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Unit converter
Pick a quantity, type a number, choose the unit you have and the unit you want, and the answer appears straight away. It covers length, weight, volume, temperature and speed, which between them handle most everyday conversions: a recipe written in cups when you cook in millilitres, a parcel listed in pounds when your scales read kilograms, a road sign in miles when your dashboard shows kilometres, an oven set in Fahrenheit when the dial is Celsius. Every result also shows the rate both ways, so you can read off the conversion in either direction without retyping. The factors used are the internationally agreed definitions, not rounded approximations, so the figures match what an official table would give.
Conversions use the internationally defined exact factors (for example 1 inch is 0.0254 m and 1 mile is 1609.344 m).
How it works
- Choose the quantity tab: length, weight, volume, temperature or speed.
- Type the value you already have in the box.
- Set the From unit to what you are converting out of, and the To unit to what you want.
- The headline tile shows the converted figure, and for everything except temperature two more tiles give the one-to-one rate in each direction.
result = value x (factor of from-unit) / (factor of to-unit)
Each linear unit carries a fixed factor to its quantity base, for example metres for length or grams for weight. To convert, the value is multiplied by the from-unit factor to reach the base, then divided by the to-unit factor to reach the target. Temperature is the exception: the input is first turned into Celsius, then into the target scale, applying the shift and the ratio that separate the scales.
- value
- the number you enter
- factor
- the exact size of a unit in its base (metre, gram, litre)
- base
- the reference unit each quantity converts through
Handy reference conversions
| 1 mile | 1.609344 km | exact by definition |
| 1 kilogram | 2.204623 lb | one pound is 453.59237 g |
| 1 US gallon | 3.785412 litres | imperial gallon is 4.546 litres |
| 100 km/h | 62.137 mph | a common motorway speed |
| 0 Celsius | 32 Fahrenheit | water freezes |
Worked example
Converting a 26.2 mile marathon into kilometres: set length, type 26.2, From miles, To kilometres. The result is 42.164928 km, because one mile is exactly 1.609344 km. Switch the units and the calculator shows that one kilometre is about 0.621371 miles, the figure you would use to read a European road sign back into miles.
Key facts
- A US gallon and an imperial gallon differ by about three quarters of a litre, a frequent source of recipe and fuel mistakes.
- Miles to kilometres is a fixed multiply by 1.609344, so 5 miles is 8.04672 km every time.
- Body temperature of 37 Celsius is 98.6 Fahrenheit, the figure most thermometers still print.
- A stone, used for body weight in the UK and Ireland, is exactly 14 pounds or about 6.35 kilograms.
Tips
- Read the reverse-rate tile when you want a quick mental shortcut, such as kilometres being roughly six tenths of a mile.
- For cooking, prefer weight over volume where a recipe allows it, since a cup of flour and a cup of sugar weigh very different amounts.
- When a parcel courier quotes pounds, convert to kilograms before comparing against a metric weight limit to avoid an overcharge.
- Double-check whether a gallon figure is US or imperial before converting fuel economy, as the choice shifts the answer by nearly a fifth.
Frequently asked questions
Are the conversion factors exact or rounded?+
They are the exact internationally defined values. One inch is 0.0254 metres, one pound is 453.59237 grams and one mile is 1609.344 metres, all by definition. Only the final display is rounded, so the underlying maths carries full precision.
Why is temperature handled differently?+
Length, weight and the rest scale from a shared zero, so doubling the input doubles the output. Temperature scales do not share a zero point, so Celsius to Fahrenheit both shifts and stretches the number rather than only multiplying it. That is why the converter shows no one-to-one rate for temperature.
What is the difference between a US and an imperial gallon?+
They are not the same size. A US gallon is about 3.785 litres while an imperial gallon, still used in the UK for some purposes, is about 4.546 litres. The volume tab lists both so you can pick the right one rather than assuming a gallon is a gallon.
Can I convert square metres or acres here?+
Area has its own dedicated tool because it usually starts from a length and a width rather than a single figure. For a flat number such as litres, kilograms or kilometres, this converter is the quicker route.
Why does a converted value sometimes show in scientific notation?+
Very large or very small results are shown to six significant figures so they stay readable. A figure like 1.23e-5 means 0.0000123, written compactly so the screen does not fill with zeros.
Things to watch
- The converter assumes the exact modern definitions, so very old or trade-specific units of the same name may differ slightly.
- Rounding to a readable number of digits means a converted figure is precise for practical use but not for legal metrology.
- It converts plain quantities only, not compound rates such as miles per gallon, which mix two units and need a dedicated tool.
Sources
- The International System of Units (SI), 9th edition · Bureau International des Poids et Mesures
Last updated: 2026
This is an estimate for general guidance, not financial, tax, legal or medical advice. Figures can change and individual circumstances vary. Always confirm with the official sources listed before making decisions.
Reviewed by Vikas Dulgunde.