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About Unit Conversions
Convert length, mass and volume units.
Metric conversion factors for length and volume are exact by definition, while some customary figures are approximations fixed by agreement. Temperature is the exception, since the Fahrenheit and Celsius scales have different zero points and conversion needs both a factor and an offset.
The United States keeps customary units in daily life — inches, feet, pounds, gallons, Fahrenheit — while science, medicine and most of the world work in metric, so conversions run between the two systems. A US gallon is 3.785 litres, smaller than the imperial gallon, and a US ton is 2,000 pounds against the metric tonne's 2,204.6.
Unit conversion multiplies by ratios that equal one, so the value is preserved while the unit changes. Exact factors exist by international agreement for metric units, while some customary figures are approximations fixed by statute.
Rounding between systems compounds: convert once at the end of a chain rather than at every step, and the result stays consistent.
Conversion factors for metric length, mass and volume are exact by international agreement, but customary units carry historical variation: a US fluid ounce is larger than the imperial one, and the two gallons differ by a fifth. Rounding belongs at the end of a chain, not in the middle.
Conversion Calculator sits with the rest of the library's everyday work, and it rests on the same measurement arithmetic the way a manufacturer's label or a timetable does. Its answer is only as good as the assumptions standing behind it.
The quantities it works from — value, from unit and to unit — are the ones a real measurement question actually turns on, and each carries a convention of its own, set out below.
Everyday arithmetic in the United States carries conventions that are easy to miss: dates run month before day, distances and weights follow customary units, and prices are quoted before tax. Those conventions make a surprising share of daily sums come out differently from the same question asked elsewhere.
The figures in this section follow American practice unless the subject itself is international.