How this is calculated
Each category defines a base unit and a factor for every other unit against it. Converting from A to B multiplies by A's factor to reach the base, then divides by B's factor. One inch is 0.0254 metres and one foot is 0.3048, so 12 inches is 0.3048 metres, which is exactly one foot.
Temperature works differently because the scales have different origins. Celsius to Fahrenheit is (°C × 9/5) + 32; Fahrenheit to Celsius is (°F − 32) × 5/9. The subtraction has to come first going one way and the addition last going the other — reversing the order is the classic mistake.
Kelvin shares Celsius's degree size but starts at absolute zero, so K = °C + 273.15 with no multiplication at all.
Units that share a name but not a size
A US liquid gallon is 3.785 litres; an imperial gallon is 4.546 — about 17% larger. This propagates down through quarts, pints and fluid ounces, so a US pint is 473 ml against an imperial 568 ml. Recipes and fuel economy figures crossing the Atlantic are both routinely misread because of it.
The US fluid ounce and the imperial fluid ounce differ too, though only by about 4%, which makes the error small enough to survive undetected in a recipe. Tons are worse: a US short ton is 907 kg, an imperial long ton is 1,016 kg, and a metric tonne is 1,000 kg — three different masses sharing one word.
Data sizes: 1,000 or 1,024?
Storage manufacturers use decimal units, where a gigabyte is 1,000³ bytes. Operating systems have historically used binary units, where the same word meant 1,024³ — about 7.4% more. This is why a 1 TB drive shows as roughly 931 GB once formatted, and why nothing is actually missing.
The IEC introduced separate names to resolve this: kibibyte, mebibyte and gibibyte for the binary powers, leaving kilobyte and megabyte to the decimal ones. Adoption has been partial, so the ambiguity persists in practice and it remains worth checking which convention a figure uses before drawing conclusions from it.
How to use the unit converter
- Pick a category. Length, weight, volume, temperature, area, speed or data. The available units change with the category.
- Choose your source and target units. Where US and imperial variants both exist, they are listed separately — check you have the right one.
- Type the value. The conversion updates as you type, so you can adjust the input and watch the result move.