Converting in three steps
- Pick the category that matches what you are measuring (length, mass, temperature, volume, speed, digital storage and so on).
- Choose the unit you have and the unit you want. Both fields are searchable: type "gal" or "mile" to jump straight to it.
- Type a value in either box. The other box updates instantly, and you can edit the result field to convert in the opposite direction.
Every conversion page also shows the conversion factor, a reference table of common values and the definition and history of both units, so you can check the logic instead of trusting a bare number.
Why there are so many unit systems
Most of the world uses the metric system (SI), where units scale by powers of ten: a kilometre is 1,000 metres and a millimetre is one thousandth of a metre. The United States, and in a few contexts the United Kingdom, still rely on customary and imperial units such as miles, pounds, gallons and degrees Fahrenheit. Those units grew out of practical, regional measures long before international standards existed.
Because both systems are in daily use, recipes, road signs, product labels, engineering drawings and scientific papers often disagree on units. A converter removes that friction, but it helps to know the exact definitions behind it: since 1959 the international inch is exactly 25.4 millimetres and the pound is exactly 0.45359237 kilograms.
Common mistakes to avoid
- US versus imperial volumes. A US gallon is about 3.785 litres, an imperial gallon about 4.546 litres. Check which one your source uses.
- Temperature is not a simple ratio. Converting Celsius to Fahrenheit needs a multiplication and an offset (°F = °C × 9/5 + 32), which is why 20 °C is not simply "twice" 10 °C in Fahrenheit.
- Bits versus bytes. Network speeds are quoted in megabits per second, file sizes in megabytes: divide by eight to compare them.
- Rounding too early. Convert with full precision and round only the final value to the number of digits you actually need.