Unit Converter

Everything except temperature is a ratio

Units of length, mass, volume, area, speed and data all measure a quantity along a scale that starts at zero. That shared zero is what makes conversion a plain ratio: one inch is 0.0254 metres, so any number of inches multiplied by 0.0254 gives metres, and dividing reverses it. There is no offset to remember, which is why a conversion factor is enough.

The factors themselves are not arbitrary. Since 1959 the international yard and pound have been defined by agreement as exact multiples of metric units: one inch is exactly 25.4 millimetres and one pound is exactly 0.45359237 kilograms. Conversions between imperial units are therefore exact rather than approximate, and any rounding you see in a result comes from the display, not the definition.

Temperature is different: it has offsets

Celsius and Fahrenheit do not share a zero point — 0 °C is the freezing point of water while 0 °F is a much colder reference — so conversion needs both a scale factor and an offset: F = C × 1.8 + 32. Kelvin, by contrast, shares the Celsius scale but starts at absolute zero, so it needs only an offset of 273.15 and no multiplication at all. This is why the temperature mode here uses formulas while every other category uses a single factor.

US and imperial units are not the same

A frequent source of real-world error is that "gallon", "pint" and "fluid ounce" mean different things on either side of the Atlantic. A US liquid gallon is 3.785411784 litres; an imperial gallon is 4.54609 litres, roughly 20 percent larger. A British pint is about 568 millilitres against the US pint's 473. Recipes, fuel economy figures and beverage sizes all depend on which one is meant, so this converter labels the system explicitly instead of quietly assuming.

Precision: do not invent digits

A calculator will happily return ten decimal places, but the honest answer depends on the input. If you measured something as 2.5 metres, the measurement itself carries two significant figures, so quoting 8.20 feet is defensible while 8.2020997 feet is not — it claims precision that was never there. The practical rule is to keep roughly the same number of significant figures as the value you started with, and to round only at the end rather than at each step.

Decimal and binary data units

Digital storage mixes two systems. Drive makers quote decimal SI units, where 1 kB is 1000 bytes and 1 TB is one trillion bytes; operating systems traditionally use binary units, where 1 KiB is 1024 bytes. Because 1024³ is about 7 percent larger than 1000³, the same physical drive appears smaller in a file manager than on its packaging. The data category here includes both spellings so you can match whichever figure you were given.

Cooking and baking

Recipes cross borders more often than they used to, and a US cup is 236.6 millilitres while a metric cup is 250. Weighing in grams removes the ambiguity entirely, which is why professional recipes increasingly list mass rather than volume.

Travel, property and fitness

Road signs in kilometres, property listed in square metres, gym plates in pounds and body weight in stones are all the same problem in different clothing. Convert the figure you were given rather than the one you prefer, then sanity-check it against something familiar: 100 kilometres is a bit over 60 miles, and 80 kilograms is about 176 pounds.

Technical and engineering checks

Specifications arrive in whichever system the author used. Converting both sides of a comparison into the same units before comparing prevents the classic mistake of treating a 0.5 inch clearance as smaller than a 12 millimetre one — they are, in fact, nearly identical.