Menu

Money & investing8 Math19 Unit converters171 Health & fitness7 Time & work5 School & grades3 Images & PDFs11 Text & generators14 Career1 Everyday life2

Guides Glossary Templates & planners

US Customary Units

Also called: US customary system, US standard units, English units

Definition: US customary units are the inch, foot, mile, ounce, pound, cup and gallon used for everyday measures in the United States; each is defined by an exact or fixed value in SI units.

Updated October 28, 2026 · How we check facts

US customary units are the length, mass and volume units used in everyday life in the United States: inch, foot, yard, mile, ounce, pound, fluid ounce, cup, pint, quart and gallon. They use irregular ratios (12 inches per foot, 16 ounces per pound, 4 quarts per gallon) rather than powers of ten. The units are tied to the International System of Units: NIST lists the inch as exactly 2.54 cm, the foot as 0.3048 m and the mile as 1.609344 km.

Example

A 6 ft 2 in person is 6 × 12 + 2 = 74 in, and 74 × 2.54 = 188 cm. A 26.2 mile marathon is 26.2 × 1.609344 = 42.16 km.

The inches to cm tool and kg to lbs tool do these conversions. The tape measure guide covers reading inch fractions.

US customary vs imperial

The two systems share names but not every unit. The US gallon is about 3.785 L; the British imperial gallon is 4.54609 L. See metric vs imperial for the comparison, and for a case where mixing systems caused a real failure (NASA's Mars Climate Orbiter, 1999).

Calculate it

Free toolInches to CM ConverterInches to cm, cm to inches and height in feet.Open →Free toolKg to Lbs ConverterKilograms to pounds and back, with lb + oz and stone.Open →

Guides that explain it

Related terms

International System of Units (SI)The International System of Units (SI) is the modern metric system: seven base units, including the meter, kilogram, second and kelvin, with prefixes that scale by powers of ten.Fluid Ounce (fl oz)A fluid ounce is a unit of volume; one US fluid ounce is 29.57353 milliliters, and 8 of them make a US cup. It is not the same as the ounce used to weigh things.Ounce (avoirdupois)The avoirdupois ounce is the US and imperial unit of weight equal to 1/16 pound, or 28.34952 grams. It measures mass, unlike the fluid ounce, which measures volume.Absolute ZeroAbsolute zero is 0 K, the lowest point of the kelvin temperature scale. On the Celsius scale it equals −273.15 °C, and on the Fahrenheit scale about −459.67 °F.Greatest Common Factor (GCF)The greatest common factor (GCF) of two or more positive integers is the largest whole number that divides each of them with no remainder; it is also called the GCD or HCF.Kelvin (K)The kelvin is the SI base unit of temperature. It starts at absolute zero (0 K), and one kelvin is the same size as one degree Celsius, with 0 °C equal to 273.15 K.Least Common Multiple (LCM)The least common multiple (LCM) of two or more positive integers is the smallest positive number that each of them divides into evenly; it gives the lowest common denominator.Mixed NumberA mixed number combines a whole number and a proper fraction, such as 4 9/100; it equals an improper fraction, here 409/100, and is what a decimal with a nonzero whole part converts to.

Sources