Weight / Mass Converter
Converts between metric and imperial mass units. Two things in this area cause real errors rather than trivial ones: there are three different tons in common use, and the ounce means two different quantities depending on what you are weighing.
How to use it
- Enter a value and pick its unit.
- Pick the target unit.
- Read the converted value.
Mass and weight are not the same quantity
Mass is how much matter something contains and does not change with location. Weight is the force gravity exerts on that mass, so it varies with local gravity. The SI unit of mass is the kilogram; the SI unit of force is the newton.
In everyday use the distinction is ignored because everything happens in roughly the same gravitational field, and a kilogram of mass weighs a consistent amount anywhere on Earth to within a fraction of a percent. Even so, gravity varies by about 0.5 percent between the equator and the poles, and by altitude, which is why precision balances are calibrated locally.
The distinction becomes real in physics and engineering, where the pound is genuinely ambiguous: the pound-mass and the pound-force are different quantities that happen to have the same numeric value at standard gravity. Mixing them in a calculation is a classic error, and the reason the slug and the pound-force exist as separate units at all.
For this converter everything is treated as mass, which is what scales actually measure and what shipping, cooking, and commerce use.
Three tons, differing by ten percent
This is the conversion most likely to cause a genuinely expensive mistake, because the word is the same and the quantities are not.
- The short ton, used in the United States, is 2,000 pounds or about 907.18 kg.
- The long ton, historically British, is 2,240 pounds or about 1,016.05 kg.
- The metric tonne is 1,000 kg or about 2,204.62 pounds.
Troy and avoirdupois ounces
Precious metals are weighed in troy ounces, everything else in avoirdupois ounces, and they differ by about 10 percent in the opposite direction to what the naming suggests.
An avoirdupois ounce is about 28.35 grams. A troy ounce is about 31.10 grams — roughly 10 percent heavier. But a troy pound contains only 12 troy ounces against 16 avoirdupois ounces, so a troy pound is lighter than a standard pound at about 373 grams versus 454.
This matters directly to anyone pricing metals. A gold price quoted per ounce is per troy ounce, so converting it using 28.35 grams overstates the price per gram by about 10 percent. Silver, platinum, and palladium are quoted the same way. Jewellery weights, however, are frequently given in grams or in carats — where a carat is exactly 0.2 grams for gemstones, which is a different thing again from karat as a measure of gold purity.
This converter uses avoirdupois ounces and pounds, which is correct for shipping, food, and general commerce and wrong for bullion.
Cooking by weight
Weight is reproducible and volume is not, which is why baking recipes in metric are usually given in grams while American recipes are given in cups.
The variation in a cup of flour is the standard illustration: scooped and packed it can be 150 grams, spooned and levelled it is about 120, and sifted about 110. That is a 35 percent spread on the primary ingredient, which is enough to change the result substantially. Weighing removes the variable entirely.
A related trap is the fluid ounce, which measures volume rather than mass and is not interchangeable with the ounce despite the shared name. Eight fluid ounces of water weighs about 8.3 ounces, but 8 fluid ounces of honey weighs about 12 and 8 fluid ounces of flour about 4.4. Any recipe conversion that treats fluid ounces as ounces will be wrong by whatever the ingredient density happens to be. Note also that the United States fluid ounce and the imperial fluid ounce differ by about 4 percent, so British and American recipes do not share the unit either.
At a glance
| Pound | Exactly 0.45359237 kg |
|---|---|
| Ounce | Avoirdupois, about 28.35 g |
| Ton | Short ton, 2,000 lb or about 907.18 kg |
| Transmitted | Nothing |
Frequently asked questions
Which ton does this use?
The US short ton of 2,000 pounds, about 907.18 kg. A metric tonne is 1,000 kg and a long ton is 2,240 pounds, so the three differ by roughly 10 percent.
Is a troy ounce the same as an ounce?
No. A troy ounce is about 31.10 g against 28.35 g for the avoirdupois ounce used here. Precious metal prices are quoted per troy ounce.
What is the difference between mass and weight?
Mass is quantity of matter and does not change with location; weight is the gravitational force on it. This converter handles mass, which is what scales measure.
Can I convert fluid ounces to ounces?
Not without knowing the density. Eight fluid ounces of water weighs about 8.3 ounces, of honey about 12, and of flour about 4.4.
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Unit conversion done properly — Most conversions are exact multiplications. The ones that are not cause the expensive mistakes.