Two Different Quantities
A weight ounce (avoirdupois ounce) is about 28.35 grams and measures mass. A fluid ounce is about 29.57 milliliters and measures volume. They are close in number but measure completely different things.
Why Recipes Confuse Them
A recipe that says '8 oz of flour' might mean 8 ounces by weight (about 227 g) or 8 fluid ounces by volume (about 1 cup). For water these nearly coincide, but for flour, sugar, or oil they differ a lot. When precision matters — especially baking — weigh in grams instead.
Quick Reference
| Unit | Measures | Equivalent |
|---|---|---|
| 1 oz (weight) | mass | ≈ 28.35 g |
| 1 fl oz (volume) | volume | ≈ 29.57 mL |
| 1 fl oz of water | ≈ mass | ≈ 29.57 g |