Grams is mass, fluid ounces is volume — density connects them
You cannot go straight from grams to fluid ounces. First divide the mass by the density to get a volume in millilitres, then divide by the size of a fluid ounce. Water is the only substance where the density term drops out.
The 29.5735 is the millilitres in one US fluid ounce; use 28.4131 for imperial. Density is in g/mL, so water is 1.00 and the formula simplifies to grams ÷ 29.5735.
Same 100 g, different volume
Denser things take up less room. One hundred grams of honey is well under three fluid ounces, while the same mass of oil is more than three and a half.
Honey (1.42 g/mL) packs the same 100 g into a third less volume than oil (0.92 g/mL).
Grams to US fluid ounces, common substances
| Substance | Density (g/mL) | 100 g | 250 g | 500 g |
|---|---|---|---|---|
| Water | 1.00 | 3.38 | 8.45 | 16.9 |
| Milk | 1.035 | 3.27 | 8.17 | 16.4 |
| Vegetable oil | 0.92 | 3.68 | 9.20 | 18.4 |
| Melted butter | 0.96 | 3.51 | 8.78 | 17.6 |
| Honey | 1.42 | 2.37 | 5.93 | 11.9 |
| Soy sauce | 1.08 | 3.12 | 7.81 | 15.6 |
Values in US fluid ounces. Flour and other dry, packable ingredients have no single density — loose versus packed can differ by half, so weigh them in grams instead of converting to a volume.
Common questions
Why can't I convert grams to fluid ounces without the density?
Grams measure mass and fluid ounces measure volume, so they are different things. The same 100 grams fills more of a measuring cup as oil than as honey. You need the density of the substance to bridge the two.
How many fluid ounces is 100 grams of water?
100 grams of water is 3.38 US fluid ounces. Water has a density of 1 g/mL, so this is the only case where the "just divide by 29.57" shortcut is accurate.
Are US and UK fluid ounces the same?
No. One US fluid ounce is 29.5735 mL and one imperial (UK) fluid ounce is 28.4131 mL, a difference of about 4 percent. Set the calculator to the right system before you trust the result.
