How to use the unit converter
Pick what you are converting, type an amount, and choose the unit you have and the unit you want. The answer appears at once with the math underneath, a table of every other unit of the same kind, and a picture of two rulers lined up so you can see that both measure the same amount. For lengths, pick feet and inches to type a height like 5 ft 11 in, or to see an answer as feet, inches and a tape-measure fraction. The Imperial | Metric switch chooses which side you start from: Imperial starts with US units and shows metric, and Metric starts with metric and shows US. Nothing you type leaves your device.
How the conversion is worked out
Every unit on this page is defined as an exact amount of a base unit, so a conversion is one exact multiplication and one exact division, with no rounding in between. Your amount is multiplied by the size of the unit you have and divided by the size of the unit you want. The answer is rounded only at the end, to the number of digits you pick, and the page tells you whether the rounded answer is exact or cut off. An inch is exactly 25.4 millimeters, a foot is exactly 0.3048 meters, and a mile is exactly 1,609.344 meters, so 12 inches is exactly 30.48 centimeters and 5 miles is exactly 8.04672 kilometers.
Length
Metric lengths step by tens: 10 millimeters make a centimeter, 100 centimeters a meter and 1,000 meters a kilometer. US lengths step by 12 inches to the foot, 3 feet to the yard and 5,280 feet to the mile. The two systems meet at the inch, which is defined as exactly 25.4 millimeters, and everything else follows from it. A nautical mile is a separate unit, exactly 1,852 meters. The converter shows inches as a tape-measure fraction too, rounded to the nearest 1/16 inch, so a metric measurement becomes something you can find on a tape.
Area
An area unit is a length unit squared, so a square foot is 0.3048 meters on each side, or exactly 0.09290304 square meters. An acre is 4,840 square yards, which is 43,560 square feet, and a hectare is exactly 10,000 square meters. One hectare is about 2.471 acres. This page uses the international acre built from the international foot of exactly 0.3048 meters, so an acre is exactly 4,046.8564224 square meters. The old US survey foot was retired on January 1, 2023, and the survey acre built on it was about 4,046.8726 square meters, so older land records can differ from this page in the sixth digit.
Volume
A cubic yard is 27 cubic feet, and a cubic foot holds about 7.481 US gallons. A US gallon is 231 cubic inches, which is exactly 3.785411784 liters, and the smaller US liquid units split it evenly: 4 quarts, 8 pints, 16 cups or 128 fluid ounces make a gallon, a tablespoon is half a fluid ounce and a teaspoon is a third of a tablespoon. A liter is a thousandth of a cubic meter, exactly. These are US customary units, so the cup here is the US customary cup of 236.588 milliliters, not a metric cup and not a United Kingdom measure.
Weight
A pound is defined as exactly 0.45359237 kilograms and an ounce is a sixteenth of a pound. Pay attention to the word ton. A US ton is 2,000 pounds, which is about 907.18 kilograms. A metric tonne is exactly 1,000 kilograms, about 2,204.6 pounds. They differ by roughly 10 percent, so the page labels them US tons and metric tonnes.
Temperature
Temperature is different because the scales start at different places, so you cannot just multiply. Kelvin is Celsius plus 273.15, and Fahrenheit is Celsius times 9/5 plus 32, which is the same as Celsius times 1.8 plus 32. To go the other way, subtract 32 from Fahrenheit and multiply by 5/9. The two scales agree at exactly minus 40, so minus 40 degrees Celsius is minus 40 degrees Fahrenheit. Room temperature of 72 degrees Fahrenheit is about 22.22 degrees Celsius, and 20 degrees Celsius is exactly 68 degrees Fahrenheit.
Speed and pressure
A mile per hour is exactly 0.44704 meters per second, so 60 miles per hour is exactly 96.56064 kilometers per hour. A knot is one nautical mile per hour, about 1.1508 miles per hour. For pressure, a bar is exactly 100,000 pascals and a standard atmosphere is exactly 101,325 pascals, about 14.696 pounds per square inch. One pound per square inch is about 6.8948 kilopascals.
Mistakes that give the wrong answer
- Mixing up the US ton and the metric tonne. They are not the same weight.
- Using a gallon from another country. This page uses the US gallon.
- Forgetting to square the length conversion for an area. One meter is about 3.28 feet, but one square meter is about 10.76 square feet, not 3.28.
- Multiplying to convert a temperature. Fahrenheit and Celsius need the offset of 32 as well as the 9/5.
- Rounding in the middle of a calculation. Convert once at the end, which is what this page does.
- Reading a ruler marked in 1/16 inch as a decimal. Use the tape-measure row to find the right mark.
Where the numbers come from
- Inch, foot, yard, mile and nautical mile: NIST SP 811, Appendix B, where the inch is exactly 25.4 millimeters and the others are exact.
- Hectare, liter, metric ton, mile per hour, foot per second, bar and standard atmosphere: exact in NIST SP 811 Appendix B.
- Pound, ounce, US ton, US gallon, quart, pint, cup, fluid ounce, tablespoon, teaspoon, cubic units, knot, kilometer per hour and pound per square inch: NIST prints these to 7 digits, and this page's exact values match every printed digit.
- Temperature: Kelvin is Celsius plus 273.15, and Celsius is (Fahrenheit minus 32) divided by 1.8, as given in NIST SP 811.
- Pound: exactly 0.45359237 kilograms since July 1, 1959 (the US and Commonwealth agreement published in the Federal Register; NIST Handbook 44, Appendix C).
- US gallon: exactly 231 cubic inches (NIST Handbook 44, Appendix C), which makes it exactly 3.785411784 liters.
- Acre: 43,560 square feet, or 4,840 square yards (NIST Handbook 44, Appendix C), on the international foot. NIST and the National Geodetic Survey retired the US survey foot on January 1, 2023.
Questions
How many millimeters are in an inch?
Exactly 25.4. So 1/2 inch is 12.7 millimeters and 1/16 inch is 1.5875 millimeters.
How many feet are in a mile?
5,280. A mile is exactly 1,609.344 meters, about 1.609 kilometers.
How do I convert Fahrenheit to Celsius?
Subtract 32, then multiply by 5/9. For 72 degrees Fahrenheit that is 40 times 5/9, about 22.22 degrees Celsius. To go the other way, multiply by 9/5 and add 32.
How many gallons are in a cubic foot?
About 7.481 US gallons. A US gallon is 231 cubic inches and a cubic foot is 1,728 cubic inches.
Is a ton the same as a tonne?
No. A US ton is 2,000 pounds (about 907.18 kilograms) and a metric tonne is 1,000 kilograms (about 2,204.6 pounds).
Is the answer rounded?
Only at the end, to the digits you choose, and the page says whether the shown answer is exact or rounded. The default is 6 significant digits.
Why does the picture have two rulers?
Each ruler counts the same amount in its own unit. The red line shows your amount on both so you can see that the two numbers describe one length, one area, one weight or one temperature.
Can I share a conversion?
Yes. Share saves only the category, the two units and your amount for 90 days so the link can show the answer, and makes a picture for Facebook.