United States customary units are the non-SI measurement units used in the United States, including the inch, foot, pound, gallon, and degree Fahrenheit. They are a collection with roots in several older measurement traditions, not one perfectly unified system. Their names overlap with British Imperial units, but some values differ, so the two systems should not be treated as interchangeable.
What are United States customary units?
The term U.S. customary units describes the non-metric units used in the United States. The National Institute of Standards and Technology (NIST) recommends this terminology because the collection draws on multiple historical systems, including avoirdupois, troy, and apothecary measures. Common examples include:
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- Length: inch, foot, and yard
- Mass: ounce and pound
- Capacity: fluid ounce and gallon
- Temperature: degree Fahrenheit
These units are now defined in terms of the International System of Units (SI), even though customary measures remain in everyday use across many U.S. applications. See NIST’s U.S. Metrication FAQs and U.S. Survey Foot: Frequently Asked Questions.
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Is the U.S. customary system the same as British Imperial?
No. The systems share many unit names and some relationships, but particular measures differ. For example, 12 inches make a foot and 3 feet make a yard in both systems, while capacity units and some mass denominations are not equivalent. NIST’s 2026 Handbook 44, Appendix B cautions against using “Imperial” or “British” to describe U.S. customary units.
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| Measure | U.S. customary value | British value | Why the distinction matters |
|---|---|---|---|
| Short ton / British ton | 2,000 pounds | 2,240 pounds | The same general name can denote different masses. (NIST, 2026 Handbook 44, Appendix B) |
| Gallon | The U.S. gallon descends from the 231-cubic-inch English wine gallon used by colonists. | The British Imperial gallon, defined in 1824, is approximately 277.42 cubic inches. | A volume stated simply as “gallons” may be ambiguous across jurisdictions. (NIST, 2026 Handbook 44, Appendix B) |
Not every similarly named unit differs. When precision matters, identify the jurisdiction and the specific unit—especially for gallons, fluid ounces, bushels, and tons.
Why does the U.S. use feet and pounds?
U.S. customary measures descend from English measures used before U.S. independence. The United States did not replace them wholesale when metric standards became part of modern measurement. Instead, the customary units acquired precise definitions tied to metric standards, while continuing to be used in many fields.
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The Metric Act of 1866 legalized metric measurement in the United States. A 1988 amendment made SI the preferred system for U.S. trade and commerce, while leaving customary use permitted. Federal policy encourages voluntary metrication; the pace of adoption is left to companies, organizations, and individuals. NIST explains the legal position on its Metrication in Law page.
Are U.S. customary units defined in metric?
Yes. NIST states that U.S. customary units are defined in terms of SI. For example, the international foot is exactly 0.3048 meter. A 1977 National Bureau of Standards history of U.S. weights and measures records that the yard and pound had been defined in terms of the meter and kilogram since 1893; see Weights and Measures Standards of the United States: A Brief History.
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Which foot applies to new measurements?
Use the international foot for new applications. NIST says the U.S. survey foot should be avoided for new use after December 31, 2022, while historical and legacy applications may retain it. Older survey records can therefore encode a different foot definition; check the dataset’s stated units before converting or combining it with newer measurements. Details are in NIST’s survey-foot FAQs.
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How should you convert U.S. customary measurements?
- Identify the exact unit and variant. For example, distinguish a U.S. gallon from an Imperial gallon, or a short ton from a British ton.
- Check for legacy definitions in historical data, particularly whether a foot is the international foot or the U.S. survey foot.
- Use an authoritative conversion table or calculator, and retain the units in the result so the conversion is auditable. NIST’s conversion resources provide a starting point.
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