Trade mathematics and unit conversions · Lesson 2 of 4

Trade mathematics and unit conversions: materials, standards and design constraints

NIST, ASTM and manufacturer sources behind the numbers, why a table can print a rounded factor, rounding down to fit and up to order, AFS limits in inches and millimetres, 24 ga weight, and metal expansion coefficients with their units.

You will be able to: Name who defines the unit factors and the rounding rules, choose a rounding direction by purpose, write AFS limits in inches and millimetres, and match a material coefficient to its temperature unit.

14 min · 11 slides · 12 questions

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Slide 1 of 11

Who defines the units

NIST publishes the definitions: Handbook 44 Appendix C and the Guide to the SI (SP 811), where factors in boldface are exact.

Handbook 44 cites the July 1, 1959 Federal Register notice for the exact inch, foot and pound relationships.

IEEE/ASTM SI 10 is the American National Standard for metric practice. It is paid text, so this course points to it and does not reproduce it.

Sources

NIST Handbook 44 (2026 edition), Appendix C: General Tables of Units of Measurement - NIST (U.S. Department of Commerce)
NIST Guide to the SI, Appendix B.9: Factors for units listed by kind of quantity (SP 811, 2008 web edition) - NIST (U.S. Department of Commerce)
IEEE/ASTM SI 10-16, American National Standard for Metric Practice (scope page only) - ASTM International

Glossary

Significant digits: The digits of a number that carry meaning. A converted value keeps the precision of the original measurement, so 8 ft converted to metres is written 2.4 m, not 2.438400 m (NIST SP 1038).
Round down, round up, round to nearest: Three rounding directions chosen by purpose. AFS rounds a fit limit down so it is never over, billed linear feet up so a quote never under-orders, and a shop reading to the nearest 1/16 in.
Blank: The flat piece of sheet cut before bending. AFS limits a blank to 48 in of total flat width with hems included, and a piece to 126 in long.
Coefficient of linear expansion: The change in length per unit of length per degree of temperature change, written E or alpha. It differs by metal and must be used with a temperature change in the matching unit, such as °F.
Temperature change versus reading: A reading, such as 200 °F, converts with the 32 offset. A change, such as a 100 °F rise, converts by 1.8 only: 100 °F of change is 55.56 K or 55.56 °C.