The conversions people actually look up
Every category shows a few one-click conversions underneath, because most visits are one of a handful of questions: inches to centimetres, kilograms to pounds, Celsius to Fahrenheit, litres to gallons.
The line under the input is the part worth reading. It tells you the rate, not just the answer, so you can carry it in your head: one inch is 2.54 centimetres, one kilogram is about 2.2 pounds.
Two traps worth knowing
Gallons. A US gallon is 3.785 litres, an imperial gallon is 4.546. That is a 20 percent difference, and recipes, fuel economy figures and older British texts do not always say which they mean. Both are listed separately here so nothing is assumed on your behalf.
Megabytes. Storage manufacturers count a megabyte as 1,000,000 bytes. Operating systems usually count 1,048,576 and often still call it a megabyte. That is the entire reason a “1 TB” drive shows up as about 931 GB, and why both units appear in the data category.
Temperature is not a factor
Length, weight and volume all scale from zero, so converting is a multiplication. Temperature scales start at different points, so Fahrenheit needs the offset of 32 and Kelvin the offset of 273.15. This is why a “temperature conversion factor” does not exist, and why doubling 10 °C does not give you double the heat.
Questions
Why do both boxes update?
Because conversion goes both ways and people arrive wanting either direction. Typing in the right-hand box converts backwards.
US or imperial gallons?
Both, listed separately. They differ by about 20 percent, which is enough to ruin a recipe or a fuel calculation, so the tool never guesses which you meant.
Why are there two kinds of megabyte?
Because the industry never agreed. A megabyte is 1,000,000 bytes by the SI definition used on storage packaging; a mebibyte is 1,048,576 bytes, which is what most operating systems show. That is why a 1 TB drive appears as roughly 931 GB.
How exact are the conversions?
The factors are the exact definitions, for instance an inch is exactly 0.0254 metres. Results are displayed to a sensible precision, not rounded internally.
Is temperature handled differently?
Yes. Temperature scales have different zero points, so they need an offset rather than a simple factor. Celsius, Fahrenheit and Kelvin are converted with the proper formulas.