Unit converter

Every unit is held as an exact fraction of the family's base unit, and the arithmetic is done in whole numbers with BigInt rather than in floating point. That matters because the definitions themselves are exact: an inch is 25.4 mm by definition, a pound is 0.45359237 kg, a US gallon is 3.785411784 litres, an acre is 4046.8564224 m². So 5 miles is exactly 8.04672 km, not 8.046719999999999 — the page prints a result as exact when it ends, and marks it ≈ when it genuinely does not, rounded to twelve significant figures. A value that runs very large or very small is written in scientific notation rather than a screen of zeros.

Temperature is the odd family: its scales have different zero points, so a reading is converted with both a factor and an offset — °C to K adds 273.15, °F is five ninths of a degree with the offset that puts 32 °F at freezing. That also means a temperature difference does not convert the way a temperature does: 10 °C warmer is 18 °F warmer, not 50. Rankine is included because it is the absolute scale that matches Fahrenheit's degree size.

Two families carry a trap that is worth naming. Data sizes come in both the decimal units a drive maker prints (kB = 1000 B) and the binary ones an operating system counts in (KiB = 1024 B); both are here and the page never silently mixes them, which is the whole reason a "500 GB" disk shows as 466 GiB. The year in the time family is 365 days exactly — a plain multiplier, not a calendar, and no use for counting real dates, which is what the date difference and age pages are for. US fluid measures are US customary throughout, with the UK gallon and pint listed separately, since they differ by about a fifth.

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