Unit Converter

Convert between metric and US units — length, weight, temperature, volume, area, and speed. Results update as you type.

Checking the factor yourself

The converter above uses the exact defined factors, but it is worth knowing which ones are exact. An inch is defined as exactly 2.54 cm and a pound as exactly 0.45359237 kg, so those conversions carry no rounding error at all. Going the other way — metres to feet, kilograms to pounds — gives a number that repeats forever and has to be cut off somewhere.

Temperature is the exception to everything on this page: it converts with a formula rather than a factor, because Celsius and Fahrenheit do not share a zero point. °F = °C × 9/5 + 32. Multiplying by 1.8 alone is the most common mistake, and it is off by 32 degrees everywhere on the scale.

The conversion reference lists every factor this tool uses at full precision, marks which are exact by definition, and covers the traps that catch people out — US versus imperial gallons, avoirdupois versus troy ounces, and statute versus nautical miles.

Metric vs. US customary: a 60-second primer

Nearly every country uses the metric system (meters, kilograms, liters, Celsius). The United States is the major exception, using customary units (feet, pounds, gallons, Fahrenheit) for daily life. The UK sits in between — metric officially, but miles on road signs and stone for body weight.

Handy anchors: a meter is a bit over 3 feet · a kilogram is about 2.2 pounds · a liter is about a quarter of a US gallon · 20°C is a pleasant 68°F room.

Three conversions people get wrong most often

Ounces: there are two of them. Weight ounces (28.35 g) and fluid ounces (29.57 ml) share a name but measure different things — a recipe's "8 oz of flour" is weight, "8 fl oz of milk" is volume, and converting one with the other's factor is the most common kitchen-math mistake.

Temperature: it's the only conversion here that isn't a simple multiply. °F = °C × 9/5 + 32, and the offset trips people up — doubling 20°C does not double the "warmth." Quick sanity check: −40° is the same in both scales.

Miles per gallon: because a UK gallon is 20% bigger than a US gallon, a car rated 40 mpg in the UK is only about 33 mpg in US terms. Fuel-economy comparisons across the Atlantic need the gallon spelled out.

Worked example

Converting a 5 km run to miles: 5 × 0.621371 = 3.11 miles. Going back, 3.11 × 1.60934 ≈ 5.005 km — the tiny drift is rounding, which is why this tool always converts from your original number rather than chaining conversions.

Every factor here is exact, and two of them still have a catch

None of the numbers this converter uses are rounded approximations. An inch is defined as exactly 0.0254 metres and a pound as exactly 0.45359237 kilograms — not measured, defined, by the international yard and pound agreement of 1959 that the US, UK, Canada, Australia, New Zealand and South Africa all signed. Everything else on the page follows from those two: a mile is 5,280 feet, an acre is 43,560 square feet, a knot is exactly 1,852 metres per hour. The converter prints the factor it used with every answer so you can check it against any other source.

Exactness is not the same as being the unit you meant, and there are two places where it is easy to convert the wrong thing correctly:

• US and Imperial volumes share names and differ by about 20%. A US pint is 473.18 mL; an Imperial pint is 568.26 mL. Same word, same recipe book, different quantity of milk. This converter offers US measures only, and says so on screen whenever a US volume unit is selected.
• The foot had two legal definitions in the US until the end of 2022. Land surveying used the "survey foot", longer than the international foot by two parts per million. Across a room that is invisible; across a 300 km state plane coordinate system it is about 0.6 m, which is enough to move a property line. This converter uses the international definition, which is the right one for everything except old survey data.

Sources

• NIST — U.S. Survey Foot — the official notice retiring the survey foot at the end of 2022, and the definition of both feet. This is the source for the two-parts-per-million figure above.
• NIST Office of Weights and Measures — Unit conversion — the published factors, against which the table in this page's source can be checked line by line.