Mass and Weight Converter: Troy Ounces, Short Tons and Long Tons
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Mass and Weight Converter
Converts between metric, imperial, troy and engineering mass units. Keeps short tons, long tons and tonnes apart, and includes the troy ounce used for assay and bullion.
The same value in every unit
Factors are exact. The pound has been exactly 0.45359237 kg since 1959, and the grain is exactly 64.79891 mg, which makes a pound exactly 7000 grains. A troy ounce is 480 grains, not 437.5 like the avoirdupois ounce, so the two differ by about 10 percent. The slug is the imperial unit of mass that makes F equals ma work in pounds force and feet.
How to use this tool
Enter a mass, pick the unit you have and the unit you want. The table shows all fifteen units at once. The groups matter here more than on most converters, because avoirdupois, troy and engineering units share names without sharing values.
The trap is the word ounce. An avoirdupois ounce is 28.35 grams, a troy ounce is 31.10 grams. They differ by about 10 percent, and gold, silver and assay results are always quoted in troy. If a number came from a laboratory or a bullion dealer, it is troy.
What the conversion is actually doing
Everything is stored as kilograms and converted by a multiply and a divide. All the imperial factors are exact, because the pound was fixed at exactly 0.45359237 kilograms by the international yard and pound agreement of 1959, and the grain was fixed at exactly 64.79891 milligrams.
Those two exact numbers make the whole system exact. A pound is exactly 7000 grains. A troy ounce is exactly 480 grains. An avoirdupois ounce is exactly 437.5 grains. The grain is the common ancestor, which is why it still turns up in assay reporting long after it vanished from everyday use.
Three different tons, and why it matters
A short ton is 2000 pounds, about 907 kilograms, and it is what North American plant and haulage quotes mean. A long ton is 2240 pounds, about 1016 kilograms, still used in some shipping and older British mining records. A tonne is exactly 1000 kilograms.
The spread between the smallest and largest is 12 percent. On a single truck that is a rounding annoyance. On an annual production figure or a bulk shipment it is a material difference, and because all three are written as “ton” in casual use, the error is easy to carry a long way before anyone notices.
A worked check you can repeat on paper
An assay returns 3.5 grams per tonne of gold. Convert to troy ounces per tonne: 3.5 divided by 31.1034768 gives 0.11253 troy ounces per tonne. That is the number the resource statement will use.
Now the check that proves the troy system hangs together. One troy ounce should be exactly 480 grains and exactly 20 pennyweight. Put 1 into the tool with troy ounce selected and both come back as whole numbers, not 479.98 or 20.001.
Mistakes that catch people out
Mixing troy and avoirdupois ounces. A 10 percent error, and both are just called ounces. Fix: assume troy for precious metals and assay, avoirdupois for everything else, and check if the source is ambiguous.
Assuming ton means tonne. Three units, 12 percent apart. Fix: find out whether the source is North American, British or metric before converting.
Confusing mass and weight. A kilogram is mass, a kilogram force is a force. This tool converts mass. Fix: if you need force, that is a different conversion.
Using the slug carelessly. A slug is the imperial mass unit that makes force equals mass times acceleration work with pounds force and feet per second squared. It is 32.174 pounds, not one. Fix: only reach for it inside a consistent imperial dynamics calculation.
How this was checked
Every factor was tested against its legal definition. The exact ones return zero error: the pound at 0.45359237 kg, the grain at 64.79891 mg, the carat at exactly 200 mg. The whole number identities all hold, so a pound returns exactly 16 ounces and exactly 7000 grains, a stone returns exactly 14 pounds, a short ton exactly 2000 pounds, a long ton exactly 2240, a troy pound exactly 12 troy ounces, and a troy ounce exactly 20 pennyweight.
All 225 unit pairs were round tripped with worst case drift at floating point level rather than factor level.
Related Calculators and Reference
If all you need is the everyday kilogram to pound conversion, the Kilograms to Pounds Converter is the faster route. It splits the result into pounds and ounces and into stones, and carries a full conversion chart.
For density rather than mass, the Rock Density and Specific Gravity Calculator covers minerals, and the API Gravity Converter handles petroleum. For grade and tonnage work the Cutoff Grade Calculator and the Grade Tonnage Curve Calculator use these units directly, and the Drillhole Assay Compositing Tool does the length weighted averaging. Definitions follow the NIST Office of Weights and Measures.