Free Unit Converters
Unit conversion is the everyday maths problem of living with two measurement systems: metric in most of the world, imperial or US customary in the United States and, for some measures, the UK. These converters use exact international definitions — one inch is exactly 2.54 centimetres, one mile exactly 1,609.344 metres, one pound exactly 0.45359237 kilograms — and show the conversion factor with every answer so you can check or reuse it by hand.
One tool for any unit
The all-in-one converter covers every category in a single page.
Dedicated converters
Each has a table of common conversions and notes on the pitfalls of that measurement type.
Length ConverterConvert metres, kilometres, centimetres, miles, yards, feet and inches instantly.
Weight ConverterConvert kilograms, pounds, ounces, stone, grams and tonnes quickly and exactly.
Temperature ConverterConvert Celsius, Fahrenheit and Kelvin with the formula shown, including the below-absolute-zero check.
Area ConverterConvert square metres, square feet, acres, hectares, square miles and more.
Volume ConverterConvert litres, millilitres, US gallons, quarts, pints, cups and fluid ounces for cooking and fuel.
Speed ConverterConvert mph, km/h, m/s, knots and ft/s with the exact factors.Money
Convert between currencies using European Central Bank reference rates.
Kitchen
Convert cups, tablespoons, grams and ounces for common ingredients.
Number-system converters
Conversions between ways of writing numbers rather than between physical units.
Number Base ConverterConvert between binary, octal, decimal, hexadecimal and any base up to 36.
Scientific Notation ConverterConvert numbers to scientific, engineering and standard notation.
Roman Numeral ConverterConvert numbers to Roman numerals and back, from 1 to 3999.
Decimal to Fraction CalculatorTurn any decimal into a simplified fraction or mixed number.Metric vs imperial vs US customary
The metric system (SI) is decimal: every unit is a power of ten away from the next, so converting between millimetres, centimetres, metres and kilometres is a matter of moving the decimal point. Imperial and US customary units are older and less regular — 12 inches to a foot, 3 feet to a yard, 1,760 yards to a mile — and where they share names, they can differ. A US gallon is 3.785 litres but a UK imperial gallon is 4.546 litres; a US pint is 473 mL but a UK pint is 568 mL. Always check which system a recipe, spec sheet or product label uses.
A quick method for converting by hand
- Find the factor linking the two units (for example 1 mile = 1.609344 km).
- Multiply by the factor to go from the first unit to the second; divide to go back.
- Check that the answer makes sense: kilometres should be a larger number than miles for the same distance.
- For area and volume, square or cube the length factor — 1 m = 3.28084 ft, but 1 m² = 10.7639 ft².
Temperature is the exception: it needs an offset as well as a multiplier, which is why the temperature converter has its own formula.
Guides for this topic
Frequently asked questions
Are the conversion factors exact?
The international inch, foot, yard, mile and pound are defined exactly in terms of metric units, so those factors are exact. Other results are shown to ten significant figures.
Which gallon do you use?
US customary (3.785411784 litres). UK imperial gallons are about 20% larger.
Why is temperature converted differently?
Celsius, Fahrenheit and Kelvin start from different zero points, so conversion needs addition as well as multiplication.
Can I convert area and volume too?
Yes. Use the area and volume converters; they use squared and cubed length factors correctly.
Reviewed September 26, 2026 by the SolveCalcPro editorial team. See our editorial policy. All tools →