Three Different Ounces
Definitions from NIST Handbook 44, Appendix C:
Default meaning
| Unit | Measures | Definition | Metric value |
|---|---|---|---|
| Ounce (avoirdupois), oz | mass | 1/16 of a pound | 28.349523125 g (exact) |
| Troy ounce, oz t | mass | 1/12 of a troy pound | 31.1034768 g (exact) |
| US fluid ounce, fl oz | volume | 1/128 US gallon | 29.57 mL |
| Imperial fluid ounce, fl oz | volume | 1/160 Imperial gallon | 28.41 mL |
Ounces vs Fluid Ounces
An ounce is a mass; a fluid ounce is a volume. They cannot be converted into each other without the density of the substance. For water, 1 US fl oz weighs roughly 29.5 g; for oil, honey or flour it is different. If a recipe says "8 fl oz", convert it as volume with the fluid ounces to millilitres converter; if it says "8 oz", convert it as mass. Cup and spoon measures are covered in the kitchen measurements guide.
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⚖️Convert oz → gTroy Ounces
Precious-metal prices are commonly quoted per troy ounce. A troy ounce (31.1034768 g) is about 9.7% heavier than an avoirdupois ounce, so using 28.35 g for a gold or silver weight understates it. Check whether a figure says "troy ounce" or "oz t" before converting.
Rounding in Practice
- 1 oz ≈ 28.35 g. Using 28 is about 1% low; using 30 is about 6% high.
- 4 oz ≈ 113 g; 8 oz ≈ 227 g; 16 oz (1 lb) ≈ 454 g.
- If a package prints both oz and g, use the gram figure rather than converting the rounded ounce figure.
- For recipes, round to what your scale displays; a 1 g scale cannot show 113.4 g.
Common Mistakes
- Converting fluid ounces as if they were mass ounces.
- Using the avoirdupois factor for precious metals.
- Treating US and Imperial fluid ounces as the same (they differ by about 4%).
- Reading pounds-and-ounces as decimals: 1 lb 8 oz is 1.5 lb, not 1.8 lb.