Free Online Unit Converter — Every Measurement, One Place
Whether you're converting kilograms to pounds for a recipe, or Celsius to Fahrenheit for a weather report, this free unit converter gives you an exact answer in seconds. Pick a category below, or jump straight to the specific conversion pair you need, and every result shows the formula so you can check the maths yourself.
This converters cover the metric system, the imperial system, and US customary units side by side, which makes them handy for students, engineers, cooks, travellers, and anyone who has ever stared at a label in the wrong units.
Length Converter
10Convert between cm, m, km and more
Temperature Converter
4Convert between °C, °F, °C and more
Mass Converter
4Convert between kg, lbs, kg and more
Volume Converter
4Convert between L, ml, L and more
Area Converter
4Convert between m², ft², ha and more
Speed Converter
4Convert between km/h, mph, m/s and more
Most searched conversions right now
These are the conversions people look up most often, the ones you half remember from school but never quite trust. Each one takes you straight to a converter that shows the result and the exact conversion factor behind it.
How unit conversion actually works
Every conversion on this site follows the same simple rule: you multiply the value you have by a conversion factor, and the unwanted unit cancels out. If you're converting a larger unit to a smaller one, you multiply, a metre becomes 100 centimetres, a kilogram becomes 1,000 grams. If you're going the other way, from smaller to larger, you divide, 100 centimetres become one metre, 1,000 grams become one kilogram. That single rule covers the entire metric system, because each step between kilo, hecto, deca, base, deci, centi, and milli is a clean power of ten. Imperial and US customary units do not share that tidy ladder, which is exactly why people reach for a converter instead of doing the arithmetic in their head, 1 inch is 2.54 centimetres, 1 mile is 1.609344 kilometres, and 1 pound is 0.45359237 kilograms. Our converters show you the factor being used, so you can verify the answer rather than trust it blindly.
What each converter on this page does
Not every category solves the same kind of problem, so here is a quick rundown of what you will find inside each one.
Length converter — centimetres, metres, kilometres, inches, feet, miles
Length is the conversion people search for most, and it is usually because a measurement arrived in the wrong unit. Furniture listed in inches, a running route measured in kilometres, a sewing pattern in centimetres, a road sign in miles, all of it needs translating before it means anything. This converter handles centimetres to metres, metres to centimetres, kilometres to metres, metres to kilometres, centimetres to inches, inches to centimetres, metres to feet, feet to metres, kilometres to miles, and miles to kilometres. The metric pairs are pure powers of ten; the metric-to-imperial pairs use fixed factors, so the answer is exact to the decimal place shown rather than rounded.
Temperature converter — Celsius, Fahrenheit, Kelvin
Temperature is the one category where you cannot just multiply, Celsius, Fahrenheit, and Kelvin are offset from each other as well as scaled differently, so the conversion needs both a multiplication and an addition. That is why guessing at temperature conversions so often goes wrong, and why a weather forecast or an oven setting from another country is a common reason to look one up. This converter covers Celsius to Fahrenheit, Fahrenheit to Celsius, Celsius to Kelvin, and Kelvin to Celsius.
Mass converter — kilograms, pounds, grams
Mass and weight are used interchangeably in everyday speech, even though they are technically different quantities. In practice, what people want is usually a straightforward swap between metric and imperial, kilograms to pounds for a luggage allowance, pounds to kilograms for a gym programme, kilograms to grams for a recipe, grams to kilograms for postage. All four are covered here, using the exact international pound definition of 0.45359237 kilograms.
Volume converter — litres, millilitres, gallons
Volume conversions come up constantly in the kitchen and at the fuel pump, and they are the category most often done wrong by eye. Litres to millilitres is a clean factor of 1,000, but litres to gallons depends on which gallon you mean, the US gallon and the imperial gallon are not the same size, so a US recipe and a UK recipe using the same word will give you different amounts. Our litre to gallon converter uses the US gallon, which is the one most online recipes and product labels assume. Litres to millilitres, millilitres to litres, litres to gallons, and gallons to litres are all available.
Area converter — square metres, square feet, hectares, acres
Area is length squared, which means the conversion factors are squared as well, a mistake that catches people out because the number looks much larger than expected. One square metre is roughly 10.76 square feet, not 3.28. Land listings are the most common reason to reach for this converter, since plot sizes are given in hectares in some countries and acres in others. This converter covers square metres to square feet, square feet to square metres, hectares to acres, and acres to hectares.
Speed converter — km/h, mph, m/s
Speed conversions matter when you cross a border, watch foreign motorsport, or read a physics problem. Kilometres per hour to miles per hour is the classic one, but metres per second to kilometres per hour shows up in schoolwork and engineering alike. The factor between km/h and mph is not a round number, so the exact value matters if you are comparing speed limits or calculating travel time. This converter covers km/h to mph, mph to km/h, m/s to km/h, and km/h to m/s.
Why manual conversions go wrong, and how to avoid it
Most conversion errors are not arithmetic errors; they are direction errors. People multiply when they should divide, or divide when they should multiply, and the answer ends up out by a factor of ten, a hundred, or a thousand. The cause is usually that the unit being converted to is larger than the unit being converted from, and the instinct to "make the number bigger" kicks in. A useful check is to ask whether the answer should be a larger or smaller number than the one you started with, a metre is bigger than a centimetre, so the same length expressed in centimetres will always be a larger number, and a length in kilometres will always be a smaller number. If your result goes the wrong way, you used the factor upside down. The second common error is mixing up area and length, where the factor needs squaring. Our area converter applies the squared factor for you, so square metres to square feet comes out at roughly 10.76 rather than 3.28. The third is using an approximate factor where an exact one is available, treating a kilogram as 2.2 pounds is fine for a rough estimate but drifts noticeably over larger quantities, which is why every converter here uses the published exact factor and states it alongside the result.
All converters, A to Z
If you already know which pair you want, say, millimetres to inches, or kilojoules to calories, skip the category pages and go straight to it. Every converter on this list works the same way: enter a value, get the converted measurement, and see the formula used so you can reproduce the result by hand.
Frequently asked questions about unit conversion
What is the difference between the metric system and the imperial system?
The metric system is built on powers of ten, so every step between units is a clean multiple, 1,000 metres to a kilometre, 100 centimetres to a metre, 1,000 grams to a kilogram. The imperial system and US customary units use historical, non-decimal relationships instead: 12 inches to a foot, 3 feet to a yard, 5,280 feet to a mile, 16 ounces to a pound. That is why metric conversions can be done mentally and imperial ones usually cannot.
Are the results rounded?
Each converter uses the full published conversion factor rather than a short approximation. Results are exact to the number of decimal places shown. Where a factor is an irrational or repeating decimal, the converter states the factor it used so you can see exactly what was applied.
Which gallon does the litre-to-gallon converter use?
It uses the US gallon, which is 3.785411784 litres. The imperial gallon used in the UK is larger, at 4.54609 litres. If you are following a recipe or reading a product label online, the US gallon is almost always the one intended.
How do I convert a length into an area or a volume?
You need two or three measurements, not one. Length converts to area by multiplying length by width, and to volume by multiplying length by width by height. Once you have the result in squared or cubed units, use the area or volume converter to change it into another unit system. Do not apply a plain length factor to an area figure, the factor needs to be squared.
Why does temperature conversion need a formula instead of a simple multiplier?
Because the three temperature scales do not share a zero point. Celsius and Fahrenheit both place zero somewhere other than absolute zero, so converting between them requires a multiplication and an offset, Fahrenheit is 1.8 times the Celsius value plus 32. Kelvin shares Celsius's step size but starts at absolute zero, so Celsius to Kelvin is a pure addition. Our temperature converter applies the correct combination for each pair.