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About Mass and Weight
Convert mass between kilograms, pounds and stone.
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.
Mass and weight are different quantities: mass is the amount of matter, weight is the force gravity exerts on it. A body keeps its mass on the Moon while weighing a sixth as much, which is why conversions between kilograms and pounds assume Earth gravity.
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.
Mass Weight Calculator sits with the rest of the library's health work, and it rests on the same measurement arithmetic the way a clinical reference table does. Its answer is only as good as the assumptions standing behind it.
The quantities it works from — mass — are the ones a real measurement question actually turns on, and each carries a convention of its own, set out below.
Health figures in the United States are read against reference tables published by federal agencies and professional bodies, and those tables describe populations rather than individuals. One measurement is a data point; a series taken the same way over months is information.
Clinical guidance also shifts as evidence accumulates, which is why a range that was standard a decade ago may no longer be quoted.