Weight converter

Convert to kilograms, then to the target unit. Twelve ounces to grams: 12 × 0.028349523125 kg = 0.34019 kg, which is 340.19 g. Because the pound and the ounce are both defined from the kilogram, the result is exact before rounding — there is no measured factor involved.

Not in physics. Mass is the amount of matter and is measured in kilograms; weight is the force gravity exerts on that mass and is measured in newtons. Everyday language, and every kitchen scale, uses “weight” for mass. The difference shows up in a spring scale: gravity is about 0.5% stronger at the poles than at the equator, so the same object reads slightly heavier there. A balance, which compares one mass against another, reads the same everywhere.

A constant of nature. From 1889 to 2019 the kilogram was the mass of one platinum-iridium cylinder kept at the BIPM near Paris. Comparisons showed it drifting by tens of micrograms against its official copies, and since a standard cannot drift, the General Conference on Weights and Measures redefined the kilogram by fixing the Planck constant at exactly 6.62607015 × 10⁻³⁴ J·s. The change took effect on 20 May 2019.

The ounce on this site is the avoirdupois ounce of everyday weighing. Gold and silver are priced in troy ounces of 31.1034768 g, about 10% heavier, and a fluid ounce is not a weight at all but a volume. Tons split the same way: the metric tonne is 1,000 kg, the US short ton 2,000 lb (907.18 kg) and the British long ton 2,240 lb (1,016.05 kg).

The cost of mixing units is not hypothetical. In 1983 Air Canada Flight 143 ran out of fuel halfway through its flight after the fuel load was calculated with a pounds-per-liter factor instead of kilograms-per-liter, leaving it with about half the fuel it needed; the crew glided the Boeing 767 to a landing at Gimli, Manitoba. The most used conversions here are pounds to kg and ounces to grams.