How to use the unit converter
- Pick a quantity at the top: length, weight, temperature, area, volume, speed, time, data, pressure or energy. Each has its own list of metric, US and imperial units.
- Type a number in the From box and choose the two units. The answer appears in the To box as you type. You can also type in the To box to convert the other way, and the ⇄ button swaps the units.
- The line under the boxes shows the factor, such as “1 cm = 0.3937007874 in”, or the formula for temperatures.
- The table converts your value into every other unit of the same kind at once. Click a row to make that unit the target. Your last quantity and units are remembered in this browser.
The exact definitions behind the numbers
Most US and imperial units are now defined in terms of metric ones, so many conversions are exact rather than measured. The converter uses these definitions:
| 1 inch | exactly 2.54 cm, so 1 foot = 30.48 cm and 1 mile = 1,609.344 m |
|---|---|
| 1 nautical mile | exactly 1,852 m; 1 knot = 1.852 km/h |
| 1 pound | exactly 0.45359237 kg (16 ounces; 1 ounce ≈ 28.35 g) |
| 1 stone | 14 pounds ≈ 6.35 kg |
| 1 US gallon | 231 cubic inches ≈ 3.785 L (128 US fluid ounces) |
| 1 UK gallon | exactly 4.54609 L (160 UK fluid ounces) |
| 1 acre | ≈ 4,046.86 m²; 1 hectare ≈ 2.47 acres |
| 1 kcal (food Calorie) | 4.184 kJ |
| 1 bar | 100 kPa ≈ 14.5 psi; 1 standard atmosphere = 101.325 kPa |
Worked examples
- Height in feet and inches. 180 cm is 5.905511811 ft. The converter gives decimal feet, so keep the whole number (5 ft) and multiply the rest by 12: 0.9055 × 12 ≈ 10.9 in, about 5 ft 10.9 in. The other way, 5 ft 10 in is 70 inches, and 70 in = 177.8 cm.
- An American recipe. 2 US cups = 473.176473 mL, and a US tablespoon is about 14.79 mL. An oven at 350 °F is 176.67 °C, which most cooks round to 175 or 180 °C.
- Tyre pressure. 32 psi is about 2.21 bar or 220.6 kPa.
- Speed limits abroad. 100 km/h is 62.14 mph, and 65 mph is 104.6 km/h.
Metric, US customary and imperial units
The metric system (SI) is used almost everywhere for science and trade. The United States mostly uses US customary units – feet, pounds, gallons and degrees Fahrenheit – and the United Kingdom still uses miles, pints and stones alongside metric units. Since the international yard and pound agreement of 1959, the inch, foot, yard, mile and pound are identical in both systems.
Volume is where they split. A UK gallon is about 20% larger than a US gallon, a UK pint holds 20 fluid ounces against 16 in a US pint, and even the fluid ounce differs (28.41 mL versus 29.57 mL). That is why the volume list includes both; if a recipe or fuel figure is British, choose the UK unit. Tons differ too: a metric ton is 1,000 kg, a US short ton 2,000 lb (907.18 kg) and a UK long ton 2,240 lb (1,016.05 kg).
Temperature works differently
Celsius, Fahrenheit and Kelvin have different zero points, so they can't be converted by multiplying alone. The converter uses °F = °C × 9/5 + 32 and K = °C + 273.15. Water freezes at 0 °C (32 °F) and boils at 100 °C (212 °F) at sea level, and −40 is the same on both scales.
These formulas convert a reading, not a difference. A rise of 10 °C is a rise of 18 °F, not 50 °F: for a difference, multiply by 9/5 and skip the +32.
Data, time and energy
Drive makers count in powers of 1,000 (1 GB = 1,000,000,000 bytes), while Windows and many programs count in powers of 1,024 and still write “GB” for what is really a gibibyte (GiB). Both families are listed separately. Network speeds are quoted in bits: with 8 bits in a byte, a 100 Mbit/s connection moves at most 12.5 MB per second.
Months and years in the time list are Gregorian averages (30.436875 and 365.2425 days). They are fine for estimates; to count the real days between two dates, use the Date Calculator. On food labels, a “Calorie” is a kilocalorie: a 2,000 kcal day is 8,368 kJ. The Calorie Calculator estimates how many you need.
Tips and common mistakes
- Ounces can be weight (oz) or volume (fl oz). They only match roughly for water, so don't swap them for flour or oil.
- Results show up to 10 decimal places, and very large or tiny values switch to scientific notation such as 1.000000e-9. Round to the precision your measurement actually has.
- For one common conversion with a ready-made table, use the quick pages: cm to inches, meters to feet, km to miles, kg to lbs, grams to ounces, Celsius to Fahrenheit, mL to fl oz, liters to gallons, km/h to mph and m² to ft².
FAQ
How accurate are the results?
The factors are the official definitions above, so inch–centimeter or pound–kilogram conversions are exact. Results are rounded to 12 significant digits to hide floating-point noise (0.30000000000000004 becomes 0.3).
Why does a 1 TB drive show about 931 GB?
The drive holds 1,000,000,000,000 bytes. Divided by 1,073,741,824 bytes per GiB, that is about 931.32 – the operating system just labels GiB as “GB”.
Why do ships and planes use knots?
The nautical mile was originally based on one minute of latitude, which makes it convenient for navigation. One knot is exactly 1.852 km/h, or about 1.15 mph.