SD-10 

GUIDE FOR IDENTIFICATION 

AND DEVELOPMENT OF 

METRIC STANDARDS 

DECEMBER 2003 

OFFICE OF THE UNDER SECRETARY OF DEFENSE 

ACQUISITION, TECHNOLOGY, & LOGISTICS 

SDMP 
FOREWORD

This guide is intended to help organizations in the Department ofpefense (DoD) determine

when it is necessary to develop metric specifications and standards, and the best way to express

metric units of measurement in those documents. The availability of metric documents is a

prerequisite to the development of metric-based weapon systems and equipment. This document

will help DoD program offices, standardization management activities, engineering offices,

contractors, and others meet the requirements of Section 5164, Metric Usage, of the Public Law

100-418, the Omnibus Trade and Competitiveness Act of 1988.

If you have any questions or comments about this document or metric use in the DoD, please

contact the Defense Standardization Program Office, 8725 John I. Kingrnan Road, ATTN: 1-307, Stop 6233, Fort Belvoir, V A 22060-6221 or email .

/

~-

Detense Standardization Progranl Office

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GUIDE FOR IDENTIFICATION AND DEVELOPMENT OF METRIC1 STANDARDS 

1. Scope. This document provides guidance for the identification and development of standards 

and specifications using the International System of Units (SI), commonly known as the metric 

system. The guide does not cover metric practice, such as methods of converting and rounding, 

as addressed in IEEE/ASTM SI 10 (see 3.7.1). 

2. Background. The Weights and Measures Law of the United States (15 U.S.C. § 205(b)(1)) 

states that it is the policy of the United States to designate the SI as the preferred system of 

measurements for trade and commerce. Another policy established under section (3) is that 

Federal agencies are to seek out ways to increase understanding of the SI through educational 

information and guidance and in their publications. 15 U.S.C. § 205(c)(4) defines the metric 

system of measurement as the International System of Units established by the General 

Conference of Weights and Measures2

 as interpreted or modified for the United States by the 

Secretary of Commerce. 

3. Definitions. 

 3.1 Metric specification. Metric specifications have requirements given in rounded, 

rational, metric units, usually as a result of being originally developed in metric. The magnitudes 

are meaningful and practical (for example, 10 grams, not 0.35273 ounces). Documents 

containing only electrical units that are used in both the metric and inch-pound systems (for 

example, volts, amps, and ohms) are classified as metric documents. Documents also containing 

dimensional interfaces must have these interfaces in metric sizes to be classified as metric 

documents. Metric specifications are developed for items to interface or operate with other 

metric items. 

 3.2 Inch-pound specification. Inch-pound specifications have requirements given in 

rounded, rational, inch-pound units, usually as a result of being originally developed in inchpound. The magnitudes are meaningful and practical (for example, 1 ounce, not 28.3495 grams). 

Inch-pound specifications should include those with rounded, rational, inch-pound units only. 

NOTE: There have been instances where magnitudes expressed in metric units as a result of 

mathematical conversion from rounded, rational inch-pound units are given first (preferred units) 

with the rounded, rational inch-pound units given in parenthesis or in a non-preferred position. 

These specifications are inch-pound documents. Inch-pound specifications are developed for 

items to interface or operate with other inch-pound items. 

 3.3 Hybrid specification. A hybrid specification is one in which some requirements are 

given in rounded, rational metric units, and other requirements are given in rounded, rational 

inch-pound units. Hybrid specifications are often required for use in new designs where existing 

usable components must interface in a metric system. 

 

1

 Since 1960, it has been known as The International System of Units (7th ed. Bureau International des Poids et 

Mesures 1998). 

2

 For information on the General Conference of Weights and Measures, see 

http://www.bipm.fr/enus/2_Committees/cgpm.html. 
2

 3.4 Not measurement sensitive documents. A not measurement sensitive document is 

one in which application of the requirements does not depend substantially on some measured 

quantity. This type of document can be used with either a metric system or an inch-pound 

system. 

 3.5 Soft conversion. A soft conversion is the process of changing a measurement from 

inch-pound units to equivalent metric units within acceptable measurement tolerances without 

changing the physical configuration of the item. 

 3.6 Hard conversion. A hard conversion is the process of changing a measurement from 

inch-pound units to non-equivalent metric units which necessitates physical configuration 

changes of the item outside those permitted by established measurement tolerances. The term 

“hard conversion” is in general use in the United States, although it is technically incorrect when 

applied to specific items because no “conversion” takes place. Instead, a new metric item 

requiring a new part identification is created to eventually replace the inch-pound version of the 

item. The new item is often referred to as being in “hard metric.” 

 3.7 Metric units. 

 3.7.1 Metric units are a system of basic measures defined by the International System of 

Units based on “Le Systeme International d ́Unites (SI),” of the International Bureau of Weights 

and Measures (IBWM)3

. These units are described in IEEE/ASTM SI 10, Use of the 

International System of Unites (SI): The Modern Metric System. 

 3.7.2 In the United States, the authoritative SI source document is National Institute of 

Standards and Technology4

 NIST Special Publication 330, The International System of Units 

(SI). For use in the United States, this is the official English language translation approved by 

the IBWM of its publication Le System International d’Unites. Guidance for use of Special 

Publication 330 is provided in NIST Special Publication 811, Guide for the Use of the 

International System of Units (SI). Based on these two publications and on authority of 15 

U.S.C. § 205(c)(4), the U.S. Department of Commerce issued Metric System of Measurement: 

Interpretation of the International System of Units (SI) in the Federal Register on July 28, 1998 

(Vol. 63, No. 144) as the interpretation and modification of the SI for use in the United States. 

 

3

 For information about the International Bureau of Weights and Measures, see http://www.bipm.org. 

4

 For additional information on these publications and the SI contact: 

National Institute of Standards and Technology 

Weights and Measures Division - Laws and Metric Group 

100 Bureau Drive Stop 2600 

Gaithersburg, Maryland 20899-2600 

By email at: TheSI@nist.gov PH: 301-975-4004 FAX: 301-926-0647 

You will find the Laws and Metric Group's Website at http://www.nist.gov/metric. 

For NIST information on scientific units go to http://physics.nist.gov/cuu/Units/index.html. 
3

4. When are Metric Standards and Specifications Needed? Normally, there are two situations 

when the development of a metric document is needed: (1) simply to put requirements in metric 

measurement units, and (2) development of physical modules for international harmonization. 

With regard to the first, this can often be accomplished by soft conversion. With regard to the 

second, there are pressures in areas of international product standardization for use of metric 

modules. Before one decides on the development of a metric document, the purpose of the 

document must be determined. Decisions must be reached on which requirements in the 

document are metric, which are soft converted, and which are inch-pound. 

 4.1 Where Soft Conversion Should Be Used. If the purpose is simply to put the 

requirements in metric language, a soft conversion is the only needed change. The soft 

conversion can be in the form of stating only the converted metric units in the requirements as 

substitutions for the inch-pound units; stating the metric units in parenthesis after the inch-pound 

units in the requirements or vice versa; or giving a table of conversions and/or conversion factors 

and giving the requirements in only one system. Soft conversion should be used in the following 

circumstances: 

 a. The technologies addressed in the document are based on the inch-pound system 

internationally. Examples are most areas of electronic packaging and grid spacing, hydraulics, 

and tire rim sizes. Since the purpose of metric is usually to achieve international standardization 

or interoperability, rather than metrication per se, there is no need to have a hard conversion in 

these areas. Rather, the purpose is to include metric language, since these documents describing 

these products or processes are expressed in metric units in the other countries and in the 

international standards. 

 b. Free standing, or stand alone, items where interoperability or interfacing is not needed, 

and there is no need for international standardization. Examples are pens and pencils, many 

types of furniture, etc. 

 c. Items or processes where rounded, rational numbers are not usual in either system. In 

these cases, economic and technical considerations are balanced to achieve an optimal situation. 

Examples of these are anti-corrosion coating thicknesses, electrical insulation thicknesses, 

locations of components in electronic assemblies, etc. 

 d. Capacities or volumes such as fuels, paints, and other coatings. Metric hardware or 

equipment can be painted using gallons of paint rather than liters, and can be powered by gallons 

of fuel as well as liters. 

 e. Many test method and process standards. Adding equivalent metric dimensions will 

permit the direct use of scales, micrometers, gauges, and other instruments calibrated in either 

system of units. However, if the existing standards and test methods affect final physical 

configuration or performance requirements, careful consideration is needed in preparing metric 

standards. 
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 4.2 When Hard Conversion Is Needed. Hard conversion is often necessary when there is 

a need for international harmonization or to operate with metric hardware. New metric design 

standardization documents will generally be developed for: 

 a. Items which have universal application and which require standardization in order to 

provide metric components for designers of metric materiel. 

 b. Items such as wire sizes, screw thread forms, fasteners, tubing sizes, and dimension 

stock material used in new systems specified from the beginning in metric dimensions. 

 c. Those areas which specify existing system designs that are to be changed to permit the 

use of metric system standards, production machinery, raw materials, spare parts, and 

maintenance tools and for which the most cost effective set of such changes must be selected. 

 d. Items where a reduction in the ranges, types, styles, or classes or products will be 

facilitated by adoption of a family of metric items. 

 e. Items which are peculiar to the DoD and represent technological advances for 

operational application in new materiel. 

 f. Items which have a universal application of function and industry has now designed 

the items in the metric system of measurement. 

New metric standardization documents will often supplement specifications and standards and 

will not necessarily supersede them. As the use of the metric system grows, use of metric 

standardization documents will increase, while use of customary standardization documents will 

decrease. 

 4.3 When No Conversion Is Needed. 

 a. Items which are becoming obsolete and the change to metric is not practical. 

 b. Items which will continue in use without modifications. No useful purpose will be 

served in translating inches to millimeters for such items, its spare parts, or the tools required to 

maintain them since production and maintenance will continue to use inch-pound system 

facilities and standards. 

 c. Items of very limited applications where costs of metric modules outweigh benefits. 

 d. Not measurement sensitive documents (see 3.4). 

 4.4 Metric Approaches. The following approaches should be considered when 

developing specifications and standards to support metric usage: 

 a. New parallel document. For very complex documents filled with many conversionsusceptible measurements, the logical method is to issue a new SI metric standardization 
5

document. Great care should be used to ensure that the new document is hard metric, and that 

equivalents are carefully selected. After that, the basic document and the metric document 

would be revised concurrently, until such time as the inch-pound document is no longer required 

and is cancelled. 

 b. Metric appendix. For less complex documents, or for very complex documents where 

retention of the original document number is considered necessary, a hard metric appendix could 

be prepared. The basic document would remain in inch-pound units and refer to the appendix for 

metric information. The appendix would refer to the basic document for technical features and 

cite only the metric equivalents, exercising care to ensure that equivalents are carefully selected. 

 c. Metric notes. For relatively simple documents with only a few measurement units, 

metrication may be handled by appropriate notes or by one or more footnotes. 

 d. Contract wording. Metric requirements can be inserted in contracts in lieu of 

development of metric specifications or standards. 

 4.5 Metric Values. As far as the individual quantity requirements contained in the 

document, there are two basic ways of arriving at practical and meaningful metric values in hard 

conversion: 

 a. Size substitution - simple replacement of standard inch-pound size with existing 

accepted metric size. This is often used to conform to internationally recognized metric 

modules. Use of preferred types and sizes and parts and materials, especially those of ISO and 

IEC should be encouraged. Alteration of international or foreign standards to fit domestic needs 

before adoption is sometimes necessary. 

 b. Adaptive conversion - a change from a rounded, rational inch-pound quantity to a 

rounded, rational metric quantity which is reasonably equivalent (as, for example, speed limits). 

The decision on which to use must be based on knowledge, experience, and common sense. 

There is a need for addressing rationalization and variety reduction, and consideration of 

arithmetic or geometric series of sizes. It is desirable to use metric dimensions for physical 

interface in hybrid systems, except between inch-pound items. 

5. Dual Dimensions. The use of both metric and inch-pound measurements on drawings or 

other pictorial illustrations to be used in a specification or standard should be avoided. The use 

of tables to translate the specific inch-pound units to metric units is acceptable. For text material, 

when preference is given to inch-pound units, acceptable metric units may be shown in 

parentheses. When preference is given to metric units, inch-pound units may be omitted or 

included in parentheses. In general, where it has long been standard practice to cite metric units 

alone (such as citing temperatures only in degrees Celsius), inch-pound equivalents may be 

omitted. A specific repetitive equivalent (for example, 1 inch (25.4 mm)), need be inserted only 

the first time it appears in a paragraph of a standardization document. 
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6. Other Guidance Publications. The following organizations have developed publications to 

provide additional guidance on metric use and practices. 

Aerospace Industries Association – NAS 10000 “NA Documents Preparation and Maintenance 

in SI (Metric) Units.” Available from http://aia-aerospace.org/ 

Aerospace Industries Association – NAS 10001 “Preferred Metric Units for Aerospace.” 

Available from http://aia-aerospace.org/

American Petroleum Institute – “Guidelines for Use of the International System of Units (SI) in 

the Petroleum and Allied Industries, Third Edition” API MPMS Chapter 15-01. Available from 

http://api-ec.api.org/newsplashpage/index.cfm 

American Society of Agricultural Engineers – EP285.7, “Use of SI (Metric) Units.” Available 

from http://www.asae.org/ 

American Society of Heating, Refrigerating, and Air Conditioning Engineers – 2001 ASHRAE 

Fundamentals Handbook: Chapter 37, “Units and Conversions.” Available from 

http://www.ashrae.org/ 

American Society of Mechanical Engineers – “ASME Orientation and Guide for Use of SI- 

(Metric) Units—Ninth Edition.” Available from http://www.asme.org/ 

American Society of Mechanical Engineers – ASME B4.2, “Preferred Metric Limits and Fits.” 

Available from http://www.asme.org/

American Society of Mechanical Engineers – ASME B4.3, “General Tolerances for Metric 

Dimensioned Products.” Available from http://www.asme.org/

American Welding Society – “Metric Practice Guide for the Welding Industry; 2001.” Available 

from http://www.aws.org/ 

American Welding Society – “Everyday Pocket Handbook on Metric Practices for the Welding 

Industry.” Available from http://www.aws.org/ 

Association for Information and Image Management – ANSI/AIIM TR01, “Guidelines for 

Metrics – With 1992 Addendum.” Available from http://www.aiim.org/ 

ASTM International – IEEE/ASTM SI 10, “Use of the International System of Units (SI): The 

Modern Metric System.” Available from http://www.astm.org 

ASTM International – E621, “Standard Practice for the Use of Metric (SI) Units in Building 

Design and Construction.” Available from http://www.astm.org

ASTM International – E 713, “Standard Guide for Selection of Scales for Metric Building 

Drawings.” Available from http://www.astm.org
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ASTM International – F 1332, “Standard Practice for Use of SI (Metric) Units in Maritime 

Applications” (Supplement to IEEE/ASTM SI 10). Available from http://www.astm.org

Department of Commerce/National Institute of Standards and Technology – NIST Special 

Publication 330, “International System of Units (SI).” Available from 

http://www.nist.gov/metric

Department of Commerce/National Institute of Standards and Technology – NIST Special 

Publication 811, “Guide for the Use of the International System of Units (SI).” Available from 

http://www.nist.gov/metric

Department of Commerce/National Institute of Standards and Technology – NIST Special 

Publication 814, “Interpretation of the SI for the United States and Federal Government Metric 

Conversion Policy.” Available from http://www.nist.gov/metric

Department of Commerce/National Institute of Standards and Technology – Federal 

Register/Vol. 63, No. 144/Tuesday, July 28, 1998/Notices “Metric System of Measurement: 

Interpretation of the International System of Units for the United States.” Included in NIST 

Special Publication 814. Available from http://www.nist.gov/metric

Department of Commerce/National Institute of Standards and Technology – “The NIST 

Reference on Constants, Units and Uncertainty.” Available from 

http://physics.nist.gov/cuu/index.html

Electronic Industries Alliance – JEDEC JEP86-A-76, “Recommended Practice for Dual 

Dimensioning.” Available from http://www.eia.org 

Institute of Electrical and Electronic Engineers – IEEE 260.1, “American National Standard 

Letter Symbols for Units of Measurement (SI Units, Customary Inch-Pound Units, and Certain 

Other Units).” Available from http://www.ieee.org 

Institute of Electrical and Electronics Engineers – IEEE 945, “IEEE Recommended Practice for 

Preferred Metric Units for Use in Electrical and Electronics Science and Technology.” Available 

from http://www.ieee.org 

Institute of Electrical and Electronics Engineers – IEEE/ASTM SI 10, “Use of the International 

System of Units (SI): The Modern Metric System.” Available from http://www.ieee.org

Interagency Committee on Metric Policy – FED-STD-376, “Preferred Metric Units for General 

Use by the Federal Government.” Available from http://assist.daps.dla.mil/quicksearch/ 

International Bureau of Weights and Measures – “The International System of Units (SI)” 7th 

Edition 1998 with 2000 Supplement.” Available from 

http://www.bipm.org/enus/6_Publications/si/si-brochure.html
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International Organization for Standardization – ISO 1000, “SI Units and Recommendations for 

the Use of Their Multiples and Certain Other Units.” Available from 

http://www.iso.ch/iso/en/ISOOnline.frontpage 

National Fluid Power Association – NFPA T2.10.2, “Survey on Metric Language Usage by the 

US Fluid Power Industry.” Available from http://www.nfpa.com/ 

 

Society of Automotive Engineers – SAE AIR 1657, “Handbook of Hydraulic Metric 

Calculations.” Available from http://www.sae.org 

Society of Automotive Engineers – SAE AIR 1758, “Limits and Fits, International Metric 

Tolerance System.” Available from http://www.sae.org

Society of Automotive Engineers – SAE J390, “Dual Dimensioning Engineering Drawings.” 

Available from http://www.sae.org

Society of Automotive Engineers – SAE TSB003, “Rules for SAE Use of SI (Metric) Units.” 

Available from http://www.sae.org