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The troy ounce is a troy unit of mass.

\(\ds \) \(\) \(\ds 1\) troy ounce
\(\ds \) \(=\) \(\ds 20\) pennyweight
\(\ds \) \(=\) \(\ds 480\) grains
\(\ds \) \(=\) \(\ds 1\) apothecaries' ounce
\(\ds \) \(\approx\) \(\ds 31 \cdotp 1\) grams

Historical Note

The troy ounce is still in legal use for transacting in precious metals such as gold.

It is seen that a troy ounce is the same size as the apothecaries' ounce, while weighing approximately $1 \cdotp 1$ ounces avoirdupois.