Guide for the Use of the International System of Units (SI)

Guide for the Use of the International System of Units (SI) 
 
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NIST Policy on the Use of the SI 
In accordance with various Federal Acts, the Code of Federal Regulations, and Executive Order 
12770 (see Preface), it is NIST policy that the SI shall be used in all NIST publications.
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When the field of 
application or the special needs of users of NIST publications require the use of other units, the values of 
quantities shall first be expressed in acceptable units, where it is to be understood that acceptable units 
include the SI units and those units recognized for use with the SI; the corresponding values expressed in 
the other units shall then follow in parentheses. (For precise definitions of the terms “SI units” and 
“acceptable units” as used in this Guide, see Sec. 5.4.) Exceptions to this policy require the prior approval 
of the NIST Director. The following three sections—2.1 Essential data, 2.1.1 Tables and graphs, and 
2.2 Descriptive information—elaborate upon this policy. 
2.1 Essential data 
Essential data express or interpret quantitative results. All such data shall be given in acceptable 
units. In those cases where
— the sole use of acceptable units would compromise good communication, or 
— units other than acceptable units have been specified as a contractual requirement, 
values of quantities shall be given in acceptable units followed, in parentheses, by the values of the same 
quantities given in the other units.
Exceptions may sometimes be necessary for commercial devices, technical standards, or quantities 
having special legal significance; examples include commercial weights and measures devices and the 
related laws and regulations. However, even in such cases, values of quantities expressed in acceptable 
units should be used when possible with the same values expressed in other units following in parentheses.