Conversion

Conversions usually fail for one of two reasons: the wrong factor, or the wrong unit family. This category covers the common families — length, mass, temperature, volume, angle and digital storage — plus currency and number-to-words conversion.

Where a conversion is exact by definition, the standard factor is used. Where it depends on outside data, such as currency, the tool is explicit about the source.

Browse conversion calculators

General units, length & mass

Temperature & cooking

Which conversion calculator do you need?

Convert a height or a distance between metric and imperial units.
- Height Converter
- Unit Converter
Convert a weight between kilograms and pounds.
- kg to lbs Converter
Switch a temperature between Celsius and Fahrenheit.
- Celsius to Fahrenheit Converter
Scale a recipe between cups, millilitres and other kitchen measures.
- Cooking Converter
Convert an angle between degrees and radians.
- Unit Converter
Convert an angle, length, mass, volume, area or temperature in one place.
- Unit Converter
Convert between bits and storage units such as megabytes and gigabytes.
- Data Storage Converter
Convert a currency amount using cached reference rates.
- Currency Converter
Write an integer out in English words for a document or check.
- Numbers to Words Converter

Exact factors and approximate ones

Most length and mass conversions use factors fixed by definition, so the arithmetic is exact up to rounding. Cooking measures are different: a cup of flour and a cup of water do not weigh the same, so volume-to-weight depends on the ingredient and how it is packed.

Currency is the other dependent case. The converter uses cached reference rates, which are suitable for estimates and comparisons but not for settlement, accounting or trading.

Temperature and meaningful precision

Temperature is an affine scale, not a ratio: Fahrenheit equals Celsius times nine-fifths plus thirty-two. That is why a temperature conversion cannot be done by multiplying alone, and why a temperature increase converts differently from a temperature value.

Precision also matters in the other direction. A conversion cannot create accuracy that was not in the input; if a measurement was rounded, the result is only as reliable as that rounding.

Reading converted values

Converting does not add precision — the output inherits the uncertainty of the input.
Cooking conversions by volume vary with the ingredient and how tightly it is packed.
Digital-storage and currency conventions (decimal vs binary prefixes, mid-market vs bank rates) can differ from what a device or statement reports.