How to use the unit converter
- Choose a category: length, weight, volume, area, temperature, data, speed or time. The two unit menus refill with the units that belong together.
- Type your number in the Value box and pick the unit you are converting from. The converted number appears in the Result box immediately, with no button to press.
- Choose the unit you want in the second menu, or press the ⇄ button to swap the two units. You can also type in the result box and read the answer in the first box, which makes the tool two-way.
- Change Significant digits if you need a rougher or more precise number, then use Copy to put a ready-made sentence such as "1 in = 2.54 cm" on your clipboard.
The address bar remembers your category and units, so you can bookmark a favorite conversion or send the link to a colleague. The shortcut chips under the converter jump straight to the conversions people search for most often.
Metric and imperial units in one place
Anyone who reads recipes, product specifications, flight data or fitness trackers from more than one country eventually has to switch between the metric system and the customary units used in the United States and, partly, in the United Kingdom. The two systems rest on different ideas. Metric units are decimal and linked to each other: a liter is a thousand milliliters, and a milliliter of water weighs about one gram. Customary units grew out of everyday measures such as feet, pounds and gallons, and the relationships between them are irregular, with 12 inches to a foot, 3 feet to a yard and 1,760 yards to a mile.
Since the international yard and pound agreement of 1959, the customary units have been defined exactly in terms of metric ones. One inch is exactly 2.54 centimeters, one pound is exactly 0.45359237 kilograms and one mile is exactly 1.609344 kilometers. The converter uses those exact definitions, so the only rounding you see is the number of significant digits you ask for.
| Quantity | Exact or standard relationship |
|---|---|
| Length | 1 in = 2.54 cm · 1 ft = 0.3048 m · 1 yd = 0.9144 m · 1 mi = 1.609344 km · 1 nautical mile = 1,852 m |
| Weight | 1 lb = 0.45359237 kg · 1 oz = 28.349523125 g · 1 stone = 14 lb |
| Volume | 1 US gallon = 3.785411784 l · 1 UK gallon = 4.54609 l · 1 US cup = 236.5882365 ml |
| Area | 1 acre = 4,046.8564224 m² = 43,560 ft² · 1 hectare = 10,000 m² |
| Temperature | °F = °C × 9/5 + 32 · K = °C + 273.15 |
| Speed | 1 mph = 0.44704 m/s · 1 knot = 1.852 km/h |
Categories explained
Length and area
Everyday trips between inches, centimeters, feet, meters, yards, miles and kilometers are covered, as well as nautical miles for sailing and aviation. For area you can move between square centimeters, square meters, hectares, square kilometers, square inches, square feet, square yards, acres and square miles. Remember that area units scale with the square of length: a square meter is not 3.28 square feet but about 10.76, because both sides are multiplied by 3.28.
Weight and volume
Weight here means mass, the quantity that kitchen and postal scales report. Ounces, pounds and stones sit next to milligrams, grams, kilograms and metric tonnes. For volume the converter offers milliliters, liters and cubic meters alongside US teaspoons, tablespoons, fluid ounces, cups, pints, quarts and gallons, plus the UK imperial gallon. Be careful with recipes: US and imperial fluid units differ, a UK imperial gallon is about 20 percent larger than a US gallon, and some countries cook with a 250 ml metric cup instead of the 236.6 ml US cup.
Temperature
Temperature is the one category that is not a simple multiplication, because the scales start at different zero points. The tool converts Celsius, Fahrenheit and Kelvin with the correct offset formulas. Water boils at 100 °C, which is 212 °F or 373.15 K; normal body temperature of about 37 °C is 98.6 °F; and the two scales cross at −40. Values below absolute zero (−273.15 °C) are rejected with an explanatory message because they are physically impossible.
Data units: why 1 GB is sometimes 1,000 MB and sometimes 1,024 MB
The prefix "kilo" means a thousand in every other field of science, but computers work in powers of two, and 1,024 is the power of two closest to a thousand. For decades both meanings were written as KB, MB and GB, which is the root of a famous confusion. Drive and phone manufacturers, and also network speeds, use the decimal convention: a 1 TB drive holds 1,000,000,000,000 bytes. Many operating systems report capacity in the binary convention instead, so the same drive shows up as roughly 931 GB. Nothing is missing; the two sides are simply dividing by different numbers.
The converter lets you choose. Select 1024 base to see binary sizes, which is what most file managers and memory chips use, or 1000 base for the decimal sizes quoted on packaging and in data plans. Standards bodies also define the unambiguous names kibibyte (KiB), mebibyte (MiB) and gibibyte (GiB) for the binary sizes, though they are far from universal in everyday speech. Bits and bytes are included as well, with eight bits to the byte, because internet speeds are quoted in bits per second while file sizes are quoted in bytes.
Speed and time
Road signs show miles per hour in the United States and the United Kingdom and kilometers per hour nearly everywhere else, so a quick check such as 100 km/h being about 62 mph is useful when you rent a car abroad. Scientists use meters per second, and sailors and pilots use knots, which are nautical miles per hour. For time, the converter handles milliseconds up to weeks. Months and years are intentionally left out, because their length varies between 28 and 31 days and between 365 and 366 days, and a converter that picked one value silently would give misleading answers.
Why precision matters: two costly mix-ups
Unit mistakes are not just academic. In 1999 NASA lost the Mars Climate Orbiter because one team's software produced thrust data in pound-force seconds while another team expected newton seconds. In 1983 an Air Canada Boeing 767 ran out of fuel mid-flight and glided to a landing at Gimli, Manitoba, after fuel had been loaded in pounds instead of kilograms. Everyday errors are milder, such as a cake that fails because tablespoons were read as milliliters, but the lesson is the same: always keep the unit next to the number, and double-check conversions that matter.
When you copy the result, the unit symbols travel with it, which helps prevent the "bare number" problem. For critical work such as medicine, engineering or aviation, follow your organization's approved procedures and verify the figure with a second method.
Private and works offline
The converter is a small script that runs in your browser. The numbers you enter are never uploaded or logged, and after the first visit the page can be installed as an app and used without a connection, for example in a kitchen, a workshop or on a plane.
Frequently asked questions
How many centimeters are in an inch?
There are exactly 2.54 centimeters in one inch. To convert inches to centimeters, multiply by 2.54; to go the other way, divide by 2.54. Select Length, set the units to inch and centimeter, and the tool does it for you.
How do I convert Celsius to Fahrenheit?
Multiply the Celsius value by 9/5 (1.8) and add 32. For example, 100 °C × 1.8 + 32 = 212 °F, and 20 °C is 68 °F. The temperature category does this automatically and also converts to Kelvin by adding 273.15.
Is a gigabyte 1,000 or 1,024 megabytes?
Both conventions exist. Storage manufacturers use 1,000 (decimal), while many operating systems and memory specifications use 1,024 (binary). The converter shows a switch for the data category so you can pick either one and see both results.
What is the difference between a US gallon and a UK gallon?
A US gallon is 3.785 liters and a UK imperial gallon is 4.546 liters, about 20 percent larger. Fuel economy figures, recipes and paint coverage can differ for that reason, so check which gallon a source uses.
How accurate are the conversions?
Factors for inches, pounds, miles, gallons and acres use their exact definitions, so the results are accurate to the number of significant digits you choose, up to 12. Temperature uses the standard offset formulas. Very large or very small results switch to scientific notation.
Why does the tool refuse negative numbers?
Length, weight, volume, area, data, speed and time cannot be negative in everyday use, so a minus sign is almost always a typing slip. Temperature is the exception and accepts negatives, down to absolute zero at −273.15 °C.
Can I type in the result box and convert backwards?
Yes. Both boxes are live. Type in the Result box and the top Value box is calculated, using the same units, so you never have to swap the menus to go the other way.
Does this converter send my numbers anywhere?
No. All conversions run in your browser, nothing is uploaded or stored, and the page works offline after it has loaded. The tool is free, requires no sign-up and has no usage limit.
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