A standard U.S. cup holds 8 fluid ounces, which equals about 237 milliliters in American recipes and 240 milliliters on nutrition labels.
Knowing how much water a cup holds matters more than you’d think. One wrong conversion can turn a good batch of iced tea bitter or throw off a garden spray mixture. The honest answer is that a cup isn’t one single number — it depends on whether you’re following a recipe or reading a nutrition label. Here’s the breakdown.
The Two Official Cup Sizes in the U.S.
The United States uses two different cup standards, and confusing them causes most measurement mistakes. The U.S. customary cup used in American recipes holds 8 fluid ounces, which works out to 236.588 mL. The U.S. legal cup used for nutrition labeling is slightly larger at 240 mL.
For everyday cooking, the difference between 236.588 mL and 240 mL is negligible — about a teaspoon and a half. But knowing which standard you’re using keeps conversions accurate when scaling recipes up or down.
Cup Measurements at a Glance
The best way to keep these numbers straight is to see them side by side. The table below compresses the essential facts into one quick reference.
| Cup Type | Fluid Ounces | Milliliters |
|---|---|---|
| U.S. customary cup (recipes) | 8 fl oz | 236.588 mL |
| U.S. legal cup (nutrition labels) | 8.12 fl oz | 240 mL |
| Common kitchen rounding | 8 fl oz | 237 mL |
| Metric cup (other countries) | 8.45 fl oz | 250 mL |
| Half U.S. pint | 8 fl oz | 236.588 mL |
NIST’s kitchen equivalency chart lists 1 cup as 240 mL, while its official SI conversion tables specify 236.588 mL for the customary cup. Both are correct — they just describe different standards.
Why a Measuring Cup Isn’t a Drinking Cup
The number on a measuring cup reflects a precise volume unit, not the size of the mug in your cabinet. A drinking glass holds anywhere from 8 to 16 ounces depending on its design. That’s why recipes fail when someone fills a random tumbler and calls it a cup.
The U.S. customary cup is defined as half of a U.S. liquid pint. It’s a standardized reference for cooking, not a description of tableware. When a recipe says “1 cup of water,” it means a level measuring cup, not a coffee mug.
Converting Water Measurements Accurately
For water, fluid ounces and weight ounces are identical — 8 fluid ounces of water weighs 8 ounces. That neat trick doesn’t apply to other ingredients, so keep it reserved for water and water-based liquids.
When a recipe calls for cups and you only have a scale, use 237 grams per U.S. customary cup. If you’re reading a nutrition label, use 240 grams. Either choice lands close enough for most household uses, from mixing plant food to brewing coffee.
If you’re choosing a vessel for daily water intake, the cup you drink from matters less than how many ounces it actually holds. Our tested roundup of the best cups for water compares real capacities and materials side by side.
NIST’s own kitchen measurement equivalencies chart remains the most reliable U.S. government reference for household cooking conversions.
FAQs
Is a cup of water 8 ounces or 240 milliliters?
Both numbers are correct depending on context. A U.S. customary cup used in recipes holds 8 fluid ounces, which is 236.588 mL. The U.S. legal cup used on nutrition labels rounds to 240 mL. For home cooking, the two are close enough that either works.
Why do some countries use 250 mL for a cup?
Metric-system countries standardized their own cup measurements, with 250 mL being the common round number in places like Australia and parts of Europe. That’s about 2% larger than the U.S. legal cup and 5.7% larger than the U.S. customary cup.
Does a cup of water weigh 8 ounces?
Yes, for water specifically. One fluid ounce of water weighs approximately one avoirdupois ounce, so 8 fluid ounces weighs about 8 ounces. This shortcut only works for water and liquids with similar density — honey or oil won’t match.
References & Sources
- NIST. “Metric Kitchen: Cooking Measurement Equivalencies.” Official U.S. government reference listing 1 cup = 240 mL for kitchen conversions.