Ounces Converter

An ounce is exactly 28.349523125 grams and one sixteenth of a pound, so sixteen ounces make one pound. That definition makes it the avoirdupois ounce, the unit used for ordinary trade.

Ounces appear on postal and courier weight bands, on food packaging and recipe cards, and in the jewellery trade, though precious metals are quoted in the troy ounce.

The gram and kilogram factors are exact decimal relations, and the imperial factors are exact by definition. Display rounding can still change the last digit on a round trip.

The usual error is confusing the troy ounce with the avoirdupois ounce: the troy ounce is 31.1035 grams, while the avoirdupois ounce is 28.349523125 grams. Ounces suit small parcels and portions, not bulk freight.

Ounces are most often confused with grams in recipes and postal weights, because the factor is 28.349523125 rather than a simple power of ten. Precious metals are quoted in the troy ounce, which is not the same as this ounce.

Between the metric units a weight conversion is a power of a thousand, so grams to kilograms and kilograms to tonnes are decimal shifts. The imperial units are all defined against the kilogram, which makes every metric-to-imperial pair a multiplication with the same factor at any size.

The dimension where this goes wrong is the ounce, because the same word covers two different weights. An avoirdupois ounce is one sixteenth of a pound and is the one used for food and parcels; a troy ounce is about 10% heavier (31.1035 g) and is used for precious metals. A price per ounce is meaningless until you know which ounce is meant.

To check a weight conversion, convert the result back, or go through the pound or the kilogram as a middle step. The commonest error is converting a figure stated in pounds into kilograms and then comparing it with a figure that was already in kilograms, which doubles or halves nothing and merely hides the mistake.