How the conversion works
Every unit in a category stores one number: its size relative to that category's base unit — the metre for length, the square metre for area, the litre for volume, the kilogram for weight, the second for time. Converting is then a single operation: multiply by the source factor to reach the base, divide by the target factor to leave it. Going through a base unit rather than storing every pair means the converter never accumulates rounding error across chained conversions, and adding a new unit takes one number instead of dozens.
Temperature is the exception, and the reason is worth understanding. The other categories are ratio scales: zero means none of the quantity, so a single multiplier converts correctly. Celsius and Fahrenheit are interval scales — their zeros are arbitrary points, not the absence of heat — so conversion needs a multiplication and an offset. This is also why 20 °C is not "twice as warm" as 10 °C, and why a temperature difference of 10 °C is 18 °F rather than 50 °F.
The conversions that trip people up
- US vs UK gallon. 3.785 L against 4.546 L — a 20% difference. American recipes, fuel economy figures and UK pub measures do not share a gallon.
- Ounces: weight or volume? A fluid ounce measures volume; an ounce measures mass. They are only interchangeable for water, and only approximately. This converter keeps them in separate categories on purpose.
- Ton, tonne, short ton. The metric tonne is 1,000 kg. The US short ton is 907 kg. The UK long ton is 1,016 kg. Cargo and emissions figures rarely say which.
- Square units are not linear. A square metre is not 3.28 square feet but 10.76, because the conversion factor is squared. The same applies to cubic units.
Metric and imperial, briefly
The metric system is decimal by construction: prefixes shift the decimal point, so converting inside it is moving a comma. Imperial units grew from human-scale references — a foot, a hand, a grain of barley — and their relationships are irregular: 12 inches to a foot, 3 feet to a yard, 1,760 yards to a mile. That irregularity is why unit conversion is a tool and not mental arithmetic.
Since 1959, imperial units have been legally defined in terms of metric ones. An inch is exactly 25.4 mm by international agreement, not by measurement. That is why the factors here are exact rather than approximate.
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Frequently asked questions
Why is temperature handled differently from other units?
Because temperature scales have offset zero points. Length is proportional — 0 m is 0 ft, and doubling one doubles the other — so a single multiplier works. But 0 °C is 32 °F, so converting requires both a multiplication and an addition. That is also why you can convert a temperature reading but not a temperature difference with the same formula.
Which gallon does the volume converter use?
Both are listed separately. The US liquid gallon is 3.785 litres; the UK imperial gallon is 4.546 litres, about 20% larger. Recipes and fuel figures from the US and the UK are not interchangeable, and this is the single most common source of error in volume conversion.
How precise are the results?
The factors are the exact internationally defined values — an inch is exactly 0.0254 m, a pound exactly 0.45359237 kg — so the only error is floating-point rounding, well below any practical measurement. Results are shown to eight decimal places and switch to scientific notation at the extremes.
Why is a month 30 days?
Because a calendar month has no fixed length. We use 30 days for months and 365 days for years so the conversion is deterministic. For anything where the exact calendar matters — contracts, interest, deadlines — count actual dates instead.