Unit Converter
Quickly convert between length, weight, volume and temperature units.
How it works
For length, weight and volume, every unit in a group converts to one base unit and out again. Length goes through the metre, weight through the kilogram, volume through the litre. That needs one factor per unit instead of a cross table, and — more importantly — it gives consistent results in every direction.
The factors here are exact definitions, not approximations. An inch is exactly 0.0254 m, a pound exactly 0.45359237 kg, a mile exactly 1609.344 m: these were agreed internationally in 1959. Using "an inch is about 2.5 cm" is faster and 1.6% wrong, which is enough to ruin a drawing.
Temperature is the exception and is handled differently. Celsius, Fahrenheit and Kelvin do not share a zero, so no multiplier is correct. Conversions go through Celsius with addition and subtraction.
That is also why "twice as hot" is meaningless in Celsius or Fahrenheit. 20 °C is not twice 10 °C; only Kelvin, which starts at absolute zero, makes a ratio physically meaningful.
Two Vietnamese units are included: tạ (100 kg) and tấn (1,000 kg). Note that tạ is not interchangeable with "quintal" as used in some other countries.
The gallon here is the US gallon (3.785411784 litres). The imperial gallon is about 20% larger at 4.54609 litres — confusing the two is the usual reason fuel-economy figures fail to line up.
Formula
Target = source value × source factor ÷ target factor • Temperature: °C = (°F − 32) × 5/9 = K − 273.15
A factor is how many metres, kilograms or litres one unit represents. Temperature uses no factor because the three scales do not share a zero.
Worked example
Converting 5.9 feet to centimetres, and 100 °F to Celsius:
- 5.9 ft × 0.3048
- 1.79832 m
- To centimetres (÷ 0.01)
- 179.832 cm
- 100 °F − 32
- 68
- × 5 ÷ 9
- 37.7778 °C
A height of 5 ft 11 in is 180 cm, not the 177 cm the mental shortcut "5.9 × 30" suggests. And 100 °F — a number that sounds alarming — is 37.8 °C, which is body temperature.
Frequently asked questions
Why so many decimal places?
Because the factors are exact definitions. Results keep up to six decimals so you can round as your work requires, rather than being pre-rounded into a value that will not convert back.
Why can temperature not use a multiplier?
The three scales have different zeros: 0 °C is 32 °F and 273.15 K. Any conversion therefore involves addition as well as multiplication. It is the basic difference between a quantity with an absolute zero and one measured on a conventional scale.
How much is a tạ?
100 kg in Vietnamese usage. Do not treat it as identical to "quintal" in foreign sources, where the amount has varied by country and period.
Is the gallon US or imperial?
US, at 3.785411784 litres. The imperial gallon is 4.54609 litres, about 20% larger. Check which one a "miles per gallon" figure uses before comparing.
Why is an inch 0.0254 m rather than a round number?
In 1959 the English-speaking countries agreed to define the international inch as exactly 25.4 mm, ending a situation where each country had a slightly different one. That figure is a definition, so it is exact rather than measured.
Can I convert litres to kilograms?
Not directly — litres measure volume and kilograms measure mass, so you need the density. Water is about 1 kg per litre, petrol about 0.74 kg and cooking oil about 0.92 kg.
Does Kelvin take a degree sign?
No. Write "300 K", not "300 °K". Kelvin is an absolute scale, and SI dropped the degree symbol for it in 1967.