Unit Converter

How to convert units: multiply by 1

Every conversion factor says two amounts are equal: 1 ft = 12 in. Written as a fraction, 12 in / 1 ft, it is equal to 1, so multiplying by it changes the units without changing the amount. Put the unit you are changing from on the bottom, and it cancels:

5 ft × (12 in / 1 ft) × (2.54 cm / 1 in) = 5 × 12 × 2.54 cm = 152.4 cm

This is called dimensional analysis. If the units don’t cancel, the fraction is upside down, which is a handy check. The converter uses the fewest factors it can, and only exact ones, so the only rounding is in the last digit shown.

The exact definitions

Most customary units have been defined in metric units since 1959, so these factors are exact, not measured:

Temperature is different

Temperature scales start at different points, so a factor alone isn’t enough. Water freezes at 0 °C = 32 °F and boils at 100 °C = 212 °F: 100 Celsius degrees span 180 Fahrenheit degrees. So F = C × 9/5 + 32 and C = (F − 32) × 5/9. Kelvin uses Celsius-sized degrees but starts at absolute zero: K = C + 273.15.

US and UK measures aren’t the same

• Gallons and pints: a UK gallon is 4.546 L and a US gallon 3.785 L. A UK pint is 20 UK fluid ounces (568 mL); a US pint is 16 US fluid ounces (473 mL).
• Tons: a US ton is 2,000 lb (907 kg), a UK ton 2,240 lb (1,016 kg), and a metric tonne 1,000 kg.
• Cups: a US cup is 236.6 mL, while recipes from Australia, New Zealand and Canada usually use a 250 mL metric cup.

Why a 500 GB drive shows as 465 GB

Drive makers count in 1,000s (1 GB = 1,000,000,000 bytes), but many computers count in 1,024s (1 GiB = 1,073,741,824 bytes) and still call it GB. 500 × 1,000,000,000 ÷ 1,073,741,824 ≈ 465.66, so nothing is missing.