How much does a cup of water weigh in grams
A standard US cup of water at typical room temperature weighs about 236.59 grams. This value comes from the conventional definition of a US legal cup as 236.5882365 milliliters and the density of water near 20°C, which is approximately 0.998 g/mL. For everyday cooking, nutrition labels, and most recipe use cases, 236.6 grams (often rounded to 240 grams) is a reliable reference point.
Standard cup volume and water density basics
The weight of a cup of water depends on two factors: the volume of the cup and the density of the water. Metric and US customary systems define a cup differently, and water density changes slightly with temperature, pressure, and purity. Understanding these relationships helps you convert between volume and weight accurately in the kitchen, in labs, and in nutrition calculations.
What is a cup in milliliters
Different regions use different definitions of a cup. In the United States nutrition labeling and everyday cooking, a cup is legally defined as exactly 236.5882365 milliliters. This is sometimes called a US customary or legal cup. A metric cup, used in some other countries, is commonly standardized to 250 mL. Knowing which cup standard applies is essential when converting to weight.
Why water density is close to 1 g/mL but not exactly
At 4°C, pure water reaches its maximum density of about 1.000 g/mL. At typical room temperature (around 20°C), density is approximately 0.998 g/mL. For most practical purposes, treating density as 1 g/mL gives a close estimate, but for precise work, small temperature differences and impurities can matter.
| Condition | Water Density | Resulting Weight for 1 US Cup |
|---|---|---|
| 4°C (density maximum) | 1.000 g/mL | Approximately 236.59 g |
| 20°C (typical room temp) | 0.998 g/mL | Approximately 236.12 g |
| Boiling (~100°C) | 0.958 g/mL | Approximately 226.36 g |
Effect of temperature on weight and volume
As temperature rises, water expands and becomes slightly less dense, so a cup of warm water weighs marginally less than a cup of cold water, even though the cup’s volume stays the same. For most culinary tasks, this difference is small, but it can matter in scientific experiments or when precise formulation is required.
- Cold water near 4°C is denser and therefore slightly heavier per cup.
- Warm water near 40–60°C is less dense and marginally lighter per cup.
- Boiling water is significantly less dense, reducing weight per cup noticeably.
Altitude, pressure, and dissolved substances
At higher altitudes, lower atmospheric pressure can slightly affect how water behaves in open containers, but it does not change the defined volume of a cup. Dissolved salts or minerals increase water’s density very slightly, so a cup of seawater or heavily mineralized water will weigh more than a cup of pure distilled water. For most diet, nutrition, and cooking labels, these differences are considered negligible.
Practical conversions for common use cases
When you need to convert between a cup measurement and grams, match the cup standard to your context. Use these practical reference values for quick estimation and consistent results in recipes, nutrition tracking, and basic lab work.
| Cup standard | Volume in milliliters | Weight of water in grams (approx.) |
|---|---|---|
| US legal cup | 236.588 mL | 236.6 g |
| Metric cup | 250 mL | 250 g |
| UK (imperial) cup | 284.131 mL | 284.1 g |
| Japanese cup | 200 mL | 200 g |
When exact weight matters
In baking, chemistry, and some medical or industrial settings, small differences can be important. If you need highly precise measurements, use a calibrated scale, account for water temperature, and confirm which cup definition your protocol or instrument uses. For nutrition facts on food labels, manufacturers typically refer to the standard US cup weight for ingredient listings.
Common myths and clarifications
Myth: One cup of any liquid always weighs the same as water in grams. Fact: This only holds for water-like liquids at similar temperature and purity; oils, alcohols, and acids have different densities. Myth: A cup is always 240 mL in every country. Fact: Regional definitions vary; always check whether a recipe or label uses a US, metric, or other cup standard.
Quick takeaways
- A US cup of water at room temperature weighs about 236.6 grams.
- A metric cup of water is 250 grams by definition of volume.
- Temperature, pressure, and dissolved substances can cause small variations.
- For reliable results in recipes and experiments, note the cup standard and, when precision matters, measure by weight.