Frequently asked questions

Unit conversion, explained

The questions people ask before they reach for a converter — what conversion actually is, the formula underneath it, and the ones that have no answer until you say which unit you mean.

What is a unit converter?

A unit converter is a tool that restates a measurement in a different unit without changing the quantity itself. 12 inches and 1 foot are the same length written two ways; the converter does the arithmetic that moves between the two. This one differs from most in that it does not ask you to pick a destination — type a value into a category and every unit in that category fills in at once.

What is unit conversion?

Unit conversion is expressing one quantity in terms of a different unit of the same kind. It is a change of description, not a change of amount: 5 kilometres and 3.106856 miles describe the identical distance. Conversion is only meaningful within a category — length converts to length and mass converts to mass, but there is no conversion from a metre to a kilogram.

How do you calculate conversion units?

Multiply by a conversion factor, which is a ratio between two units whose value is 1. Because 1 mile = 1609.344 metres, the ratio 1609.344 m / 1 mile equals 1, and multiplying by it changes the units without changing the quantity. Arrange the ratio so the unit you are leaving cancels: 5 miles × (1609.344 m / 1 mile) = 8046.72 m. This site does the same thing in two steps — it converts your value into the SI base unit for the category, then out of that base unit into every other unit at once.

What is the formula for conversion?

For any unit that scales from zero, the formula is: value in base unit = value × factor, and then value in target unit = value in base unit ÷ target factor. Combined, that is target = value × (source factor ÷ target factor). Temperature needs one extra term because its scales do not share a zero point, so each unit carries an offset as well: base = value × factor + offset, and the reverse is (base − offset) ÷ factor. That is why 0 °C is 273.15 K rather than 0 K, and why converting temperature with a single multiplier is quietly wrong.

How to do conversion step by step?

Step one: identify the category — length, mass, volume, temperature and so on. Step two: write down the unit you have and the unit you want. Step three: find the factor that relates them, ideally through the SI base unit, so 1 inch = 0.0254 m and 1 foot = 0.3048 m. Step four: multiply by the factor for the unit you have, then divide by the factor for the unit you want, and check that the answer moved in the direction you expected — a larger unit always gives a smaller number. Converting 30 inches to feet: 30 × 0.0254 = 0.762 m, then 0.762 ÷ 0.3048 = 2.5 ft.

What is an example of a unit conversion?

Converting 5 kilometres to miles. A kilometre is 1000 metres and a mile is exactly 1609.344 metres, so 5 km = 5000 m, and 5000 ÷ 1609.344 = 3.106856 miles. Everyday examples run the same way: 1 inch = 25.4 mm exactly, 1 pound = 0.45359237 kg exactly, 1 US gallon = 3.785411784 litres exactly, and 100 °C = 212 °F.

Why do we convert units?

Because measurements have to be comparable before they can be used. Different countries, trades and instruments record the same quantity in different units — a recipe in cups, a spec sheet in millimetres, a scale in stones — and comparing or combining them requires putting them on one footing first. It also matters more than it sounds: the Mars Climate Orbiter was lost in 1999 because one team supplied thrust figures in pound-force seconds while the software expected newton-seconds.

How do you convert SI units?

Within the metric system, conversion is a shift of the decimal point, because SI prefixes are powers of ten: kilo is 10³, centi is 10⁻², milli is 10⁻³, micro is 10⁻⁶. So 1 km = 1000 m, 1 m = 100 cm, and 1 g = 1000 mg. Going from a smaller unit to a larger one you divide; the other way you multiply. Converting between SI and non-SI units — metres to feet, kilograms to pounds — needs a defined factor instead, and every one of those on this site is written as its published definition rather than as a copied decimal.

How to understand SI units?

SI is built from seven base units — the metre for length, kilogram for mass, second for time, ampere for electric current, kelvin for temperature, mole for amount of substance and candela for luminous intensity. Every other SI unit is derived from those by multiplication or division: a newton is a kg·m/s², a pascal is a newton per square metre, a joule is a newton-metre. Prefixes then scale any of them by powers of ten. Once you know the base unit for a category, the rest of the category is that base unit with a number attached — which is exactly how this site stores every unit it converts.

What is 200 cm in SI units?

2 metres. The SI base unit for length is the metre, and ‘centi’ means one hundredth, so 200 cm = 200 × 0.01 m = 2 m. In scientific notation that is 2 × 10⁰ m. The centimetre is itself a perfectly valid SI unit — it is the metre with a prefix — so 200 cm is already an SI measurement; expressing it ‘in SI units’ normally means restating it in the unprefixed base unit.

What is 1 unit in inches?

There is no answer, because ‘unit’ on its own is not a length. A unit is whichever measure has been named — an inch is a unit, a metre is a unit, a litre is a unit — so ‘1 unit’ has a size only once you say what unit it is. If you meant the common metric-to-imperial lookups: 1 cm = 0.393701 inches, 1 mm = 0.0393701 inches, 1 metre = 39.3701 inches, and 1 foot = 12 inches exactly. If you meant a billed unit of electricity, that is one kilowatt-hour, which is energy and has no length at all.

Is 1 unit 1 cm?

No — not unless something has explicitly defined it that way. In CAD drawings, game engines and graph paper, ‘1 unit’ means whatever the drawing‘s scale declares, and it is often a metre rather than a centimetre. In electricity billing, 1 unit is 1 kilowatt-hour. Outside of a context that defines it, ’1 unit' has no size, which is why every value on this site is stored against a named SI base unit instead.

How to measure 1 unit?

Decide which unit you mean first, then use the instrument that reads it: a ruler or tape for a metre or an inch, a scale for a kilogram or a pound, a measuring jug for a litre, a thermometer for a degree, a meter for a kilowatt-hour. There is no instrument that measures ‘units’ in general, because the unit is the thing you choose before measuring, not the thing you discover afterwards.

How do I calculate units?

For electricity, a unit is one kilowatt-hour, and you calculate it as power in kilowatts multiplied by hours of use: a 2 kW heater run for three hours uses 2 × 3 = 6 units. If the appliance is rated in watts, divide by 1000 first — a 500 W device for four hours is (500 ÷ 1000) × 4 = 2 units. For any other kind of unit, calculating means converting: take the quantity you measured, multiply by the factor for the unit you measured in, and divide by the factor for the unit you want.

What is the unit of 1 foot?

The foot is itself the unit — it is a unit of length. One foot is defined as exactly 0.3048 metres, which is also exactly 12 inches, 30.48 centimetres, 304.8 millimetres and one third of a yard. That definition has been exact since the 1959 international yard and pound agreement, so foot-to-metre conversions are not approximations.

Can I use my phone as a unit converter?

Yes. This site is a website rather than an app, so it runs in your phone's browser with nothing to install, no account and no advertising. Add it to your home screen and it opens like an app. Because every page is a complete conversion table before any script runs, and pages you have opened are cached, it keeps working on a phone with no signal — on a plane, in a basement, or abroad without data.