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Rules&Regs

Using SI in ASTM Standards

A Guide to Form and Style for ASTM Standards, Part H

Part H of Form and Style for ASTM Standards describes how to use the International System of Units (SI) in ASTM standards. SI units are to be included in all ASTM standards. A discussion early in Part H notes the importance of SI:

“Given ASTM’s mission to be the foremost developer and provider of voluntary consensus standards with global recognition and use, ASTM technical committees are urged to give diligent consideration to the use of rationalized SI (metric) units in their standards.”

According to H1.2.1, each technical committee has the option of using rationalized SI units, or rationalized inch-pound units, or both, as the standard units of measure. H1.2.2 instructs committees to follow procedures given in SI 10, American National Standard for Metric Practice. SI 10, jointly sponsored by ASTM Committee E43 on SI Practice and IEEE Standards Coordinating Committee 14 (Quantities, Units and Letter Symbols), is available in Vol. 14.04 of the Annual Book of ASTM Standards, and as a separate publication.

Some ASTM committees may have special considerations in the use of SI units in their standards. H1.2.2.1 allows such committees to develop specific technical guidance for clarification. Examples include the Outline of Form of Specifications used by Committee B05 on Copper and Copper Alloys, and A994, Guide for Editorial Procedures and Form of Product Specifications for Steel, Stainless Steel and Related Alloys, under the jurisdiction of Committee A01 on Steel, Stainless Steel and Related Alloys.

Section H2 presents relevant definitions for terms, including SI unit, inch-pound unit and rationalization. When writing a standard, the following two terms are important to understand:

  • Solely SI Standard, an ASTM standard in which only rationalized SI units are cited; inch-pound units are not provided in the standard.
  • Combined Standard, an ASTM standard in which rationalized SI units and inch-pound units are included in the same standard, with each system of units to be regarded separately, as standard.

For solely SI standards, the following paragraph must be included in the scope:

Units — The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.

The following paragraph must be included in combined standards:

Units — The values stated in either SI units or inch-pound units are to be regarded separately as standard. The value stated in each system may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from the two systems may result in non-conformance with the standard.

H3.1.2.2 recommends that SI units appear first in a combined standard, followed by the inch-pound units in brackets. However, a committee can reverse this order if the following conditions are met:

  • All units appear in a consistent order throughout the text of the standard; and
  • All combined standards under the committee’s jurisdiction apply the same convention.

When a standard cites inch-pound units as the standard units of measurement, the following paragraph is to be included in the scope:

Units — The values stated in inch-pound units are to be regarded as standard, The values given in parenthesis are mathematical conversions to SI units that are provided for information only and are not considered standard.

Further information regarding special format considerations is included in Section H5. H6 on Tables notes that instructions on including SI in tables can be found in Section G27 of Form and Style.

This article appears in the issue of Standardization News.