Exact conversion factors
U.S. customary units are legally defined in terms of the metric system, so most conversions are exact rather than approximate: an inch is exactly 2.54 cm and a pound exactly 0.45359237 kg. This converter stores every unit as an exact multiple of one base unit (meter, kilogram, liter, square meter, meter per second), converts through that base, and rounds only for display.
The formulas
value in B = value in A × (factor of A ÷ factor of B) 1 in = 2.54 cm 1 ft = 0.3048 m 1 mi = 1.609344 km 1 lb = 0.45359237 kg 1 U.S. gal = 3.785411784 L 1 acre = 43,560 ft² 1 mph = 0.44704 m/s °C = (°F − 32) × 5/9 °F = °C × 9/5 + 32 K = °C + 273.15
Worked example
26.2 miles to kilometers: 26.2 × 1.609344 = 42.16481 km (a marathon is officially 42.195 km = 26.219 mi).
350 °F oven to Celsius: (350 − 32) × 5/9 = 176.67 °C. 2 cups to milliliters: 2 × 236.5882365 = 473.18 mL.
Notes
- Results show up to 8 significant digits; factors are exact except the derived ones, which are exact to double precision.
- U.S. tons are short tons (2,000 lb). Nautical miles and knots use the international nautical mile of 1,852 m.
Sources: NIST, Handbook 44, Appendix C: General Tables of Units of Measurement; NIST SP 811, Appendix B (conversion factors).
Frequently asked questions
How many centimeters are in an inch?
Exactly 2.54. The inch has been defined as 25.4 mm since the 1959 international yard and pound agreement, so 1 foot = 30.48 cm, 1 yard = 0.9144 m and 1 mile = 1.609344 km — all exact.
How do I convert Fahrenheit to Celsius?
Subtract 32, then multiply by 5/9: (100 °F − 32) × 5/9 = 37.8 °C. The other way, multiply by 9/5 and add 32: 20 °C × 9/5 + 32 = 68 °F. −40 is the same in both scales. Temperature needs a formula rather than a single factor because the scales have different zero points.
How many pounds are in a kilogram?
About 2.20462. The pound is defined as exactly 0.45359237 kg, so 1 kg = 1 / 0.45359237 ≈ 2.2046226 lb. Strictly, pounds and kilograms measure mass; everyday "weight" on a bathroom scale is the same number.
Are U.S. and UK gallons the same?
No. A U.S. liquid gallon is exactly 231 cubic inches = 3.785411784 L; an imperial (UK) gallon is exactly 4.54609 L, about 20% larger. U.S. cups, pints, quarts and fluid ounces here are all U.S. customary units (1 cup = 8 fl oz).