How to use the unit converter
Pick a category, choose the unit you have on the left and the unit you want on the right, and type. The converter works in both directions: type in the right-hand box and the left-hand box updates instead, which saves swapping when you want to know how many kilometres make 26.2 miles. If you don't know which category a unit belongs to, type its name or symbol into the search box — psi, fl oz, GiB, knot — and press From or To next to the right match.
The input is more forgiving than most converters. It accepts grouping commas (1,250), a decimal comma (2,5), scientific notation (6.02e23), fractions and mixed numbers (3/4, 1 1/2, ¾) and simple sums such as 12*3+4 or 2^10, so you can convert the total of several measurements without reaching for a calculator.
Under the result you get the formula for the pair, a conversion table of common values (your own value is highlighted in it), and your value expressed in every unit of the category. Set the precision to fixed decimal places, significant figures or scientific notation; Auto shows up to ten significant figures and trims floating-point noise, so 0.1 + 0.2 shows as 0.3. Save pairs you use often with the star, and use Copy link to share a conversion — the address carries the units and value, for example /unit-converter/?from=km&to=mi&v=5.
The most common conversions, with formulas
Four conversions account for most searches. Each has a one-line formula you can do on a phone calculator, and the converter above shows the same working when you pick the units.
Km to miles
Miles = km × 0.621371 (and km = miles × 1.609344), so 5 km is 3.11 miles and a 10 km run is 6.21 miles.
Kg to lbs
Pounds = kg × 2.20462 (and kg = lb × 0.45359237), so 70 kg is 154.3 lb, or 11 st 0.3 lb.
Celsius to Fahrenheit
°F = °C × 9 ÷ 5 + 32 (and °C = (°F − 32) × 5 ÷ 9), so 20 °C is 68 °F and a 180 °C oven is 356 °F.
Cups to grams
Grams = cups × 236.6 ml × the ingredient's density in g/ml, so a US cup of plain flour (about 0.53 g/ml) is roughly 125 g and a cup of water is 237 g.
The exact definitions behind the numbers
Most imperial and US units have been defined in terms of metric ones since the 1959 international yard and pound agreement, so conversions between them are exact rather than approximate. The converter uses these legal definitions directly, which is why 1 mile shows as exactly 1.609344 km and not a rounded figure.
| Unit | Exact definition |
|---|---|
| Inch | 25.4 mm |
| Foot / yard / mile | 0.3048 m / 0.9144 m / 1,609.344 m |
| Nautical mile | 1,852 m |
| Pound (avoirdupois) | 0.45359237 kg |
| Stone | 14 lb = 6.35029318 kg |
| UK gallon | 4.54609 litres |
| US gallon | 231 cubic inches = 3.785411784 litres |
| Kilocalorie (food Calorie) | 4,184 J (thermochemical) |
| Kilowatt-hour | 3.6 MJ |
| Standard atmosphere | 101,325 Pa |
A few units are conventions rather than definitions, and the converter says so when you use them: an average month (30.436875 days) and year (365.2425 days) come from the 400-year Gregorian cycle, Mach 1 assumes 340.294 m/s at sea level in the standard atmosphere, and horsepower comes in a mechanical (745.7 W) and a metric (735.5 W, PS) version that differ by about 1.4%.
UK imperial vs US customary: the common traps
The United Kingdom and the United States share unit names but not always unit sizes, and mixing them up is the most common conversion mistake.
- Pints and gallons. A UK pint is 568 ml (20 UK fluid ounces); a US pint is 473 ml (16 US fluid ounces). A UK gallon is about 20% bigger than a US gallon.
- Fluid ounces. Oddly, the UK fluid ounce (28.41 ml) is smaller than the US one (29.57 ml), even though the UK pint is larger.
- Miles per gallon. Because the gallons differ, 40 mpg in a British car review equals about 33.3 mpg in an American one. Choose mpg (UK) or mpg (US) deliberately.
- Tons. A UK long ton is 2,240 lb (1,016 kg), a US short ton is 2,000 lb (907 kg), and a metric tonne is 1,000 kg.
- Body weight. In the UK people often give weight in stones and pounds; a stone is 14 lb, so 11 st 4 lb is 158 lb or 71.7 kg.
When you convert between UK and US volume units the converter adds a reminder, so a recipe or a fuel figure doesn't silently come out 20% wrong.
Temperature, fuel economy and data storage
Three categories don't work by simple multiplication, and many converters get the details wrong.
Temperature scales have different zero points, so you multiply and add: °F = °C × 1.8 + 32, and K = °C + 273.15. A difference of 10 °C is a difference of 18 °F, not 50 °F. Values below absolute zero (−273.15 °C, −459.67 °F, 0 K) are flagged because they cannot exist.
Fuel economy comes in two opposite kinds of unit. Miles per gallon and km per litre measure distance per fuel, so bigger is better; litres per 100 km measures fuel per distance, so smaller is better. Converting between the kinds means dividing a constant by the value: L/100 km = 282.481 ÷ mpg (UK), or 235.215 ÷ mpg (US). The table beneath shows why this matters — improving from 20 to 25 mpg saves far more fuel than improving from 50 to 55.
Data storage has two systems. Decimal SI prefixes (kB, MB, GB, TB) multiply by 1,000, which is how drive makers and network speeds count. Binary IEC prefixes (KiB, MiB, GiB, TiB) multiply by 1,024, which is what Windows actually measures while still labelling it “GB”. That is why a 1 TB drive appears as about 931 GB. Network speeds are in bits: 100 Mbit/s is 12.5 MB/s. For screen measurements in centimetres and inches at true size, use the online ruler.
Converting cups to grams for baking
A cup is a volume and a gram is a weight, so turning one into the other needs the ingredient's density — how many grams fit in a millilitre. A US cup of water weighs about 237 g, but the same cup of plain flour weighs roughly 125 g and of honey about 336 g. Choose the ingredient in the Cooking category and the converter applies its typical density, showing the figure it used.
| Ingredient | 1 US cup (237 ml) | 1 tbsp (15 ml) |
|---|---|---|
| Plain or self-raising flour | ≈ 125 g | ≈ 8 g |
| Granulated or caster sugar | ≈ 200 g | ≈ 13 g |
| Butter | ≈ 227 g | ≈ 14 g |
| Rolled oats | ≈ 90 g | ≈ 6 g |
| Honey | ≈ 336 g | ≈ 21 g |
These are averages for ingredients spooned into the cup and levelled. Scooping flour straight from the bag can pack in 20–30% more, which is enough to turn a sponge into a brick; weighing is always more reliable. Note also that an Australian or metric cup is 250 ml, and a UK tablespoon in modern recipes is 15 ml. If you're scaling a recipe up or down by a percentage, the percentage calculator does the arithmetic.
Frequently asked questions
How accurate is this unit converter?
Conversions between metric, imperial and US units use the exact legal definitions — an inch is exactly 25.4 mm and a pound exactly 0.45359237 kg — so the only limit is the precision you choose to display. Calculations use double-precision floating point, which is exact to about 15 significant figures. Auto mode shows ten and trims tiny rounding artefacts. Cooking conversions between volume and weight are estimates, because ingredient density varies.
Why does my 1 TB hard drive show as 931 GB?
Drive manufacturers use decimal units, where 1 TB is 1,000,000,000,000 bytes. Windows measures in binary units, where a “GB” is really a gibibyte of 1,073,741,824 bytes, so the same drive reads as about 931 GiB. Nothing is missing. Convert terabytes (TB) to gibibytes (GiB) in the Data storage category to see the exact figure for any size.
How do I convert mpg to litres per 100 km?
Divide 282.481 by UK miles per gallon, or 235.215 by US miles per gallon. So 50 mpg (UK) is about 5.65 L/100 km. Because the two scales are reciprocals, the same division converts back the other way. Choose Fuel economy, set mpg (UK) or mpg (US) on one side and L/100 km on the other, and the converter shows the constant it is using.
Is a pint 568 ml or 473 ml?
Both, depending on the country. A UK (imperial) pint is 568.26 ml and is still the legal measure for draught beer and cider in British pubs. A US pint is 473.18 ml. The same applies to gallons and quarts. The converter lists UK and US units separately and warns you when you convert between the two systems.
Can I type fractions or sums into the converter?
Yes. You can type fractions such as 3/4, mixed numbers like 1 1/2, the fraction characters ½ ¼ ¾ ⅓, and simple arithmetic using + − × ÷ and brackets, such as (12+18)*2 or 2^10. The converter works out the value, shows it in the notes, and converts it. When you leave the box, a sum is replaced with its result.
Does the converter work offline and is anything stored?
Once the page has loaded, every conversion is calculated in your browser, so it keeps working without a connection. Nothing you type is sent anywhere. Saved pairs, recent pairs and your precision setting are kept in your own browser's local storage and can be removed with the × on each chip or by clearing site data.