NIST Guide to the SI, Chapter 2: 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.¹ 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, Sec. 2.1.1 Tables and graphs, and Sec. 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.

2.1.1 Tables and graphs

In tables, values of quantities expressed in acceptable units and the corresponding values expressed in other units may be shown in parallel columns, with the acceptable-unit column preceding the other-unit column. In graphs, axes labeled in other units shall be given secondary status. This may preferably be done by placing scale marks on and labeling the left-hand ordinate and bottom abscissa in acceptable units, and placing scale marks on and labeling the right-hand ordinate and top abscissa in other units. Alternatively, lighter-weight scale marks and smaller type may be employed to indicate other units using the same ordinate and abscissa as is used for the acceptable units.

2.2 Descriptive information

Descriptive information characterizes arrangements, environments, the generalized dimensions of objects, apparatus, or materials, and other attributes that do not enter directly into calculations or results. When necessary for effective communication, such information may be expressed using customary terms that are widely used and recognized. Examples include common drill sizes and traditional tools used in the United States, U.S. standard fastener sizes, commercial pipe sizes, and other common terms used in the trades, the professions, the marketplace, sports, and various social activities. When such descriptive information is given, values in acceptable units are not required. For example, it is permissible to refer to a "36-inch pipeline" or a "half-inch drill" without first giving the value in an acceptable unit.