Converters calculators

Every converters calculator here follows the same rules: results update live as you type, the formula is shown with a worked example, and the math runs entirely in your browser — nothing you enter is uploaded anywhere. Each one is unit-tested against published reference values before it ships.

Exact definitions, not rounded factors

Most conversion errors are inherited from a rounded factor someone memorised. One inch is exactly 25.4 millimetres — that is a definition agreed internationally in 1959, not a measurement — and one pound is exactly 0.45359237 kilograms. Every converter here uses those exact definitions, which is why the results agree with a metrology table to whatever precision you ask for. Using 2.2 pounds to the kilogram instead of the exact 2.2046226 is fine for a bag of flour and wrong by nearly a kilogram on a 200 kg load.

Temperature is the exception to the pattern, because Celsius and Fahrenheit have different zero points as well as different step sizes. You cannot convert a temperature by multiplying alone, which is why converting a temperature difference is not the same operation as converting a temperature — a 10°C rise is an 18°F rise, not 50°F.

The gallon problem, and other units with two meanings

Some unit names mean different amounts in different places, and nothing on the label warns you. A US gallon is about 3.785 litres; an imperial gallon is about 4.546 litres, roughly 20% larger. A US fluid ounce and an imperial fluid ounce differ by about 4%. A US cup is 240 ml by nutrition-labelling convention, while Australian recipes use a 250 ml cup. Our converters state which variant they use on the page rather than leaving you to guess, because a recipe scaled with the wrong cup fails quietly and a fuel-economy figure converted with the wrong gallon is off by a fifth.

Stone is a British and Irish habit that maps awkwardly onto decimal weight: one stone is exactly 14 pounds, so 11 stone 4 is not 11.4 stone, and that misreading is the single most common conversion error in body-weight questions.

Weight, mass and the honest answer about kilograms

Strictly, kilograms measure mass and pounds-force measure weight, and the two are different physical quantities that happen to be proportional on the surface of the Earth. In every everyday context — luggage, groceries, body weight, freight — people mean mass, and the conversion between kilograms and the avoirdupois pound is exact and unambiguous. We convert on that basis and say so rather than pretending the distinction does not exist, because it does matter in physics and engineering, and nowhere else you are likely to be standing.

Reference charts, because you often want the neighbours

Each converter page carries a reference chart of common values alongside the live conversion. That is deliberate: people converting units are usually orienting themselves rather than seeking one number. Someone converting 180 cm to feet generally wants to know what 175 and 185 look like too. The charts are generated from the same exact factors as the calculator, so the table and the tool can never disagree — a failure mode that is surprisingly common on conversion sites where the chart was typed by hand.