Ounces to grams: the kitchen conversions worth memorising
One ounce is 28.3495 grams — not the 25 or 30 grams most recipes round to. Here is where the rounding is harmless and where it changes a bake.
The exact figure, and the two shortcuts people use
One avoirdupois ounce — the one used for food weight — is exactly 28.349523125 grams, a figure fixed by the same 1959 international agreement that defined the pound. In practice, two shortcuts circulate: rounding to 25 g, which is fast mental maths and understates weight by about 12 per cent, and rounding to 30 g, which is closer and overstates by about 6 per cent. Neither is exact, and which one a recipe author used is rarely stated.
Mass ounces versus fluid ounces
The bigger trap is not the rounding — it is that "ounce" means two different kinds of quantity depending on context, and cookbooks do not always say which. A weight ounce (oz) is mass, converting at 28.3495 g regardless of what is being weighed. A fluid ounce (fl oz) is volume, and a US fluid ounce is 29.5735 ml while a UK (imperial) fluid ounce is 28.4131 ml — a five per cent difference between the two fluid-ounce standards alone, before you even compare fluid ounces to weight ounces.
Water is the one ingredient where a fluid ounce and a weight ounce happen to coincide closely (1 fl oz of water weighs almost exactly 1 oz), which is precisely why the two get confused for everything else — a fluid ounce of honey weighs about 1.4 oz, and a fluid ounce of flour weighs about a third of an ounce, because density is doing the work that the unit name obscures.
A reference table for common kitchen amounts
| Ounces (weight) | Grams | Common rounding seen in recipes |
|---|---|---|
| 1 oz | 28.35 g | 25 g or 30 g |
| 4 oz (¼ lb) | 113.4 g | 115 g or 125 g |
| 8 oz (½ lb) | 226.8 g | 225 g |
| 16 oz (1 lb) | 453.6 g | 450 g |
The 450 g rounding for a pound is common enough in British recipe writing that it is worth recognising on sight — it is about 0.8 per cent light, which is irrelevant for almost anything except a recipe that has been scaled up many times, where the small error compounds with every multiplication.
Where exactness actually matters
- Bread baking. Hydration percentages are calculated from flour and water weights, so a rounded ounce figure shifts the ratio the recipe was actually tested at.
- Confectionery. Sugar-syrup and chocolate work is sensitive to small proportion changes because the chemistry has narrow tolerances.
- Scaling a recipe up. Multiplying a rounded figure by ten multiplies the rounding error by ten as well.
For a single portion of most everyday cooking, either shortcut is fine — the exact factor earns its keep specifically when a recipe is precise, scaled, or repeated at volume.
The takeaway
Use 28.3495 g per ounce when it counts, check whether a recipe means weight or volume before converting, and treat "1 fl oz ≈ 1 oz" as true for water and nothing else.
Filed under Baking · Tags: ounces to grams, oz to g conversion, fluid ounces, cooking measurements. Figures verified against the official definitions — see our editorial policy.