acalculator

How do I convert a measurement?

Pick what you are measuring, enter the amount, and pick the two units. The answer and the formula update as you type.

Your numbers

Converted amount
2.54

1 in = 2.54 centimeters (cm).

From
1 in
To
centimeters (cm)
Formula
cm = in × 2.54

Converted amount: 2.54. 1 in = 2.54 centimeters (cm).

How to calculate

Converts a measurement of length, weight, volume, temperature, area, or speed from any unit to any other.

Example with the default inputs (Measure Length, Amount 1, From Inches (in), To Centimeters (cm)): 1 in = 2.54 centimeters (cm).

Method: converted = amount × (base units per "From" unit) ÷ (base units per "To" unit); temperatures add an offset

  • Every factor is the exact definition in NIST SP 811 (2008), Appendix B.8: 1 in = 2.54 cm, 1 lb = 0.45359237 kg, 1 US gallon = 231 in³.
  • Cups, pints, quarts, gallons, and fluid ounces are US customary units.
  • Temperatures convert through kelvins: K = °C + 273.15 = (°F + 459.67) × 5 ÷ 9. Nothing is colder than 0 K.
  • A length, weight, volume, area, or speed cannot be negative.
  • The answer shows at most 6 decimal places; the calculation keeps full precision.

Machine-readable copies: Markdown, JSON.

Worked examples

Each example is checked against the calculator on every build.

  1. Measure Length, Amount 1, From Inches (in), To Centimeters (cm) gives Converted amount 2.54, Formula cm = in × 2.54.Source: NIST SP 811 (2008) B.8: 1 in = 2.54 cm exactly
  2. Measure Weight, Amount 150, From Pounds (lb), To Kilograms (kg) gives Converted amount 68.038856.Source: NIST SP 811 (2008) B.8: 1 lb = 0.45359237 kg exactly
  3. Measure Volume, Amount 1, From Gallons (gal), To Liters (L) gives Converted amount 3.785412.Source: NIST SP 811 (2008) B.8: 1 US gallon = 231 in³ = 3.785411784 L
  4. Measure Temperature, Amount 98.6, From Degrees Fahrenheit (°F), To Degrees Celsius (°C) gives Converted amount 37, Formula °C = (°F − 32) × 5 ÷ 9.Source: NIST SP 811 (2008) 4.2.1.1: t/°C = (t/°F − 32) ÷ 1.8
  5. Measure Area, Amount 1, From Acres (ac), To Square feet (ft²) gives Converted amount 43,560.Source: NIST SP 811 (2008) B.8: 1 acre = 43,560 ft²
  6. Measure Speed, Amount 60, From Miles per hour (mph), To Kilometers per hour (km/h) gives Converted amount 96.56064.Source: NIST SP 811 (2008) B.8: 1 mi = 1.609344 km exactly

How it works

Every unit of a measure has an exact size in that measure's base unit: metres for length, kilograms for weight, cubic metres for volume, square metres for area, and metres per second for speed. The converter turns the amount into the base unit, then into the unit you want:

converted = amount × (base units per "From" unit) ÷ (base units per "To" unit)

The formula line shows the one factor for your two units as "To = From × factor", or as "To = From ÷ factor" when that factor is shorter (for example "in = cm ÷ 2.54"):

  • Each factor is rounded to 12 significant figures; a factor that ends exactly on a 5 at the 13th figure rounds up (1 tbsp = 14.78676478125 mL shows 14.7867647813). Factors are written out in full, never in exponent form (mil = km ÷ 0.0000000254).
  • "Shorter" means fewer significant digits, not counting leading or trailing zeros: 2.54 has 3, 0.393700787402 has 12, 1000 and 0.001 have 1.
  • On a tie, the formula shows the factor that is 1 or more: g = kg × 1000, and kg = g ÷ 1000.
  • The same unit on both sides has no factor: km² = km².

The units and their exact sizes

  • Length (metres): mm 0.001, cm 0.01, m 1, km 1,000, µm 0.000001, nm 0.000000001, in 0.0254, ft 0.3048 (12 in), yd 0.9144 (3 ft), mi 1,609.344 (5,280 ft), nautical mile 1,852, mil 0.0000254 (1/1,000 in).
  • Weight (kilograms): µg 0.000000001, mg 0.000001, g 0.001, kg 1, metric ton 1,000, grain 0.45359237 ÷ 7,000, oz 0.45359237 ÷ 16, lb 0.45359237, US short ton 2,000 lb.
  • Volume (cubic metres): mL and cm³ 0.000001, L 0.001, m³ 1, US gallon 231 in³ = 0.003785411784, quart 1/4 gallon, pint 1/8, cup 1/16, fluid ounce 1/128, tablespoon 1/256, teaspoon 1/768, in³ 0.0254³, ft³ 0.3048³, yd³ 0.9144³.
  • Area (square metres): mm² 0.000001, cm² 0.0001, m² 1, hectare 10,000, km² 1,000,000, in² 0.0254², ft² 0.3048², yd² 0.9144², acre 43,560 ft² = 4,046.8564224, mi² 1,609.344².
  • Speed (metres per second): m/s 1, km/h 1,000 ÷ 3,600, mph 1,609.344 ÷ 3,600 = 0.44704, ft/s 0.3048, knot 1,852 ÷ 3,600.
  • Temperature: through kelvins. K = °C + 273.15, and K = (°F + 459.67) × 5 ÷ 9. So °F = °C × 9 ÷ 5 + 32 and °C = (°F − 32) × 5 ÷ 9.

Assumptions

  • A length, weight, volume, area, or speed below zero has no answer. A temperature below absolute zero (0 K) has no answer.
  • The answer shows at most 6 decimal places; the calculation keeps full precision.

Worked examples by hand

1 inch to centimetres. 1 × 0.0254 m ÷ 0.01 m = 2.54 cm.

150 pounds to kilograms. 150 × 0.45359237 = 68.0388555 kg.

1 US gallon to litres. 231 in³ × 0.0254³ = 0.003785411784 m³, and ÷ 0.001 = 3.785411784 L.

98.6 °F to Celsius. (98.6 − 32) × 5 ÷ 9 = 66.6 × 5 ÷ 9 = 37 °C.

1 acre to square feet. An acre is 43,560 ft² by definition.

60 mph to km/h. 60 × 1,609.344 m ÷ 1,000 = 96.56064 km/h.

Other questions people ask

Which units can I convert?

Length: millimeters to miles, nautical miles, and mils. Weight: micrograms to short tons, including grains and ounces. Volume: milliliters, liters, cubic units, and US teaspoons, tablespoons, fluid ounces, cups, pints, quarts, and gallons. Temperature: Celsius, Fahrenheit, and kelvins. Area: square millimeters to square miles, hectares, and acres. Speed: m/s, km/h, mph, ft/s, and knots.

Are the conversions exact?

Yes, for every unit here. Since 1959 the inch is exactly 2.54 cm and the pound exactly 0.45359237 kg, and the US gallon is exactly 231 cubic inches. The calculator uses these definitions (NIST SP 811) and rounds only the number it shows.

Why can a temperature be negative but a length cannot?

Celsius and Fahrenheit put zero at a chosen temperature, so colder temperatures are negative. Nothing is colder than absolute zero, 0 K, which is −273.15 °C and −459.67 °F. A length, weight, volume, area, or speed starts at zero.

Are cups and gallons US or UK units?

US customary units. A US cup is 236.5882365 mL and a US gallon 3.785411784 L. A UK (imperial) gallon is larger, 4.54609 L, and is not in this list.