under consideration.

NIST Handbook 130, “Uniform Laws and Regulations in the Areas of Legal Metrology and Fuel 

Quality,” provides the Method of Sale Regulation 2.32 “Retail Sales of Hydrogen Fuel” that 

specifies the kilogram (kg) as the unit of measurement for sales of gaseous hydrogen vehicle fuel 

and for related labeling and advertising requirements. More information on these codes and 

regulations are available on the NIST OWM website at: http://www.nist.gov/pml/wmd/. 

Handbook 130 includes fuel quality standards and references to test methods. 

2. Where did the legal metrology standards for hydrogen come from?

The legal metrology requirements in the National Institute of Standards and Technology (NIST) 

Handbook 44 and Handbook 130 were developed by the U.S. National Work Group (USNWG) for 

the Development of Commercial Hydrogen Measurement Standards. Startup of the USNWG 

was sponsored by the U.S. Department of Energy (DOE) and the U.S. Department of Commerce’s 

(DOC) National Institute of Standards and Technology (NIST). This group is comprised of 

stakeholders from private industries, standards organizations, and state and federal 

government. Broad participation was sought so as to include hydrogen dispenser and 

component manufacturers, fuel suppliers, standards developers, and weights and measures 

device type evaluation laboratories and regulators in developing its recommendations. The 

USNWG adhered to the weights and measures standards development process that has been in 

place since 1905. 

The draft standards created by the USNWG were adopted by the National Conference on 

Weights and Measures (NCWM) in 2010 and first published by NIST in the 2011 editions of 

Handbook 44 and Handbook 130. Handbook requirements are adopted by the states, in part or 

entirety, as law and/or regulation. More information on work efforts by the USNWG is available 
NIST Weights and Measures 6 of 12 September 2019

on the NIST Office of Weights and Measures (OWM) website available at: 

http://www.nist.gov/pml/wmd/lmdg/usnwg.cfm.

3. What types of hydrogen fueling systems are in use in the U.S.A.?

Both stationary and portable hydrogen refueling systems are in use. Dispensers are 

permanently installed in service stations capable of generating hydrogen through natural gas 

reformation, an electrolyzer powered by electricity supplied by the grid, or renewable sources 

such as solar or wind power. Stationary installations may also be supplied with hydrogen 

through deliveries from tanker or tube trailer trucks. There are platform and vehicle-mounted 

refueling systems also in use to refuel road tours and to temporarily fuel fleet operations based 

in a certain geographic location.

4. Why is the recognized method of sale for hydrogen vehicle fuel on the basis of price per 

kilogram ($/kg)?

Industry pre-market practice has been to dispense hydrogen in mass units of measurement. 

Because the hydrogen is delivered in a compressed gaseous form, volume measurement would 

not be suitable due to the dependency on pressure and temperature of the gas. To ensure 

accuracy in the amount of hydrogen fuel delivered, mass units have been the preferred method 

of delivery in the weights and measures and hydrogen communities. The corresponding 

international standard developed by the International Organization of Legal Metrology (OIML) in 

Recommendation (R) 139 “Compressed gaseous fuel measuring systems for vehicles” also 

recognizes mass as the basis for dispensing compressed gases. The latest version of OIML R 139, 

is available at: http://www.oiml.org/en. 

The “Gasoline Gallon Equivalent” (GGE) was considered as a specialized mass unit for dispensing 

hydrogen, but not supported. With a standardized unit of measurement (the kilogram) used for 

advertisement, dispenser labeling, and hydrogen gas deliveries through retail dispensers, 

consumers can make value comparisons between competing retailers, and the unit is traceable. 

The U.S. has a national policy to establish the SI (International System of Units, commonly 

known as the metric system) as the preferred system of weights and measures for U.S. trade 

and commerce. Most commercial gaseous hydrogen metering systems measure in mass, an 

equitable, efficient means of measurement that is based on a traceable unit of measure. 

Coincidentally, a kilogram of hydrogen has virtually the same energy content as one gallon of 

gasoline. Consequently, the kilogram was adopted as the quantity unit for commercial sales of 

hydrogen vehicle fuel.

5. What criteria are used to obtain U.S. type evaluation approval of a hydrogen vehicle fuel 

dispenser? 

A set of general evaluation criteria (checklist) based on General Code and device specific design 

requirements in Section 3.39. of the NIST Handbook 44 was developed for inclusion in National 

Conference on Weights and Measures (NCWM) Publication (Pub) 14, “Measuring Devices 

Technical Policy, Checklists, and Test Procedures.” Checklists are typically developed by the 

NCWM National Type Evaluation Technical Committee (NTETC) Measuring Sector Subgroup. 

The Department of Energy/NIST U.S. National Work Group for the Development of Commercial 

Hydrogen Measurement Standards as well as the NTETC all provided input to fully develop the 

performance test procedures for the NCWM Pub 14, Field Evaluation and Permanence Tests 

Section specific to hydrogen dispensers.
NIST Weights and Measures 7 of 12 September 2019

The final draft of the general checklist was forwarded to the National Type Evaluation Program 

(NTEP) Committee for inclusion in NCWM Pub 14 in late 2011 resulting in their approval of the 

checklist in late 2012.

U.S. type evaluations are carried out by NCWM NTEP, whose laboratories use NCWM Pub 14. 

6. What is the accepted tolerance for the performance of a hydrogen dispenser?

The NIST Handbook 44, Section 3.39. Hydrogen Gas-Measuring Devices Code, paragraph T.2. 

Tolerances specifies that hydrogen dispensers fall within the Accuracy Class 7.0. These devices 

are required to meet a performance of ± 5.0 % for acceptance tolerance and ± 7.0 % for 

maintenance tolerance.

7. What test methods are recognized for verifying the performance of a hydrogen dispenser? 

The NIST Handbook 44, Section 3.39. Hydrogen Gas-Measuring Devices Code, paragraph N.4. 

Tests recognizes three official methods for testing the measurement performance of a hydrogen 

dispenser: the gravimetric test, master meter transfer standard test, and PVT (pressure volume 

temperature) test. More detailed procedures are available in the NIST Examination Procedures 

Outline 29 for Hydrogen Gas Retail Vehicle Fuel Dispensers (Draft 5) that address the gravimetric 

test method.

8. How many test drafts are required in a test of a hydrogen vehicle refueling dispenser? 

The type and number of test drafts required during the performance test of a hydrogen vehicle 

fuel dispenser are prescribed in the Notes section of the NIST Handbook 44, Section 3.39.

Hydrogen Gas-Measuring Devices Code. The Code forms the basis for both type evaluation test 

criteria and for testing and inspection of devices while in commercial operation. 

For hydrogen dispensers in commercial use, the master meter (transfer) standard test will 

consist of a minimum of two test drafts. One test draft shall consist of a draft size that 

represents the smallest quantity the device is designed to accurately deliver (which might be the 

amount of fuel necessary to top-off a nearly full vehicle tank), but is usually defined as the 

manufacturer’s marked Minimum Measured Quantity (MMQ). The other draft shall consist of a 

draft size of either ten times the MMQ or 1 kilogram, whichever is greater. The gravimetric and 

PVT pressure volume temperature tests will consist of a minimum of two test drafts. One test 

draft shall consist of a draft size that represents twice the manufacturer’s marked Minimum 

Measured Quantity (MMQ). The other draft shall consist of a draft size of either five times the 

MMQ or 4 kilograms, whichever is greater. Tests shall be conducted at an amount equal to the 

MMQ when it is likely a dispenser will be used to make deliveries at the declared MMQ. The 

flow rates selected for the test are to reflect normal delivery conditions. More tests may be 

performed if conditions warrant further testing. It is worth noting that dispensers must also 

meet other industry performance standards as well as legal metrology requirements. 

9. What is type evaluation and what does the process involve?

A device undergoing type evaluation is intended to represent a particular design that will be 

mass produced for commercial use. Therefore, type evaluation tests are structured to provide 

sufficient data to demonstrate a device’s performance over a lifetime of use and are more 

extensive than the periodic field performance testing that is used to determine a device’s 

performance compliance and proper maintenance while in commercial service. Type evaluation 

enables testing of a same “type” or pattern to provide confidence to weights and measures 
NIST Weights and Measures 8 of 12 September 2019

jurisdictions, facilitates acceptance, assists the manufacturer in identifying any problems or 

deficiencies and correcting them before making equipment more widely available.

10. Can a type of hydrogen dispenser be issued a National Conference on Weights and Measures

(NCWM) National Type Evaluation Program (NTEP) Certificate of Conformance (CC)?

The specifications, tolerances, and other technical requirements in NIST Handbook 44, Section 

1.10 General Code and Section 3.39 Hydrogen Gas-Measuring Devices Code apply to commercial 

hydrogen vehicle refueling devices. NTEP is administered by NCWM. NTEP determines the 

conformance of a device type to the provisions in its policies, technical checklists, and test 

procedures. More information on NTEP, the application process, and contacts if you have 

questions is available on the NCWM website at: https://www.ncwm.com/ntep-about. 

Additionally, it should be noted that the state/local weights and measures authority having 

jurisdiction over commercial dispensers may also have provisions in place for performing type 

evaluations. 

11. What is the smallest permissible quantity unit value that can be displayed for a delivery on a

hydrogen dispenser or recorded on a hydrogen fuel sales receipt?

The NIST Handbook 44, Section 3.39. Hydrogen Gas-Measuring Devices Code, paragraph S.1.4.

Value of Smallest Unit specifies 0.001 kg as the value of the smallest unit permitted and not to

be exceeded for devices with a maximum flow rate of 30 kilograms/minute (kg/min) or less; 0.01

kg is the value of the smallest unit permitted and not to be exceeded on devices with flow rates

greater than 30 kg/min.

12. What is unique about hydrogen vehicle fuel when compared to compressed natural gas

(CNG)?

The weights and measures and hydrogen communities were able to develop many hydrogen

standards by building on what they have learned from similar compressed gas applications such

as compressed natural gas (CNG). However, hydrogen has several unique properties that must

be considered when addressing safety, storage, and commercialization of the technology.

Hydrogen has 6 to 8 times lower density than CNG, even at higher service pressures. The

pressures at which hydrogen is dispensed and stored can be more than 2.5 times higher than

those for CNG. Hydrogen dispensers are equipped with safety features to regulate the fill cycle

in order to prevent the heating that occurs during the fueling process from raising the receiving

tank’s temperature above the typical maximum allowable temperature of 85 °C (185 °F).

13. Are there any requirements for hydrogen vehicle fuel quality?

Multiple organizations in the fuel standards community worked to fully develop a hydrogen fuel

quality standard. There has been a concerted effort to harmonize these standards among the

different organizations. The vehicle fuel quality standard recognized and required by many

weights and measures jurisdictions and other enforcement agencies is included in the NIST

Handbook 130, “Uniform Laws and Regulations in the Areas of Legal Metrology and Fuel

Quality.” This standard specifies that hydrogen fuel meet the requirements of the most current

version of Society of Automotive Engineers (SAE) Standard J2719, “Hydrogen Fuel Quality for

Fuel Cell Vehicles.” The SAE J2719 Standard is aligned with the corresponding International

Organization for Standardization (ISO) standard for hydrogen fuel quality (ISO/DIS 14687-2

Hydrogen fuel - Product specification - Part 2 Proton exchange membrane fuel cell applications

for road vehicles). SAE J2719 references the specific ASTM sampling and testing methods that

are necessary to ensure that hydrogen fuel meets the designated 99.97 % purity standard and

that all remaining constituents are within allowable limits. The citation for this reference
NIST Weights and Measures 9 of 12 September 2019

standard is in the NIST Handbook 130, Section IV., G. Uniform Fuels and Automotive Lubricants 

Regulation, Section 2. Standard Fuel Specifications, paragraph 2.20. Hydrogen Fuel.
NIST Weights and Measures 10 of 12 September 2019

Pressure Conversions

These conversions are based on National Institute of Standards and Technology (NIST) Special 

Publications (SP) 330 “The International System of Units (SI)” and SP 811 “Guide for the Use of the 

International System of Units (SI)” that provide guidelines on the use of the International System of 

Units (SI), the metric system. These publications are available on the NIST Office of Weights and 

Measures web site at: https://www.nist.gov/pml/weights-and-measures/publications/metricpublications. This information was prepared to provide a ready reference for commercial measurement 

systems designed to dispense hydrogen vehicle fuel at various service pressure ratings. This information 

is limited to units of measurement that might typically be used to declare service pressure on 

dispensers, street signs, and advertisements. The U.S. weights and measures and hydrogen 

communities agreed to standardize industry practices and to present information in a consistent 

manner by recognizing the “pascal” as the unit of measurement for expressing hydrogen dispenser 

service pressure.

• To convert pressure values in measurement units of pounds-force per square inch (psi) to 

megapascal (MPa)

Multiply by 0.006 894 757

▪ e.g., 10 000 psi  0.006 894 757 = 68.947 57 MPa 

(This number rounded to the nearest ten is 70 MPa)

• To convert pressure values in measurement units of bar to megapascal

Multiply by 0.1

▪ e.g., 700 bar  0.1 = 70 MPa

• To convert pressure values in measurement units of “bar” to “psi:”

Multiply by 14.503 774

▪ e.g., 700 bar  14.503 774 = 10 152.641 psi

(This number rounded to the nearest thousand psi is 10 000 psi)

The General Table for conversion of units of pressure shown below was excerpted from NIST Handbook

44 “Specifications, Tolerances, and Other Technical Requirements for Weighing and Measuring Devices,”

Appendix C and provided to assist in working with and reporting in pressure units of measurement. NIST 

Handbook 44 is available on the NIST OWM web site at: https://www.nist.gov/pml/weights-andmeasures/publications.
NIST Weights and Measures 11 of 12 September 2019

Units of Pressure 

(all underlined figures are exact)

Starting 

Unit 

←

Multiply by the Conversion Factor Below the Ending Unit:

Ending

Unit

→

Pascal

(Pa)

kilopascal

(kPa)

megapascal

(MPa)

pound-force per 

square inch 

(psi)

(lbf/in2)

millimeter of

mercury

(mm Hg (0 

°C))

Inch of water 

(in H2O (4 °C))

1 Pa = 1 0.001 0.000 001 0.000 145 037 74 0.007 5006 15 0.004 014 742 13

1 kPa = 1000.0 1 0.001 0.145 037 744 7.500 615 05 4.014 742 133

1 MPa = 1 000 000 1 000 1 145.037 744 7 500.615 05 4 014.742 13

1 psi (lbf/in2) = 6 894.757 6.894 757 0.006 894 757 1 51.714 918 1 27.680 671 4

1 mm Hg (0 °C) = 133.322 4 0.133 322 4 0.000 133 322 4 0.019 336 78 1 0.535 255 057

1 in H2O (4 °C) = 249.082 0.249 082 0.000 249 082 0.036 126 291 1.868 268 198 1
NIST Weights and Measures 12 of 12 September 2019

Hydrogen and Other Vehicle Fuels Comparison Table

Retail Vehicle 

Fuel

Method of Sale Energy Content1

(compared to 3.785 

liters (L) (1 gallon 

(gal) of gasoline)

Approximate Specific Gravity2

State of Fuel

Unit of 

Measurement

(Air = 1.0) (Water = 1.0)

Compressed 

Hydrogen Gas

Mass 1.0 kg (2.205 pound 

[lb]) has slightly 

more than 100 % of 

the energy

0.0696

Compressed gas at

35 MPa3to 

70 MPa3

(5000 psi to 

10 000 psi)

Kilogram (kg)

Liquefied 

Petroleum Gas 

(LPG) or 

Commercial 

Propane

Volume or Mass

3.785 L (1 gal) has 

73 % of the energy 0.500 to 0.580

Pressurized liquid 

at

1.6 MPa

(250 psi) Gallon4 or Pound

Liquefied Natural 

Gas (LNG)

Volume Equivalent 

Units or Mass

3.785 L (1 gal) has 

64 % of the energy 0.41 to 0.50

Liquid

(− 162 oC @ 

101.325 kPa 

[− 260  ̊F @ 

1 atm6])

Diesel Gallon 

Equivalent5

(DGELNG) or Pound 

or kg

Diesel Fuel No. 2

Volume

3.785 L (1 gal) has 

113 % of the energy

0.85

(15  ̊C @ 

101.325 kPa

[60  ̊F @ 1 atm6])

Liquid

Gallon or Liter

Gasoline

Volume

100 % of the energy

0.72 to 0.78

(15  ̊C @ 

101.325 kPa

[60  ̊F @ 1 atm6])

Liquid

Gallon or Liter

Compressed 

Natural Gas

(CNG)

Volume Equivalent 

Units or Mass

2.567 kg (5.660 lb) 

has 100% of the 

energy

0.6 to 0.8

Compressed gas at 

20 MPa to 35 MPa 

(3000 psi to 5000 

psi)

Gasoline Gallon 

Equivalent7

(GGECNG), Diesel 

Gallon Equivalent7

(DGECNG), Pound or 

kg

1The energy content values are not exact and are for information purposes since there are multiple variables that affect 

the energy content of a fuel supply.

2Specific Gravity – The ratio of the density (mass per unit volume) of a liquid to the density of water or the ratio of the 

density of a gas to the density of air at standard conditions. 

3NOTE: Liquid storage is at a pressure of 1 MPa (150 psi).

4Retail motor-fuel application

At this time the weights and measures community has only addressed retail sales of gaseous (rather than liquid) 

hydrogen for vehicle refueling applications.

5Diesel Gallon Equivalent (DGE) means 6.059 lb of liquefied natural gas.

6 atm – atmosphere 

7Gasoline Gallon Equivalent (GGE) means 5.660 lb of compressed natural gas; Diesel Gallon Equivalent (DGE) means 

6.384 lb of compressed natural gas.