Everyday Tools
Unit Converter
Pick a measurement family, then convert between its units. Length, weight, temperature, and volume are all supported.
Calculation inputs
The from and to units must belong to the same measurement family.
Results
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Complete the fields and select Calculate to show results here.
About this calculator
A unit converter changes a measurement from one unit into another within the same family, such as length, weight, temperature, or volume. It is useful for cooking, travel, schoolwork, and any situation mixing metric and US customary units.
How the calculation works
- Length, weight, and volume convert through a base unit (metre, kilogram, litre)
- Temperature uses the Celsius/Fahrenheit/Kelvin conversion formulas rather than a simple factor
Notes and assumptions
US customary volume units are used for cups, pints, quarts, and gallons.
How it works in plain English
Length, weight, and volume conversions all work by routing the value through a common base unit: metres for length, kilograms for weight, and litres for volume. Converting kilometres to miles, for example, first turns kilometres into metres, then metres into miles, using a fixed conversion factor at each step.
Temperature is different because Celsius, Fahrenheit, and Kelvin don't share a common zero point, so they can't be converted with a simple multiplication factor. Instead, each pair of temperature units uses its own formula that accounts for both the different scale size and the different starting point.
Selecting a 'from' and 'to' unit that belong to different families, such as converting a length into a weight, isn't a valid conversion, since there's no meaningful relationship between the two kinds of measurement. The tool requires both units to come from the same family before it will calculate a result.
The formula
- Length/weight/volume: value in target unit = value x (from-unit factor / to-unit factor)
- Celsius to Fahrenheit: F = C x 9/5 + 32
- Fahrenheit to Celsius: C = (F - 32) x 5/9
- Celsius to Kelvin: K = C + 273.15
Worked example
Converting 100 kilometres to miles uses the fixed factor that 1 kilometre equals 0.621371 miles, giving 100 x 0.621371 = 62.14 miles. For a temperature example, converting 20 degrees Celsius to Fahrenheit uses F = 20 x 9/5 + 32 = 36 + 32 = 68 degrees Fahrenheit. Both conversions rely on established, unchanging conversion factors or formulas rather than approximations, so the results are exact. Working the kilometre conversion in reverse, 62.14 miles multiplied by 1.609344 returns approximately 100.0 kilometres, confirming the two conversion factors are true inverses of each other. The same check applies to temperature: converting 68 degrees Fahrenheit back to Celsius using C = (68 - 32) x 5/9 returns 20 degrees Celsius.
Common exact conversion factors
| Conversion | Factor |
|---|---|
| 1 mile | 1.609344 kilometers |
| 1 kilometer | 0.621371 miles |
| 1 pound | 0.45359237 kilograms |
| 1 kilogram | 2.20462 pounds |
| 1 inch | 2.54 centimeters |
| 1 US gallon | 3.78541 liters |
Factors reflect standard international definitions.
Frequently asked questions
Why can't I convert pounds directly into inches?
Weight and length measure fundamentally different physical properties, so there's no valid conversion factor between them. The converter requires both units to belong to the same measurement family, such as length to length or weight to weight.
Why does temperature use a formula instead of a simple factor?
Celsius, Fahrenheit, and Kelvin scales start counting from different zero points and use different-sized degree increments, so converting between them requires both multiplying and adding a fixed offset rather than just multiplying by one factor.
Are US and UK gallons the same?
No, a US gallon (3.78541 liters) is smaller than a UK imperial gallon (4.54609 liters). This converter uses US customary volume units, which is worth noting if you're working with a recipe or fuel figure from outside the US.
How precise are the conversion results?
Conversions use standard, internationally defined factors, so results are mathematically exact rather than approximate. Any rounding shown is purely for display and doesn't affect the underlying calculation. Because the underlying factors are fixed international standards rather than estimates, chaining multiple conversions together, such as miles to kilometres to metres, still returns the same accurate result as converting directly in a single step.
Can I convert between metric prefixes, like millimeters to kilometers?
Yes, since both units belong to the length family, the calculator can convert directly between any two units on that list, no matter how far apart their scale is, by routing the value through the base metre unit.