The exact factor, and a shortcut you can do in your head
One US fluid ounce is 29.5735 mL, fixed by the definition of the US gallon (3785.41 mL ÷ 128). For a quick estimate, divide the millilitres by 30: 300 mL → 10 oz, against an exact 10.14. Worth memorising: 100 mL ≈ 3.4 oz (the airport liquids limit) and 1 litre ≈ 34 oz.
US and UK fluid ounces are not the same size
The two systems split their gallons differently — the US gallon into 128 ounces, the Imperial gallon into 160 — so a UK fluid ounce is the smaller of the two. The gap is about 4 percent and grows with the volume.
Same name, different size: 500 mL reads as 16.9 US fl oz but 17.6 UK fl oz. Check which system a recipe or label uses before you convert.
Common volumes in US fluid ounces
| Millilitres | US fl oz | What it is |
|---|---|---|
| 15 mL | 0.51 oz | 1 tablespoon |
| 30 mL | 1.01 oz | a single shot |
| 100 mL | 3.38 oz | TSA carry-on limit |
| 237 mL | 8.01 oz | 1 US cup |
| 330 mL | 11.16 oz | European drink can |
| 355 mL | 12.00 oz | US drink can |
| 500 mL | 16.91 oz | half a litre |
| 750 mL | 25.36 oz | wine bottle |
| 1000 mL | 33.81 oz | 1 litre |
A US drink can is a round 12 oz (355 mL); a European one is a round 330 mL (11.16 oz). Neither is a clean conversion of the other.
The mistake to avoid
- Fluid ounces measure volume, not weight. An ounce (oz) is mass; a fluid ounce (fl oz) is volume. They only line up for water — 8 fl oz of honey weighs far more than 8 oz.
- "1 fl oz = 30 mL" is a label rounding. The FDA lets bottles print 30 mL, but the real value is 29.57 mL. For liquid medicine doses, use the exact figure.
- mL is not mg. Millilitres are volume, milligrams are mass, and the link between them depends on density — 1 mL of water is about 1000 mg, but oil and syrup differ.
Common questions
How many ounces is 100 mL?
100 mL is 3.38 US fluid ounces. That is the TSA carry-on limit, which is why 100 mL rounds to the "3.4 oz" printed on travel bottles.
How do I convert mL to fluid ounces in my head?
Divide the millilitres by 30. For 750 mL that is 25 oz, against an exact 25.36 — close enough for a bottle size. The error stays around 1.4 percent.
Are US and UK fluid ounces the same?
No. A US fluid ounce is 29.57 mL and a UK Imperial fluid ounce is 28.41 mL, so the US one is about 4 percent larger. A recipe or bottle from Britain uses the smaller ounce.
