Unit Converter

This unit converter handles seven families of measurement: length, weight, area, volume, speed, digital data and temperature. Pick the family, choose the unit you are converting from and the unit you want, type a value, and the tool returns the converted figure along with a table showing the same quantity expressed in every other unit of that family. It covers metric and imperial members of each family, so length includes kilometres, metres, centimetres, millimetres, miles, yards, feet, inches and nautical miles, while weight spans kilograms through to stones and tons. The comparison table is the part people tend to use most, because it turns a single lookup into a full picture of scale. It is built for anyone reading a recipe, a spec sheet, a shipping quote or a map produced in the other measurement system.

Every family except temperature is stored as a table of multipliers against one base unit. Length uses the metre, weight the kilogram, area the square metre, volume the litre, speed metres per second, and digital data the byte. Each unit in the family carries the number of base units it represents: a mile is 1609.344 metres, a stone is 6.35029 kilograms, an acre is 4046.86 square metres, a US gallon is 3.78541 litres, a knot is 0.514444 metres per second, and a kilobyte is 1024 bytes.

A conversion is then two steps. The value you type is multiplied by the factor for the source unit, which expresses it in base units. That base figure is divided by the factor for the target unit, which expresses it in the unit you asked for. The same base figure is reused to fill in every other row of the comparison table, so the whole table costs one multiplication and one division per row.

Temperature is handled separately, because Celsius, Fahrenheit and Kelvin are offset scales rather than simple multiples of one another, so a single factor cannot describe them.

Start with length. You enter 5 and choose miles as the source and kilometres as the target. The converter multiplies 5 by the mile factor of 1609.344, giving 8046.72 metres. It then divides by the kilometre factor of 1000, returning 8.04672 kilometres. The same 8046.72 metre figure produces the other rows: divided by 0.9144 it gives 8800 yards, and divided by 0.3048 it gives 26,400 feet, which are the exact whole numbers you would expect, since a mile is defined as 1760 yards.

Now weight. You enter 12 with stones as the source and kilograms as the target. Twelve multiplied by 6.35029 gives 76.20348 kilograms. Dividing that base value by the pound factor of 0.453592 returns 168 pounds, which matches the definition of a stone as fourteen pounds, since 12 times 14 is 168.

Data behaves the same way with binary factors. Entering 2 gigabytes multiplies by 1,073,741,824 to give 2,147,483,648 bytes, which divided by 1,048,576 comes back as 2048 megabytes.

The commonest source of a wrong answer is picking a plausible but different unit. Gallons, quarts, pints and fluid ounces in this tool use US definitions, so a recipe written with UK imperial pints will be out by roughly a fifth if you convert it as a US pint. Tons here are metric tons of 1000 kilograms, not US short tons or UK long tons. Nautical miles at 1852 metres are distinct from statute miles at 1609.344 metres, and using the wrong one on a marine chart compounds quickly over distance.

Digital data uses binary multipliers, where a kilobyte is 1024 bytes. Storage manufacturers usually quote decimal units, where a kilobyte is 1000 bytes, which is why a drive sold as 1 TB shows as roughly 931 GB in a file manager. If you are reconciling a marketing figure with an operating system figure, that difference explains the gap.

Finally, watch rounding. The tool converts from the full precision base value, so chaining several conversions by hand and rounding at each step will drift away from the figure shown here.