Unit Converter (Metric & Imperial)

Britain still lives in two systems at once. Petrol is sold by the litre but fuel economy is quoted in miles per gallon, road signs show miles, babies are weighed in pounds and ounces, and recipes swing between grams, cups and gas marks. Our free unit converter handles metric and imperial units across length, weight, temperature, volume, area, speed, data size, cooking measures, oven temperatures and fuel economy. It converts both ways as you type, understands combined values such as 5 ft 10 in and 11 st 4 lb, keeps UK and US pints, gallons and cups separate, and shows a table of common values for the pair you pick. It all runs in your browser.

How to use the Unit converter

1. 1 Choose a categoryPick the kind of measurement you have, such as length, weight, volume, oven temperature or fuel economy. Each category only lists units that can sensibly be converted into one another.
2. 2 Select the two unitsChoose the unit you are converting from and the unit you want. For heights and body weights, choose the combined ft & in or st & lb options to enter two parts at once.
3. 3 Type in either boxThe conversion works in both directions. Type in the left box to convert one way, or in the right box to convert back, and the other side updates instantly.
4. 4 Use the swap button and the common values tableSwap reverses the pair in one click. The table underneath lists everyday values for the chosen pair, which is handy for printing or for checking a whole recipe.
5. 5 Copy the resultPress copy to put the converted figure on your clipboard, ready to paste into a spreadsheet, a form, a recipe or a message.

Why the UK still mixes metric and imperial

The UK officially adopted metric units for most trade decades ago, but imperial units never fully disappeared. Road distances and speed limits are signed in miles and miles per hour, draught beer and cider are still served in pints, and milk in returnable bottles can be sold by the pint. In everyday conversation most people still give their height in feet and inches and their weight in stones and pounds, even though a GP surgery or hospital will usually record centimetres and kilograms.

The result is that many ordinary tasks involve a conversion. A car advert quotes miles per gallon while the pump shows litres. A gym programme lists kilograms while the bathroom scales show stones. A family recipe written by a grandparent asks for ounces and a moderate oven, while a modern cookbook uses grams and a fan setting. A unit converter that understands British habits, not just textbook units, saves a lot of guesswork.

Most of the conversions in this tool are based on exact definitions. Since 1959 the inch has been defined as exactly 25.4 millimetres and the pound as exactly 0.45359237 kilograms, so every length and weight conversion between the two systems can be worked out precisely. The displayed figure is rounded for readability, but the underlying calculation uses the exact factors.

Length, height and distance

Length covers millimetres, centimetres, metres and kilometres alongside inches, feet, yards and miles, plus the nautical mile used at sea and in aviation. A foot is 12 inches, or exactly 30.48 centimetres, a yard is 3 feet, or exactly 0.9144 metres, and a statute mile is 1,760 yards, or exactly 1.609344 kilometres. A nautical mile is a different unit altogether: it is defined as exactly 1,852 metres, which is about 1.15 statute miles.

For heights, use the combined ft & in option. Typing 5 ft 10 in gives about 177.8 cm, and typing 180 cm on the metric side gives roughly 5 ft 10.9 in. This avoids the common mistake of typing 5.10 feet, which is not 5 feet 10 inches at all but 5 feet and about 1.2 inches, because a tenth of a foot is 1.2 inches. Running distances work the same way: a 5K is about 3.1 miles, a 10K about 6.2 miles and a half marathon about 13.1 miles.

Weight, body weight and tonnes

The weight category covers milligrams, grams, kilograms and metric tonnes alongside ounces, pounds and stones. There are 16 ounces in a pound and 14 pounds in a stone, so a stone is about 6.35 kilograms. The combined st & lb option lets you type a body weight such as 11 st 4 lb, which converts to about 71.7 kg, or start from kilograms and see the answer in stones and pounds rather than an awkward decimal.

Tons need particular care because three different units share almost the same name. A metric tonne is 1,000 kilograms. A UK long ton is 2,240 pounds, about 1,016 kilograms, and a US short ton is 2,000 pounds, about 907 kilograms. Older British documents, shipping records and some engineering specifications use the long ton, while American sources almost always mean the short ton, so always check where a figure came from before converting it.

• 1 ounce = about 28.35 grams
• 1 pound = 16 ounces = about 453.6 grams
• 1 stone = 14 pounds = about 6.35 kilograms
• 1 UK long ton = 2,240 pounds, 1 US short ton = 2,000 pounds, 1 tonne = 1,000 kilograms

Volume: UK and US measures are not the same

Volume is where most conversion errors happen, because the UK and the US use the same names for different quantities. A UK pint is 568.261 millilitres and contains 20 UK fluid ounces, while a US liquid pint is 473.176 millilitres and contains 16 US fluid ounces. The fluid ounces differ too: a UK fluid ounce is about 28.41 ml and a US fluid ounce about 29.57 ml. Quarts and gallons follow the pints, so a UK gallon is exactly 4.54609 litres and a US gallon is exactly 3.785411784 litres.

The converter keeps these as separate units so you can see the difference rather than guess. That matters when you follow an American recipe, read a US product label or compare fuel figures from an American website. For engineering and science work the tool also includes cubic centimetres, cubic metres, cubic inches and cubic feet. One cubic centimetre is exactly one millilitre, and one cubic metre is 1,000 litres.

Cooking measures and oven temperatures

The cooking category is built for recipes. A metric cup is 250 ml, a US cup is about 236.6 ml and an older imperial cup is about 284.1 ml, so the same recipe can turn out differently depending on which cup the author meant. Tablespoons and teaspoons vary slightly as well: a UK or metric tablespoon is 15 ml and a teaspoon 5 ml, while the US tablespoon is about 14.8 ml and the teaspoon about 4.9 ml. For small amounts the spoon difference rarely matters, but cups and fluid ounces can add up across a whole bake.

Cups measure volume, not weight, so a cup of flour and a cup of sugar weigh different amounts. If a recipe gives cups for dry ingredients, weighing is more reliable once you know the weight per cup for that ingredient, which good recipes often state. Liquids such as water and milk are close to one gram per millilitre, so the volume and weight figures are roughly interchangeable for them.

Oven temperatures get their own category because British recipes use four different scales: degrees Celsius for a conventional oven, Celsius for a fan oven, degrees Fahrenheit, and gas marks. A fan oven usually needs about 20 degrees less than a conventional one, so 180°C conventional becomes about 160°C fan. Gas marks follow a standard table, and the Fahrenheit and Celsius figures in it are rounded kitchen values rather than exact conversions.

• Gas mark ¼ = 110°C = 225°F, gas mark ½ = 120°C = 250°F
• Gas mark 1 = 140°C = 275°F, gas mark 2 = 150°C = 300°F, gas mark 3 = 170°C = 325°F
• Gas mark 4 = 180°C = 350°F, gas mark 5 = 190°C = 375°F, gas mark 6 = 200°C = 400°F
• Gas mark 7 = 220°C = 425°F, gas mark 8 = 230°C = 450°F, gas mark 9 = 240°C = 475°F

Temperature, area, speed, data size and fuel economy

Temperature converts between Celsius, Fahrenheit and kelvin. To go from Celsius to Fahrenheit, multiply by 9, divide by 5 and add 32, so 20°C is 68°F and 37°C is about 98.6°F. Kelvin is Celsius plus 273.15 and is used in science and for the colour temperature of light bulbs. Area covers everything from square millimetres to square miles, including hectares and acres for land: one hectare is 10,000 square metres, or about 2.47 acres, and one acre is about 4,047 square metres. Floor areas from estate agents often appear in square feet, and one square metre is about 10.76 square feet.

Speed converts between metres per second, kilometres per hour, miles per hour, knots and feet per second. 30 mph is about 48 km/h and 70 mph is about 113 km/h. A knot is one nautical mile per hour, about 1.15 mph, and is used for wind speeds in shipping forecasts and for boats and aircraft.

Data size explains a long-standing source of confusion. Storage makers use decimal units, where a kilobyte is 1,000 bytes and a terabyte is a million million bytes, while many operating systems report sizes in binary units, where a kibibyte is 1,024 bytes. That is why a new 1 TB drive appears as roughly 931 GB in Windows: it is showing about 931 GiB. Broadband speeds are given in bits rather than bytes, and there are 8 bits in a byte, so a 100 Mbit/s connection downloads at best about 12.5 megabytes a second.

Fuel economy converts between UK miles per gallon, US miles per gallon, litres per 100 kilometres and kilometres per litre. Because a UK gallon is larger than a US gallon, the same car returns a higher figure in UK mpg than in US mpg, by a factor of about 1.2. Litres per 100 km works the other way round to mpg: a lower number means a more economical car. Treat official figures as a comparison between models rather than a promise, because real driving usually gives different results.

Examples you can copy

5 ft 10 in           = 177.8 cm
11 st 4 lb           = 71.7 kg (approx.)
1 UK pint            = 568.261 ml
1 US pint            = 473.176 ml
1 UK gallon          = 4.54609 litres
30 mph               = 48.3 km/h (approx.)
1 hectare            = 2.471 acres (approx.)
1 TB (decimal)       = 931.3 GiB (approx.)

°F       = °C x 9 / 5 + 32
K        = °C + 273.15
°C fan   = °C conventional - 20
L/100km  = 282.481 / UK mpg
L/100km  = 235.215 / US mpg

50 UK mpg  ->  282.481 / 50  =  5.65 L/100km

1 US cup milk        = 236.6 ml
2 US tbsp butter     = 29.6 ml (approx.)
1 US tsp vanilla     = 4.9 ml
350°F                = gas mark 4 = 180°C (160°C fan)

Common mistakes to avoid

Frequently asked questions