Start with the fractions, not the whole cup
Most recipes never ask for a whole cup in milliliters — they ask for a third, a quarter or a half. That is where the three cup systems diverge in practice, because rounding a fraction of 236.588 ml is different from rounding a fraction of 250 ml. Keep the table below beside the recipe and you will never need to guess.
| Fraction | US cup (ml) | Metric cup (ml) | Imperial cup (ml) |
|---|---|---|---|
| 1 cup | 236.588 | 250 | 284.131 |
| ¾ cup | 177.441 | 187.5 | 213.098 |
| ⅔ cup | 157.725 | 166.667 | 189.421 |
| ½ cup | 118.294 | 125 | 142.066 |
| ⅓ cup | 78.863 | 83.333 | 94.710 |
| ¼ cup | 59.147 | 62.5 | 71.033 |
Where the numbers come from
The US cup is defined through the fluid ounce: 8 US fluid ounces at 29.5735 ml each gives 236.588 ml. The metric cup was set at a round 250 ml when Australia and New Zealand metricated their kitchens. The imperial cup is 10 imperial fluid ounces at 28.4131 ml each, giving 284.131 ml — a legacy of the imperial pint. None of these is “wrong”; they are just different standards that happen to share a name.
The 240 ml question
You will also see 240 ml quoted for a US cup. That is the legal cup used on American nutrition labels (21 CFR), chosen because it divides neatly. Recipe writers almost never mean it — cookbooks, baking books and converter tools use the customary 236.588 ml. The 3.4 ml gap is small, but across a recipe with several cup measures it can add up to a tablespoon or more.
Worked example: a pancake batter
A batter calls for 1½ cups of milk. In US cups that is 1.5 × 236.588 = 354.882 ml — call it 355 ml on a jug. In metric cups it would be 375 ml, a 20 ml difference that thins the batter noticeably. The cups to ml converter does any figure instantly, and the cups in a liter guide covers the reverse direction for larger volumes.
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