Oz vs fl oz: the four ounces
An ounce is a weight, unless it is a volume, unless it is gold. Here is how to tell which.
The four ounces
| Ounce | Exactly | Used for |
|---|---|---|
| Ounce (oz) | 28.349523125 g | Food, mail, kitchen scales, the default meaning of a bare “oz”. |
| US fluid ounce (US fl oz) | 29.5735295625 mL | US labels, bottles, cans. 16 of them make a US pint. |
| UK fluid ounce (UK fl oz) | 28.4130625 mL | UK recipes, older imperial labels. 20 of them make a UK pint. |
| Troy ounce (oz t) | 31.1034768 g | Gold, silver, platinum. About 10% heavier than the ounce on your kitchen scale. |
Why they share a name
All four come from the Latin uncia, a twelfth part. The Romans split their pound into twelve unciae, and the word stuck to anything that was a small fraction of a larger unit. Weight and volume ounces then drifted apart. The imperial fluid ounce was first the volume of one ounce of water, so it sits close to the weight ounce. The US kept the older wine-gallon measure, so the US fluid ounce is about 4% bigger. The troy ounce followed the precious-metal trade and kept its own weight.
Common mistakes
- Weighing a liquid in fluid ounces, or pouring a weight. A cup of flour is not 8 oz. It is 8 fl oz of space and about 4.4 oz of flour.
- Mixing up US and UK pints. A US pint is 473 mL. A UK pint is 568 mL, because it holds 20 ounces instead of 16.
- Using the kitchen ounce for gold. A troy ounce is 31.10 g, about 9.7% more than 28.35 g.
- Assuming 1 fl oz is 30 mL. It is 29.57 mL. Nutrition labels round it. Your measuring cup does not.
- Reading a perfume's oz as weight. 1.7 oz on a perfume bottle is 1.7 fl oz, about 50 mL.
Three questions to ask when you see "oz"
- Is it a liquid, or would you put it on a scale? Liquids that pour are usually fluid ounces. Things you weigh are weight ounces.
- Where is the label from? US and UK fluid ounces differ by about 4%, and cups and pints differ by more.
- Is it a precious metal? Then it is troy ounces, even if the label just says oz.
Questions
Is oz the same as fl oz?
No. Oz is a unit of weight, 28.35 g. Fl oz is a unit of volume, 29.57 mL in the US and 28.41 mL in the UK. They are close for water and unrelated for everything else.
How many ounces are there?
Four in everyday use: the avoirdupois ounce (weight), the US fluid ounce, the UK fluid ounce and the troy ounce.
Why do they share a name?
All four descend from the Latin uncia, a twelfth part. The imperial fluid ounce was originally the volume of one ounce of water by weight, which is why the two are so close in the UK system.