Unit Converter

Most conversion mistakes are not arithmetic errors. They come from using the wrong definition of a unit: a British gallon where a US one was meant, or a nutrition-label cup instead of a recipe cup. The converter above uses one clearly stated definition per unit, derived from exact constants, so the answer is right once you know which unit you actually have.

How it works

Choose a quantity (length, weight, temperature, volume, speed or area), then a unit to convert from and one to convert to. The result updates as you type, and the table underneath shows a handful of common values for the same pair, handy for a quick reference or a printout. The swap button reverses the direction without retyping anything. Everything runs in your browser.

Each unit is stored as its size in one base unit per quantity: meters, kilograms, liters, meters per second and square meters. Converting 5 feet to centimeters multiplies by the size of a foot in meters (0.3048) and divides by the size of a centimeter (0.01). Because every factor goes through the same base, any unit converts to any other in its family, not just the pairs someone thought to hard-code.

Temperature is the exception. Celsius, Fahrenheit and Kelvin have different zero points, so the tool converts through Celsius, applying both the scale and the offset. Entering a value below absolute zero (−273.15 °C, −459.67 °F or 0 K) shows a warning, because no physical temperature is colder.

Results are rounded to four decimal places for display. Very small results keep four significant figures instead, so a value like 0.0001235 does not collapse to zero.

Where the numbers come from

The factors are exact wherever an exact definition exists:

• Inch and pound. The 1959 international yard and pound agreement fixed the inch at exactly 25.4 mm and the pound at exactly 0.45359237 kg. The foot, yard, mile, ounce and stone all follow from those two numbers.
• US gallon. 231 cubic inches, which works out to exactly 3.785411784 liters. The US fluid ounce is 1/128 of that, the cup 8 fluid ounces, the tablespoon half a fluid ounce and the teaspoon a third of a tablespoon.
• Knot. One nautical mile per hour, where the nautical mile is exactly 1,852 meters.
• Acre and hectare. An acre is 43,560 square feet (about 4,046.86 m²). A hectare is 10,000 m², so one hectare is roughly 2.47 acres.

Traps worth knowing

Imperial is not US customary. The UK imperial gallon is about 4.546 liters, roughly 20% larger than the US gallon, and the imperial fluid ounce is slightly smaller than the US one. A British car's miles-per-gallon figure is not comparable with an American one. This tool uses US volumes throughout.

A "cup" depends on who is asking. The US customary cup here is about 236.6 ml. US nutrition labels round it to 240 ml, and recipes from Australia, New Zealand and Canada often use a 250 ml metric cup. For baking, weighing in grams avoids the question entirely.

Ounces measure two different things. A weight ounce (1/16 of a pound) and a fluid ounce (a volume) are unrelated units that happen to share a name. A US fluid ounce of water weighs about 1.04 ounces: close, but not the same. The converter keeps them in separate families.

Temperature differences are not temperatures. A 10 °C rise is an 18 °F rise, because a change carries no offset. Converting "10 °C" as a reading gives 50 °F, which is the wrong answer for a change. Multiply a difference by 1.8 instead.

Doing it in code

The same approach, a base-unit table plus a special case for temperature, fits in a few lines:

const TO_METERS = { mm: 0.001, cm: 0.01, m: 1, km: 1000, in: 0.0254, ft: 0.3048, mi: 1609.344 };

function convertLength(value, from, to) {
  return (value * TO_METERS[from]) / TO_METERS[to];
}

convertLength(26.2, "mi", "km"); // 42.1648…

Keep the factors as named constants rather than scattering 2.54 and 0.3048 through a codebase. Round only when displaying, never between steps. JavaScript numbers are 64-bit floats, so a chain of conversions can pick up tiny errors, as the note on 0.1 + 0.2 in JavaScript Fundamentals explains.

Related tools

The BMI calculator takes height in centimeters or in inches only, so convert a height in feet (or meters) here first. The ratio calculator scales a recipe or a drawing once its units match, and the percentage calculator shows how far apart two converted figures are.