Unit Converters

A recipe says pounds, the car's display shows mpg, an internet plan is sold in GB but downloads show MB/s. Converting correctly between units comes up constantly in the kitchen, the workshop, on the road and at the computer. The converters here convert to the unit you want and also show the equivalent in other common units.

Which tool for which question?

• Weight Converter: Kilograms, Pounds, Ounces: Convert weights between milligrams, grams, kilograms, tonnes, carats, ounces, pounds, stone, troy ounces and US/imperial tons.
• Pressure Converter: Bar, psi, Pa, atm, mmHg: Convert pressure between pascals, kilopascals, bar, millibar, psi, atmospheres, kgf/cm², mmHg and torr, for tyres, blood pressure and weather.
• Energy Unit Converter: Joules, Calories, kWh, BTU: Convert energy between joules, kilojoules, megajoules, calories, kilocalories, watt-hours, kilowatt-hours and BTU; compare food calories with electricity use.
• Power Unit Converter: kW, Horsepower (PS, hp), BTU/h: Convert power between watts, kilowatts, megawatts, metric horsepower (PS), mechanical horsepower (hp), BTU per hour and kcal per hour.
• Speed Converter: km/h, mph, m/s and Knots: Convert speeds between kilometres per hour, miles per hour, metres per second, knots and feet per second, and see the running pace in minutes per km.
• Temperature Converter: Celsius, Fahrenheit, Kelvin: Convert temperatures between Celsius (°C), Fahrenheit (°F), Kelvin (K) and Rankine (°R), and convert temperature differences correctly too.
• Length Converter: Metres, Inches, Feet, Miles: Convert lengths between millimetres, centimetres, metres, kilometres, inches, feet, yards, miles and nautical miles, and see a height in feet and inches.
• Data Size Converter: MB, GB, TB, GiB and Download Time: Convert data sizes between bits, bytes, kB, MB, GB, TB, PB and KiB, MiB, GiB, TiB, PiB, and see how long a file takes to download at your connection speed.
• Fuel Economy Converter: L/100 km, km/L, mpg: Convert fuel economy between litres per 100 km, kilometres per litre, US mpg and UK (imperial) mpg.
• Time Unit Converter: Seconds, Minutes, Hours, Days: Convert durations between milliseconds, seconds, minutes, hours, days, weeks, months and years, and split a duration into days, hours, minutes and seconds.

How the calculation works

Most conversions use a single multiplier: result = value × (source unit in a common unit) ÷ (target unit in the common unit). The common unit is usually an SI unit (metre, kilogram, pascal, joule, watt).

In three places this simple logic is not enough:

• For temperature the scales have different zero points, so a fixed offset is needed as well as a multiplier; the tool converts to Kelvin first and then to the target.
• In fuel consumption L/100 km and km/L are reciprocals, while mpg gives two values because US and UK gallons differ.
• In data size, SI (decimal) prefixes are based on 1,000 and binary prefixes (such as GiB) on 1,024, so "1 GB" and "1 GiB" are not the same.

Things to watch

• Horsepower has two different definitions; note which one you read the result with.
• For gold and gems, the troy ounce and carat differ from the everyday ounce and gram.
• With very large or very small values, rounding can affect the last digits.