Unit Converter

A practical guide to unit conversion

Most conversion mistakes are not arithmetic errors - they come from picking the wrong unit in the first place. Here is what each family in this converter covers and where the traps are.

Length

The metric system is decimal all the way through, so kilometres, metres, centimetres and millimetres differ only by powers of ten. Imperial length is where it gets awkward: twelve inches to a foot, three feet to a yard, 1,760 yards to a mile. The two anchor points worth memorising are 1 inch = 2.54 cm exactly, and 1 mile = 1.609 km. Everything else follows from those.

Weight and mass

One kilogram is 2.20462 pounds. In everyday use people round to 2.2, which is fine for a suitcase and not fine for a medication dose. Note that the "stone" used in the UK equals 14 pounds and does not exist in American usage, and that the US ton (2,000 lb) differs from the metric tonne (1,000 kg) and from the UK long ton (2,240 lb). If a shipping quote looks strange, this is usually why.

Temperature

Temperature is the only family here that needs an offset as well as a scale factor, because the zero points differ. Celsius to Fahrenheit is (C x 9/5) + 32; Fahrenheit to Celsius is (F - 32) x 5/9. Kelvin shares the Celsius step size but starts at absolute zero, so K = C + 273.15. The two scales cross at -40 degrees, which is the same temperature in both.

Volume

Volume hides the most dangerous trap in this converter: the US and imperial gallons are not the same. A US gallon is 3.785 litres, an imperial gallon is 4.546 litres, a difference of about 20%. The same split runs through pints and fluid ounces. When you compare fuel economy figures between American and British sources, check which gallon each one means before drawing a conclusion.

Speed

Kilometres per hour and miles per hour use the same ratio as the length units, so 100 km/h is about 62 mph. Knots appear in aviation and sailing - one knot is one nautical mile per hour, or 1.852 km/h. Metres per second is the scientific unit and is what most physics problems expect.

Digital storage

Storage is the family where two standards coexist and neither is wrong. Drive manufacturers use decimal units, where one gigabyte is 1,000,000,000 bytes. Operating systems usually report binary units, where one gibibyte is 1,073,741,824 bytes but is often labelled "GB" anyway. That mismatch is why a 1 TB drive shows up as roughly 931 GB after you plug it in. This converter uses the decimal definitions and labels them clearly.

How to use the converter

1. 1Pick a measurement family from the tabs at the top.
2. 2Choose the unit you are converting from and the unit you want.
3. 3Type a value. The result appears immediately, with the conversion factor shown underneath.
4. 4Use the swap button to reverse the direction without retyping anything.

Why precision matters more in some contexts

• Cooking tolerates rounding. A few grams either way rarely ruins a recipe, though baking is less forgiving than a stew.
• Engineering and construction need the full factor. A 1% error over a 30 metre span is 30 centimetres.
• Medicine treats conversion as a safety issue - dosing errors from pound-to-kilogram slips are a documented cause of harm.
• Travel and fuel mostly need a good estimate, as long as you know which gallon a figure is quoting.

The 1999 loss of the Mars Climate Orbiter is the famous example: one team supplied thruster data in pound-force seconds while the other expected newton-seconds, and a 125 million dollar spacecraft flew too close to the planet. Unit discipline is cheap insurance.

Related tools

The BMI Calculator accepts both metric and imperial input, so you rarely need to convert first. For proportional changes rather than unit changes, use the Percentage Calculator, and the Tip Calculator handles the currency arithmetic that usually follows a travel conversion.