Ounces vs Fluid Ounces: Weight and Volume Are Not the Same
An ounce is 28.35 grams of weight. A US fluid ounce is 29.57 millilitres of space. They only line up for water, and even then not exactly.
29 June 2026 · 5 min read · Cooking
Recipes, nutrition labels and drink bottles all use "oz", but there are really three different ounces in everyday use, and only one of them is a weight.
The three ounces
| Unit | Measures | Exact value |
|---|---|---|
| Ounce (avoirdupois), oz | Weight | 28.349523125 g |
| US fluid ounce, fl oz | Volume | 29.5735295625 mL |
| Imperial fluid ounce, fl oz | Volume | 28.4130625 mL |
A fourth, the troy ounce (31.1034768 g), is used only for precious metals.
When can you swap them?
Only for liquids with about the same density as water. One US fluid ounce of water weighs about 29.6 g, or 1.04 oz, so the error is roughly 4%. The imperial fluid ounce was historically defined as the volume of one ounce of water, so for water the imperial units line up to within about 0.2%.
For anything else, weight and volume diverge fast:
| 1 US cup (8 fl oz) of… | Approximate weight |
|---|---|
| Water | 237 g (8.3 oz) |
| Milk | 245 g (8.6 oz) |
| Granulated sugar | 200 g (7.1 oz) |
| All-purpose flour | 120–125 g (4.2–4.4 oz) |
| Honey | 340 g (12 oz) |
A cup of flour weighs about half as many ounces as its fluid-ounce volume. Ingredient weights vary with brand, packing and humidity, so treat these as guides and weigh when accuracy matters.
Reading labels
- "Net wt 16 oz" is weight, 453.6 g.
- "16 fl oz" is volume, 473 mL in the US.
- A UK "pint" of milk is 20 imperial fl oz (568 mL), while a US pint is 16 US fl oz (473 mL).
Kitchen tips
- If a recipe lists flour, sugar or butter in ounces, it means weight. Use a scale.
- If it lists milk, water or stock in ounces, it almost always means fluid ounces.
- For baking, weighing in grams is the most repeatable method.
Related conversions
This article is for general information. Please verify figures against the relevant standard before using them in critical work. About accuracy