Unit Systems: SI, US Customary, and Imperial

SI is the modern international metric system

The BIPM describes the International System of Units (SI) as the internationally agreed measurement system. It has seven base units. Since May 20, 2019, its definitions have been based on fixed values of seven defining constants.

Base units are a starting point, not a list of every useful unit. Area and speed, for example, can be expressed in square meters and meters per second. The kilogram is the mass base unit. The liter is a non-SI unit accepted for use with SI; it is not an eighth base unit. Kelvin is the temperature base unit, and Celsius is related to kelvin by an offset.

The name SI was adopted by the CGPM in 1960. The 1875 Metre Convention created the BIPM. Those are different milestones. The measurement systems history guide explains that distinction.

US customary and Imperial need separate labels

NIST recommends “US customary” for the non-SI measurement units used in the United States. It is misleading to treat every US unit as British Imperial, or to divide countries into those that use SI and those that have never adopted it. A document’s named convention is more useful than a country stereotype when selecting a factor.

The international inch, foot, and avoirdupois pound provide shared definitions for these reference entries. That does not make every similarly named unit interchangeable. The table distinguishes the US liquid gallon and pint from their Imperial counterparts, as well as the short ton, long ton, and tonne.

An ounce also needs context. An avoirdupois ounce measures mass; a fluid ounce measures volume. Moving from fluid ounces to grams requires information about the substance. A pound of mass is also different from a pound-force. Choose a mass tool, volume tool, or force tool according to the quantity being measured.

For recipes, use the cup and spoon convention comparison. A nutrition-label or Australian recipe cup must not silently inherit a US customary cup factor. The liquid-volume entries below do not describe US dry measures.

Named units and their metric equivalents

The numbers below use the same unit data and calculation engine as the converters. The NIST conversion entries are the source reference; fuller customary values can be derived from definitions and the Handbook 44 unit ratios. The acre and square-foot rows use the international foot; NIST distinguishes these from legacy survey-foot equivalents. Display rounding is separate from a measurement’s accuracy.

For amounts in a non-temperature row’s named unit, multiply by its metric factor. For the reverse direction, divide by that factor. Temperature needs both a scale and an offset: °C = (°F − 32) × 5/9. The two temperature rows give numeric pairs without claiming a universal boiling or freezing condition.

Use the metric, US customary, and Imperial reference chart for its lookup tool, or open the matching converter to calculate another amount.

Prefixes scale units; dimensions still matter

SI prefixes express decimal multiples or fractions. These are four useful examples, not the complete prefix list.

For length, 1 meter = 100 centimeters. Squaring that relationship gives 1 m² = 10,000 cm²; cubing it gives 1 m³ = 1,000,000 cm³. These are arithmetic derivations. Moving two decimal places for every metric conversion would therefore give wrong area and volume results.

Use an area converter for square units and a volume converter for cubic or capacity units. Converting length into area, or volume into mass, is a different task that needs additional information.

Choose a conversion in four steps

1. Identify the quantity: length, mass, liquid volume, area, speed, or temperature.
2. Read the full unit name and convention, including US liquid, Imperial, short ton, long ton, or mass ounce.
3. Choose a tool for the same quantity. For recipes, check the ingredient and measuring convention separately.
4. Calculate with the defined factor, then round the displayed result for the precision you need. Extra calculator digits do not improve an uncertain measurement.

If a source says only “gallon” or “ton” and the context does not resolve it, ask the author which convention they mean. A converter cannot recover missing measurement information.

Common questions

Is SI the same as every metric unit?

SI is the modern international system, with base and derived units. Everyday metric writing also uses accepted non-SI units such as the liter. Keep that classification distinct from a tool’s practical unit choices.

Is a US gallon the same as an Imperial gallon?

No. They have separate rows and factors above. The site's “US gallons and liters” task uses the US liquid gallon; select Imperial explicitly in the volume tool when that is the source convention.

Why does temperature require another formula?

Celsius and Fahrenheit have different zero points. A numerical pair in the table is not a ratio for every temperature; use the offset formula or temperature converter.

Should I memorize the rounded factors?

They help read a reference, but use the matching converter when the rounding matters. Retain the original unit, convention, and measurement precision alongside the result.

Sources and review

• BIPM SI overview: modern base units and definitions.
• CGPM Resolution 12 (1960) and Metre Convention: distinct historical milestones.
• BIPM prefix table: prefix names, symbols, and factors.
• NIST metrication FAQ: US customary terminology and SI adoption context.
• NIST SP 811 Appendix B.8: conversion factors by named unit.
• NIST Handbook 44 (2023), Appendix C: customary and Imperial unit ratios.
• NIST revised land-unit factors: international-foot acre and legacy survey-foot distinctions.
• NIST units outside SI: accepted non-SI units.
• NIST temperature guidance: Celsius/Fahrenheit/Kelvin relationships.

Reviewed October 5, 2026. Tables identify calculated values and rounding. See our conversion methodology.