Answer-first summary
For water at room temperature, 1 US cup equals approximately 236.59 grams. This is because a gram is a unit of mass and a cup is a unit of volume; to convert, you need the density of the ingredient. Water’s density is about 1 gram per milliliter at 4°C, and roughly 0.998 g/mL at 20°C, making 240 mL (8 fl oz) close to 240 g. The exact number varies slightly with temperature and measurement system. Below are standard conversions for water and notes on when these values change for other substances.
Standard cup-to-gram conversions for water
Use this table as a quick reference. Values are rounded for practical use; small temperature-driven density changes can shift results by about ±1%.
| Volume (cups) | Metric (mL) | Approximate mass (grams) | Typical use case |
|---|---|---|---|
| 1/4 cup | 60 mL | ~60 g | Small baking portions |
| 1/3 cup | 79 mL | ~79 g | Everyday portions |
| 1/2 cup | 118 mL | ~118 g | Everyday portions |
| 2/3 cup | 158 mL | ~158 g | Everyday portions |
| 3/4 cup | 177 mL | ~177 g | Everyday portions |
| 1 cup | 236.59 mL (8 fl oz) | ~236.59 g | Recipes and nutrition labels |
| 1 1/2 cups | 354.88 mL | ~354.88 g | Larger batches |
| 2 cups | 473.18 mL | ~473.18 g | Soups and beverages |
Why grams per cup isn’t always the same
Density—mass per unit volume—determines how many grams fit into a cup. Temperature changes slightly alter water’s density, and different measuring systems define cup volumes differently. Ingredients with sugar, fat, or air behave differently than water. Understanding these factors reduces conversion errors in cooking, nutrition tracking, and lab work.
Temperature effects on water density
Water is densest at about 4°C (39°F), where 1 mL ≈ 1.000 g. At 20°C (68°F), density is roughly 0.998 g/mL; at 100°C (212°F), it drops to about 0.958 g/mL. For everyday kitchen use, these shifts are small but can matter for high-precision work. Room-temperature water (20°C) means 240 mL is very close to 239–240 g, a difference of about 0.5%.
US customary vs metric cups
Recipes and nutrition labels often use different cup definitions. A US legal cup is 240 mL, common in nutrition labeling in the United States. A metric cup is 250 mL, used in many other countries. A US customary cup is about 236.59 mL. The choice of cup changes gram-per-cup values slightly, so note which standard your recipe or dataset follows.
Ingredient density matters
Dry flour packs differently than water, and oils have lower density. For example, all-purpose flour weighs roughly 120–130 g per US cup, while honey can exceed 300 g per cup. Always verify conversions for non-water ingredients using reliable sources rather than assuming the water equivalence.
Practical conversion workflow
When you need precise grams from cups, follow these steps to reduce error. These tips apply to recipes, nutrition calculations, and small-scale experiments.
- Confirm the cup standard: check whether the source uses US customary, metric, or another definition.
- Note the ingredient: use water-density approximations only for water and very dilute aqueous solutions.
- Account for temperature when precision matters: measure mass directly if possible.
- Use a calibrated scale for critical work; avoid volume-only conversions for commercial or scientific outputs.
When to measure by weight instead of volume
For consistent results, many bakers and labs prefer grams over cups. Weight is unaffected by packing, humidity, or ingredient compressibility. When accuracy, repeatability, or regulatory compliance is important, mass-based measurements are the safer choice. Reserve volume measures for quick estimates or when scales are unavailable.
Quick reference: grams per cup for water at room temperature
Use this list for fast, approximate conversions. Small temperature and standard variations may shift values by about 1%.
- 1/4 cup water: ~60 g
- 1/3 cup water: ~79 g
- 1/2 cup water: ~118 g
- 2/3 cup water: ~158 g
- 3/4 cup water: ~177 g
- 1 cup water: ~236.59 g (US customary) or ~240 g (US legal/nutrition)
- 1 1/2 cups water: ~355 g
- 2 cups water: ~473 g