Metric, US Customary, and Imperial: The Milestones Behind Today's Units

A measurement system’s history helps explain its names, but the definition attached to a unit determines the conversion. “Imperial” and “US customary” are often used loosely in everyday writing. That shorthand can hide a different gallon or pint.

For definitions and worked conversion references, start with the unit systems guide. This history guide focuses on three documented milestones in international measurement and what they mean for reading units today.

1875: an agreement for international measurement

The BIPM’s account of the Metre Convention records the treaty signed on May 20, 1875. It created the International Bureau of Weights and Measures (BIPM) and a framework for international cooperation in measurement.

That treaty is not the date the name “International System of Units” was adopted. Treating 1875 as the founding date of SI compresses different stages of standardization into one event.

1960: the name SI is adopted

Resolution 12 of the 11th General Conference on Weights and Measures adopted the name International System of Units, with the abbreviation SI, in 1960. The resolution listed six base units at that time.

The modern system has seven: meter, kilogram, second, ampere, kelvin, mole, and candela. The BIPM’s current SI overview describes that present system. A historical resolution and the modern base-unit list should therefore be read in their own contexts.

2019: definitions based on fixed constants

According to the BIPM SI overview, definitions based on seven fixed defining constants took effect on May 20, 2019. The familiar unit names remain, while the definitions provide the foundation for realizing measurements.

For someone converting a suitcase’s mass or a room’s length, the practical task is still to identify the quantity, choose the correct units, and preserve reasonable precision. A change in how a base unit is defined does not turn an uncertain measurement into an exact one.

Why US customary and Imperial need separate names

NIST’s US metrication FAQ recommends “US customary” when referring to the non-SI units used in the United States. It also explains that SI is recognized internationally, including in the United States. A list of supposedly “non-metric countries” is a poor guide to the units on an actual document.

Some unit definitions are shared: the international inch is 2.54 centimeters, and the avoirdupois pound is 0.45359237 kilograms. Liquid measures still need their convention. The named conversion entries in NIST SP 811 distinguish US and Imperial measures rather than assigning one factor to every “gallon.”

The US liquid gallon is 3.785411784 liters; the Imperial gallon is 4.54609 liters. The fuller US value follows from the gallon’s 231 cubic inches and the international inch definition. NIST Handbook 44, Appendix C supplies the customary unit relationships. These are unit definitions and arithmetic, not estimates of a particular container’s contents.

Use history to ask better conversion questions

The useful question is the convention a source uses, rather than a prediction about an entire country’s habits:

• For a length stated in international inches, use inches and centimeters.
• For pounds of mass, use kilograms and pounds. A pound-force is a different quantity.
• For liquid volume, identify US liquid or Imperial before selecting a gallon or pint factor. Use the named-unit reference chart to compare them.
• For recipes, check the cup or spoon convention and the ingredient separately in cooking measurements. Volume-to-mass conversion needs an ingredient reference or density.

“Metric” also does not mean that every everyday metric unit is an SI base unit. The liter is a non-SI unit accepted for use with SI. Celsius and Fahrenheit conversion requires an offset, as explained in NIST’s temperature guidance.

Return to the unit systems guide for the base-unit list, selected prefixes, dimensional examples, and a four-step conversion workflow.

Reviewed October 5, 2026. Historical claims above link to BIPM records; terminology and conversion references link to NIST. See our methodology for calculation and display practices.