How a unit converter turns one measurement into another
Almost every unit conversion is a single multiplication. Each category has a base unit — the metre for length, the gram for weight, the litre for volume — and every other unit is defined as a fixed multiple of it.
A kilometre is exactly 1,000 metres and a mile is exactly 1,609.344 metres, so converting kilometres to miles just divides one ratio by the other: 1 km = 0.621 mi.
The same trick powers kilograms to pounds (1 kg = 2.205 lb) and litres to gallons (1 L = 0.264 US gal).
Because the factors are defined constants rather than measured estimates, the converter multiplies your number through the base unit and returns the answer directly.
Temperature is the one exception: Celsius, Fahrenheit and Kelvin start their scales at different zero points, so a plain ratio will not work. That case needs a formula, which the next section covers.
Metric vs imperial, and why temperature needs a formula
Most of the world measures in metric — metres, kilograms, litres, degrees Celsius — while the US and, for some everyday uses, the UK still lean on imperial units like feet, pounds, gallons and Fahrenheit.
Converting between the two systems is straightforward for length, weight and volume because each unit is a fixed slice of a shared base: 1 inch is exactly 2.54 cm, 1 pound is 453.592 grams, 1 US gallon is 3.78541 litres.
Temperature is different. Water freezes at 0°C but 32°F, and the degrees themselves are sized differently, so you cannot just multiply by a factor.
Celsius to Fahrenheit uses °F = °C × 9/5 + 32, which applies both a scale factor and an offset. Kelvin adds only an offset from Celsius: K = °C + 273.15.
This is why 100°C, boiling water, comes out as 212°F rather than a tidy round number.
Exact factors, precise results — but round for real life
Unit conversion is not an estimate.
Unlike a tax or mortgage calculator, which models rates and assumptions that shift, a converter uses standard, defined constants agreed internationally: the metre, kilogram and second are fixed by definition, and units like the inch and pound are set to exact values by the 1959 international agreement.
That means kilometres to miles, kilograms to pounds and Celsius to Fahrenheit all return mathematically precise answers every time — there is no rounding baked into the factor itself. What you do with that precision is up to context.
In cooking, rounding 236.588 ml to a cup is fine; in science or engineering you may keep several decimals.
For travel, knowing that 100 km/h is about 62 mph or that a 5 km run is roughly 3.1 miles is plenty.
The converter shows full precision so you can round sensibly for the job at hand rather than losing accuracy before you start.