How Unit Conversion Actually Works
Most conversions are one multiplication. A conversion factor is simply the ratio between two units, and applying it is a single multiply: Days = Weeks × 7, Bytes = Bits ÷ 8, Milliliters = Cups (US) × 236.588. Reversing a conversion means multiplying by the reciprocal, which is why our paired tools carry factors like 236.588 in one direction and 0.00422675 in the other. Those long decimals are not false precision — they are the reciprocal carried to enough digits that round-tripping a value does not visibly drift.
Some conversions are not multiplication at all. Temperature is the classic exception, because the scales do not share a zero point. You cannot convert Celsius to Fahrenheit with a factor alone; you need °F = (°C × 9/5) + 32, where the +32 accounts for the offset between the two scales' zeros. Kelvin to Celsius is pure subtraction: °C = K − 273.15. This is why 0 °C is 32 °F rather than 0 °F, and why "twice as hot" is meaningless in Celsius but well-defined in Kelvin.
Exact definitions versus historical approximations. Some conversions are exact by international agreement: one inch is defined as precisely 25.4 mm, so inch–millimetre conversion carries no rounding error at all. Others are exact but awkward — a US cup is defined as 236.588 mL, a number that looks arbitrary because it descends from fractions of a gallon rather than anything metric. And a few are genuinely approximate: our days-to-years converter uses 365 days, which ignores leap years. That is the right call for a rough conversion and the wrong tool for a legal deadline — for exact calendar arithmetic use the days between dates calculator, which counts real dates rather than applying an average.
Watch out for units that share a name. A US fluid ounce and a UK fluid ounce are different volumes. A US cup, a UK cup and a metric cup all differ. Our cooking converters state the US standard explicitly in the formula line for exactly this reason — an unlabelled "cups to ml" tool is a silent 5% error waiting to happen in a recipe.
Data units are the messiest of all. Our Data converters use the binary convention — Megabytes = Gigabytes × 1024 — because that is what operating systems report. Drive manufacturers use decimal (1 GB = 1000 MB), which is precisely why a "1 TB" drive shows up as roughly 931 GB in your file manager. Neither is wrong; they are different conventions, and the discrepancy compounds as units get larger.