www.nist.gov/si-redefinition

SI KILOGRAM

All Photos Credit: J. L. Lee / NIST

Kilogram – Unit of Mass 

Symbol: kg

The kilogram is defined by 

taking the fixed numerical 

value of Planck’s constant h

to be 6.62607015 ×10−34 when 

expressed in the unit J s, which 

is equal to kg m2 s−1, where the 

meter and the second are defined 

in terms of c and ∆νCs.
4. When should I have my 

weights recalibrated to the new 

definition?

There is no need for an immediate 

recalibration or a change in your 

calibration interval. You can have 

your weights recalibrated at the next 

scheduled calibration as dictated by 

your quality documentation (see #1). 

5. Will my masses be calibrated 

in vacuum or put in the Kibble 

balance when I send them to NIST 

for calibration?

No, masses will not be calibrated in 

vacuum. All customer calibrations 

will be conducted at atmospheric 

pressure in typical laboratory 

conditions against working standards 

traceable to the new SI realization in 

vacuum. 

6. I’ve heard that the kilogram isn’t 

stable. Should I include a drift 

component in my uncertainty 

budget and if so how much?

The new definition of the kilogram 

has no systematic drift and is 

inherently stable as it is tied to 

Planck’s constant. The artifacts used 

in the dissemination of mass may 

be unstable and it is up to each 

laboratory to monitor stability of 

their masses and apply appropriate 

corrections internally. 

7. When will I be able to get a 

Kibble balance?

NIST and other National Metrology 

Institutes are partnering with industry 

leaders to develop tabletop Kibble 

balances. Public announcements will 

be made as they become available.

8. What uncertainty changes can 

I expect from NIST as a result of 

the redefinition? 

As of May 20, 2019, there will be a 

slight increase in the uncertainty 

of your calibrated masses at the 

kilogram level. The classification 

of your masses based on legal 

metrology standards will not be 

affected.

9. What should my traceability 

statement say?

If you are traceable to the SI now, you 

will remain traceable under the new 

definition. For masses calibrated at 

NIST, you can get your traceability 

statement from the calibration report 

for your masses. 

10. How should I change my quality 

documentation to reflect the 

new definition?

In your quality documentation 

you can state that traceability and 

continuity is maintained as detailed 

in #1 and #8.

11. Who can I contact with further 

questions about the redefinition’s 

impact on me? 

Any further questions can be 

directed to Zeina Kubarych 

(zeina.kubarych@nist.gov) or Patrick 

Abbott (patrick.abbott@nist.gov) of 

the Mass and Force Group at NIST.

1. Will the values of the corrections 

on my masses change as a result 

of the redefinition?

No. The redefinition was implemented 

such that continuity exists between 

the old and new definitions of the 

kilogram. This was proven by a pilot 

study conducted in 2016 between 

the National Metrology Institutes 

contributing to the new definition and 

the International Bureau of Weights 

and Measures. 

2. Will the values of the uncertainties 

on my masses change because of 

the redefinition?

Yes. As of May 20, 2019, the 

International Prototype Kilogram 

(IPK) will be assigned an uncertainty 

of ± 0.010 mg. It will be necessary to 

add this uncertainty in quadrature 

to all one-kilogram calibrations that 

are traceable to the SI through the 

IPK. For example, if your current 

uncertainty (k=1) for your onekilogram mass is 20 micrograms, the 

new uncertainty (k=1) becomes 

22.4 micrograms. 

3. Are you still calibrating weight 

sets? How about large masses?

Yes, we are still calibrating masses up 

to approximately 27,000 kg.

Mass Calibrations 

under the New SI