Unit Converter

How it works

Each category uses a common base unit. For length the base is the meter, for mass it is the gram, and for volume it is the liter. The source value is first multiplied by its factor to reach that base, then divided by the destination factor. Converting 2.5 meters to feet therefore means converting meters to the base meter value and then applying the exact meter-to-foot relationship.

Temperature needs a slightly different path because its scales have offsets as well as different step sizes. Celsius is the internal baseline. Fahrenheit values are shifted by 32 and scaled by 5 divided by 9 on the way in, while Kelvin values are shifted by 273.15. The reverse operation applies the destination scale's offset. This produces sensible results for negative temperatures and for values below the freezing point of water.

Precision and rounding

The displayed number keeps up to ten fractional digits, but that does not mean the original measurement is accurate to ten digits. Conversion can preserve a defined relationship without creating measurement precision that was not present in the input. Treat the result as an unrounded intermediate, then round it for the tool or document that will use it. Copy conversion includes both units so a number without context is less likely to be pasted into the wrong field.

Choosing the right unit system

Length conversions are useful for dimensions, travel distances, and design files. Mass conversions cover cooking, shipping, and laboratory quantities. Volume choices are explicitly US customary for the non-metric options, which matters for recipes and packaging. Temperature conversion is useful when comparing weather, equipment specifications, or experiments recorded on different scales. If a standard, label, or regulation specifies a particular unit system, use that named system rather than assuming two similarly named units are interchangeable.

The same name is not the same unit

The most expensive conversion errors are not arithmetic mistakes. They are two units that share a name and differ in size.

A US gallon is 3.785 liters; an imperial gallon is 4.546. That is a twenty percent gap on a word neither party thinks to qualify, and it is why a recipe or a fuel figure copied between the two countries can be badly wrong while looking entirely reasonable. Pints and fluid ounces diverge the same way, in opposite directions — an imperial pint is larger, an imperial fluid ounce is smaller.

Mass has its own version. A ton can mean 1,000 kg (metric tonne), 2,000 lb (US short ton) or 2,240 lb (UK long ton). Shipping, construction and commodity prices all use the word without always saying which.

Even the units that sound scientific carry ambiguity. A calorie on a food label is a kilocalorie, a thousand times the calorie in a physics textbook. A horsepower differs between the mechanical and metric definitions.

The volume options here are US customary, stated explicitly for that reason. If a figure came from a British source, a shipping document or a standard, find out which definition it used before converting — the conversion cannot tell, and it will produce a confident number either way.

Round at the end, and keep the precision you were given

A converted value carries the accuracy of its input, and conversion cannot improve it.

"About 30 miles" converted to 48.28032 kilometers reports seven significant figures for a number that had one. The precision is arithmetic, not knowledge, and writing it down invites the next person to treat an estimate as a measurement. Round the result to roughly the precision of the source: 48 km, or 50 if the original really was a guess.

The opposite error matters in chains. Rounding at every step compounds — convert inches to centimeters, round, convert to meters, round again, and the drift can exceed the tolerance you were working to. Carry the full precision through the calculation and round once, when you write the answer.

Temperature deserves its own note, because it is the only conversion here with an offset rather than a simple factor. A difference of 10 °C is 18 °F, not 50 °F — the offset applies to points on the scale, not to intervals between them. Converting a temperature change as though it were a temperature is a common and quiet mistake in specifications and control systems.