A cup is not one volume
Five cups are in regular use and they are not close. The Japanese cup holds 200 ml. The old imperial cup holds 284.13. That is 42 percent between them, on the same word.
| Standard | Millilitres | vs US cup | Where you meet it |
|---|---|---|---|
| US customary | 236.5882 | +0.00% | American recipes and measuring cups |
| US legal | 240.0000 | +1.44% | Nutrition labels, 21 CFR 101.9 |
| Metric | 250.0000 | +5.67% | Australia and New Zealand |
| Imperial | 284.1306 | +20.09% | Older British recipes, by convention |
| Japanese | 200.0000 | -15.46% | Japanese recipes |
Two of these come from the same agency
NIST Handbook 44 lists a measuring cup as 237 ml. The NIST kitchen equivalencies page lists a cup as 240 ml. Both are correct in context. The first rounds the customary cup, the second is the legal cup used for nutrition labels, and 21 CFR 101.9 sets that one out in law.
The exact customary figure, 236.5882365 ml, is not printed by NIST at all. It falls out of the gallon, which NIST does define exactly as 3785.411784 ml. A cup is one sixteenth of that.
What using the wrong cup costs you
The error is not academic. Here is the same three cup flour recipe measured with each cup, using this site's all-purpose flour density.
| Cup used | Error | 3 cups gives | Difference |
|---|---|---|---|
| Japanese | -15.46% | 304 g | -56 g |
| US customary | +0.00% | 360 g | +0 g |
| US legal | +1.44% | 365 g | +5 g |
| Metric | +5.67% | 380 g | +20 g |
| Imperial | +20.09% | 432 g | +72 g |
A metric cup on an American recipe adds about 20 g of flour across three cups, near enough an extra sixth of a cup. An imperial cup adds about 72 g, which is 20 percent over and enough to turn a soft dough stiff.
Weighing removes the problem completely, which is the real argument for a scale.
Millilitres to cups
The same millilitre figure in the three cups you are most likely to need.
| Millilitres | US cups | US legal | Metric |
|---|---|---|---|
| 1 | 0.0042 | 0.0042 | 0.0040 |
| 5 | 0.0211 | 0.0208 | 0.0200 |
| 15 | 0.0634 | 0.0625 | 0.0600 |
| 50 | 0.2113 | 0.2083 | 0.2000 |
| 100 | 0.4227 | 0.4167 | 0.4000 |
| 125 | 0.5283 | 0.5208 | 0.5000 |
| 150 | 0.6340 | 0.6250 | 0.6000 |
| 200 | 0.8454 | 0.8333 | 0.8000 |
| 250 | 1.0567 | 1.0417 | 1.0000 |
| 300 | 1.2680 | 1.2500 | 1.2000 |
| 500 | 2.1134 | 2.0833 | 2.0000 |
| 750 | 3.1701 | 3.1250 | 3.0000 |
| 1000 | 4.2268 | 4.1667 | 4.0000 |
One litre is 4.226753 US cups. That figure appears in NIST Handbook 44's own conversion table, so it is one of the few you can check against a government document rather than another converter.
Cups to millilitres
| Cups | US customary | US legal | Metric | Imperial |
|---|---|---|---|---|
| 1/4 | 59.15 | 60.00 | 62.50 | 71.03 |
| 1/3 | 78.86 | 80.00 | 83.33 | 94.71 |
| 1/2 | 118.29 | 120.00 | 125.00 | 142.07 |
| 2/3 | 157.73 | 160.00 | 166.67 | 189.42 |
| 3/4 | 177.44 | 180.00 | 187.50 | 213.10 |
| 1 | 236.59 | 240.00 | 250.00 | 284.13 |
| 1 1/2 | 354.88 | 360.00 | 375.00 | 426.20 |
| 2 | 473.18 | 480.00 | 500.00 | 568.26 |
Two exact relationships hide in that table. Two US cups make 473.176473 ml, which is exactly one US liquid pint. Two imperial cups make 568.26125 ml, exactly one imperial pint from the UK Weights and Measures Act.
The imperial cup is a convention, not a law
This one is worth saying plainly because every converter states 284.13 ml as though it were defined somewhere.
It is not. The UK Weights and Measures Act 1985 defines the gallon and the pint. It does not define a cup. Canada's Weights and Measures Act defines the imperial gallon, fluid ounce and pint, and its schedule contains no cup either.
The 284.13 ml figure is half an imperial pint, or ten imperial fluid ounces, which is what British cookbooks settled on. It is a sound convention with no statute behind it, and knowing that is useful when two old recipes disagree.
Questions cooks actually ask
- How many ml in a cup?
- 236.5882 ml if the recipe is American, 240 ml if you are reading a nutrition label, 250 ml if the recipe is Australian or from New Zealand, and about 284 ml for an older British cup.
- Is 250 ml a cup?
- It is exactly one metric cup, used in Australia and New Zealand. On an American recipe it is 5.67 percent too much, which compounds across every cup you measure.
- How many cups is 100 ml?
- 0.4227 US cups, so a little under half. In metric cups it is 0.40.
- Which cup should I use for an American recipe?
- The US customary cup at 236.59 ml. If your measuring cup came with a metric kitchen set it probably holds 250 ml, and you will be over on every ingredient.
- Why not just weigh everything?
- You should, for baking. Weight has no standard to get wrong. The whole problem on this page exists only because volume depends on whose cup is in your drawer.
Where these numbers come from
The cup values are exact constants from primary sources, not rounded conversions. The US customary cup is derived from the NIST gallon, one sixteenth of 3785.411784 ml. The legal cup is set in federal regulation. The imperial cup is half the pint defined in UK statute. The metric cup and the Japanese cup come from the bodies that publish them.
Percentages and gram differences on this page were computed with exact fractions rather than decimals, so the rounding you see is only in the display.
- NIST Handbook 44, Appendix C, the exact gallon and the measuring cup
- NIST, metric kitchen cooking measurement equivalencies
- 21 CFR 101.9, a cup means 240 millilitres for nutrition labelling
- UK Weights and Measures Act 1985, Schedule 1, the imperial pint
- Food Standards Australia New Zealand, the 250 ml cup
- Kagawa Nutrition University, the 200 ml Japanese cup