Differences Between an Ounce and a Troy Ounce

Measures of weight vary.

When weighing items, is an ounce or troy ounce used? "Webster's New World Finance and Investment Dictionary" says to use ounces to measure volume in pounds or cups. Use troy ounces to weigh precious metals like gold and silver.

Troy Ounce

The troy ounce supposedly got its name during the Middle Ages for an annual fair held in Troyes, France. A troy ounce weight is equal to about 1.1 avoirdupois ounces, 480 grains, or 31.04 grams.

Ounce

"Word Origins" states that ounce is derived from the Old French word, unce, and originally was used as a Troy weight. It's now part of the avoirdupois system and weighs one sixteenth of a pound.

Avoirdupois Weight

This method of weights and measures is based on a pound containing 16 ounces or 7,000 grains, and is equal to 453.59 grams.

Uses

The "American Heritage Science Dictionary" says avoirdupois weight is used in America to weigh everything except gems, precious metals, and drugs.

Writing Format

Measures for a troy ounce and an ounce go after the figures. The troy ounce format could be 25.2 troy oz. or 19.7 oz. avdp. The format for an ounce could be 16 oz.

References

• The American Heritage Science Dictionary, Houghton Mifflin Company, 2005
• Webster's New World Finance and Investment Dictionary, Wiley Publishing, 2003
• Word Origins, John Ayto, 2006