Unit Converter

What the converter supports

The converter handles 4 categories and 28 units in total. Length, weight and volume each convert between every pair of units in the same category. Temperature converts between the three scales listed below. The counts in this table are taken straight from the data the tool runs on, so they cannot drift out of date.

How the calculation works

For length, weight and volume, every value is routed through one base unit (meters, kilograms or liters). The formula is: input x (base-unit value of the input unit) / (base-unit value of the target unit). Converting 5 feet to meters is 5 x 0.3048 / 1 = 1.524 m. Using a base unit means each unit needs only one stored factor, rather than a separate factor for every pair, which removes a whole class of typing mistakes. The line under the result, such as "1 ft = 0.3048 m", shows the ratio used. All units in the same category are also shown at once so you can pick whichever reads most naturally.

The factors are exact definitions, not rounded approximations. The international inch is exactly 25.4 mm, and the international pound is exactly 0.45359237 kg (fixed by the 1959 international yard and pound agreement). From those: 1 foot = 12 inches = 0.3048 m, 1 yard = 3 feet = 0.9144 m, 1 mile = 5,280 feet = 1,609.344 m, and 1 avoirdupois ounce = 1/16 pound = 28.349523125 g. The US liquid gallon is defined as 231 cubic inches, which is exactly 3.785411784 liters. The nautical mile is exactly 1,852 m. Results are displayed to 10 significant digits, which hides floating-point noise such as 0.30000000000000004.

Reference table: every unit and its definition

This table lists each unit's size in its category's base unit, generated from the same data the converter uses.

Length conversion table

Inches and feet turn up in screen sizes, tire sizes and body height in the United States and a few other countries. Miles appear on road signs in the US and the UK, and nautical miles (1,852 m) measure distance at sea and in the air. A monitor is sold by its diagonal: a 27-inch monitor has a diagonal of 27 x 2.54 = 68.58 cm. Its width and height depend on the aspect ratio and have to be worked out separately.

Weight conversion table

The ounce here is the ordinary avoirdupois ounce, a unit of weight. The troy ounce used for precious metals (about 31.1035 g) is a different unit and is not in this tool. The "fluid ounce" in the volume list is a unit of volume, not weight. The shared word "ounce" makes this the most common mix-up in cooking conversions. Here, t is the metric tonne of 1,000 kg.

Volume conversion table

Same name, different size. The US gallon is 3.785 L and the imperial (UK) gallon is 4.546 L, about 20% larger. A car rated at 40 miles per imperial gallon gets 40 x 3.785411784 / 4.54609 = about 33.3 miles per US gallon, so fuel-economy figures from the two systems cannot be compared directly. The cup is also not universal. The cup in this tool is the US customary cup, 236.588 mL. The cup printed on US nutrition labels is rounded to 240 mL, and recipes from Australia, New Zealand and Canada often assume a 250 mL cup. For baking, where proportions matter, look for gram weights rather than cup measures.

Temperature: why multiplication alone is not enough

Length, weight and volume have a true zero that means "none", so a single multiplication converts them. Celsius and Fahrenheit place zero at different points, so converting needs an offset as well as a scale factor:

• Fahrenheit = Celsius x 9/5 + 32
• Celsius = (Fahrenheit - 32) x 5/9
• Kelvin = Celsius + 273.15

A one-degree Celsius step equals a 1.8-degree Fahrenheit step, since water's freezing and boiling points are 100 degrees apart on one scale and 180 on the other. So a temperature difference of 10 degrees C is 18 degrees F, but a temperature reading of 20 degrees C is 68 degrees F, not 36. Forgetting the 32 is the most common temperature error. Kelvin has the same step size as Celsius but starts at absolute zero, so the tool warns you if you enter something colder than that.

Worked examples

1. A US oven temperature. 350 degrees F gives (350 - 32) x 5/9 = 176.67 degrees C. Most ovens are set in 5 or 10 degree steps, so 175 or 180 degrees C is the practical setting. Going the other way, 180 degrees C is 180 x 9/5 + 32 = 356 degrees F.
2. Height and weight in US units. 5 ft 9 in is 69 inches, so 69 x 2.54 = 175.26 cm. 180 lb is 180 x 0.45359237 = 81.65 kg. In the other direction, 70 kg is 70 / 0.45359237 = 154.32 lb.
3. A running distance. A marathon of 42.195 km is 42.195 / 1.609344 = 26.22 miles, and 10 km is 6.21 miles.
4. Gallons. Two US gallons are 2 x 3.785411784 = 7.5708 L, while two imperial gallons are 9.0922 L.

Common mistakes and limits

• Temperature value versus temperature difference. Apply the offset to readings and only the 1.8 ratio to differences.
• US versus imperial volumes. Gallon, quart, pint and fluid ounce differ between the two systems. Everything in this tool is US customary except the item labeled as the imperial gallon.
• Weight ounce versus fluid ounce. 28.35 g and 29.57 mL are different kinds of quantity. Water happens to make them look similar, but flour, honey and oil do not.
• Volume to weight. You cannot convert milliliters to grams without knowing the density of the ingredient. Only water is close to 1 g per mL.
• Safety-critical work. The tool applies the defined factors correctly, but for medication doses, structural calculations or fuel loads, write the unit next to every number and have the result checked by a second method.

Unit confusion has caused real accidents. In 1983 an Air Canada jet ran out of fuel in flight after fuel was calculated in pounds when kilograms were required, and in 1999 NASA lost the Mars Climate Orbiter because thrust data in one unit system was read as another. The lesson is simply to carry the unit with the number.

Getting the most out of the results

• For items bought abroad, convert inches and pounds to centimeters and kilograms to judge the real size and shipping weight, and round up when estimating postage.
• For recipes, convert cups and spoons to milliliters first, then look for a version of the recipe written in grams if you can.
• Use Copy result to paste the converted value into a spreadsheet or message. The text includes both units.
• Swap units with the swap button to check a result by converting it back. You should land on the number you started with, to the displayed precision.

Related tools

• Cooking Measurement Converter for cups, spoons, milliliters and grams by country
• Cooking Temperature Converter for Fahrenheit, Celsius, gas mark and fan-oven adjustments
• Fuel Economy Unit Converter for mpg, km/L and L/100 km

Frequently Asked Questions

This tool is for reference. For medicine, engineering or legal metrology, confirm the original unit and the official standard.