Free Online Unit Converter

Convert length, weight, temperature, and more.

Convert Units

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Common Conversions

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What is a Unit Converter?

A unit converter translates a measurement from one unit into another by multiplying it by an agreed conversion factor. It covers length, weight, volume, temperature, area and speed, spanning both the metric system used by most of the world and the imperial and US customary units still common in the United States. Many of the factors involved are exact by international definition rather than measured approximations.

How to Use This Converter

  1. Choose the category.Length, weight, volume, temperature and so on. Units can only be converted within the same physical quantity.
  2. Enter your value and pick the units.Select what you have and what you want. The result updates as you type.
  3. Check the precision you need.For cooking, a rounded figure is fine. For engineering, medicine or trade, use the full decimal value.

Two Systems, One World

Almost every country uses the metric system, formally the International System of Units (SI). The United States is the main exception in everyday life, with Liberia and Myanmar also retaining non-metric use. This split is why conversion is a constant background task in trade, travel, science and cooking.

Metric units scale by powers of ten, which makes conversion within the system trivial: a kilometre is 1,000 metres, a centimetre is one hundredth of a metre. Imperial units grew from practical, human-scale references and scale irregularly, which is why 12 inches make a foot, 3 feet make a yard and 1,760 yards make a mile.

Exact Conversion Factors

These are defined values, agreed by international standards bodies. They are exact, not rounded measurements.

ConversionExact FactorStatus
1 inch2.54 centimetresExact by definition
1 foot0.3048 metresExact by definition
1 mile1.609344 kilometresExact by definition
1 pound0.45359237 kilogramsExact by definition
1 US gallon3.785411784 litresExact by definition
1 UK gallon4.54609 litresExact by definition

Worked Example: Converting a Recipe

A US recipe calls for 2 cups of milk and an oven at 375 degrees Fahrenheit

Volume conversion

1 US cup = 236.588 millilitres 2 cups = 2 x 236.588 = 473.2 mL Round for kitchen use: about 475 mL

Temperature conversion

C = (F - 32) x 5/9 C = (375 - 32) x 5/9 C = 343 x 0.5556 = 190.6 degrees Round to the nearest oven setting: 190 C

2 cups is roughly 475 mL, and 375 F is about 190 C

Cups are not standardised worldwide

A US cup is 236.6 mL, a metric cup is 250 mL and a UK cup historically was 284 mL. Baking recipes, where ratios matter, can fail if the wrong cup is assumed. Weighing ingredients in grams avoids the problem entirely, which is why most professional recipes specify weight.

Temperature Is the Odd One Out

Most conversions are a single multiplication. Temperature is not, because Celsius and Fahrenheit place their zero points differently. Celsius sets zero at the freezing point of water; Fahrenheit sets zero at the freezing temperature of a brine solution, a choice made by Daniel Fahrenheit in 1724.

Celsius to Fahrenheit: F = (C x 9/5) + 32
Fahrenheit to Celsius: C = (F - 32) x 5/9
Celsius to Kelvin: K = C + 273.15

The scales cross at -40 degrees, where Celsius and Fahrenheit read the same
Reference PointCelsiusFahrenheit
Water freezes0 C32 F
Comfortable room21 C70 F
Human body37 C98.6 F
Water boils100 C212 F
Scales meet-40 C-40 F

When Conversion Errors Get Expensive

Unit confusion has caused some genuinely costly failures. The Mars Climate Orbiter was lost in 1999 because one team supplied thrust data in pound-force seconds while the receiving system expected newton-seconds. In 1983, an Air Canada flight ran out of fuel mid-air after a refuelling calculation mixed pounds and kilograms during Canada's metric transition.

Sense-check every conversion

Before accepting a result, ask whether the number moved in the direction you expected. Converting kilometres to miles should give a smaller number; converting kilograms to pounds should give a larger one. This one-second check catches the majority of conversion mistakes, including inverted factors.

Everyday Conversions Worth Knowing

People Also Ask

Multiply by the conversion factor that links the two units. One inch is exactly 2.54 centimetres, one pound is 0.45359237 kilograms, and one mile is exactly 1.609344 kilometres. These are defined values rather than measurements, so they are exact by international agreement rather than approximations.
Mass is how much matter an object contains and does not change with location. Weight is the force gravity exerts on that mass, so it changes with gravitational pull. In everyday use kilograms and pounds are treated as interchangeable measures of the same thing, which is close enough on Earth's surface but technically imprecise.
They descend from different historical standards. The US liquid gallon is 3.785 litres, based on the old English wine gallon, while the UK imperial gallon is 4.546 litres, defined in 1824 from the volume of ten pounds of water. A UK gallon is about 20 percent larger, which is why fuel economy figures are not directly comparable between the two countries.
Multiply the Celsius figure by 9/5 and add 32. To go the other way, subtract 32 first and then multiply by 5/9. Temperature is unusual among conversions because the two scales have different zero points, so a simple multiplication is not enough; the offset has to be handled as well.
Several common conversions are exact by definition: 1 inch is 2.54 cm, 1 pound is 0.45359237 kg, and 1 mile is 1.609344 km. Others are rounded for convenience, such as 1 litre being roughly 0.264 US gallons. When precision matters, use the full defined value rather than a shortened one.

📚 Sources & References

  1. National Institute of Standards and Technology (NIST) — Official US conversion factors and measurement standards.
  2. International Bureau of Weights and Measures (BIPM) — The SI system and internationally defined base units.
  3. NIST Special Publication 811 — Guide for the use of the International System of Units.

Conversion factors shown are based on internationally defined standards. For engineering, medical, scientific or legal applications, verify values against the relevant official standard for your field.