Density in Unit Conversions: Why a Cup of Flour Isn’t a Cup of Water
“Convert 1 cup to grams” has no single answer — it depends on what you’re measuring. The missing piece is density. Here’s how volume-to-mass conversion really works, with densities for common ingredients.
Direct answer
A US cup is always 236.588 mL, but its mass varies with density. Water is ≈ 1.00 g/mL (≈ 237 g/cup), while all-purpose flour is ≈ 0.53 g/mL (≈ 120 g/cup). To convert any volume to a mass: mass (g) = volume (mL) × density (g/mL).
Densities of common ingredients
| Ingredient | g per US cup | Density (g/mL) |
|---|---|---|
| Water | 237 g | ≈ 1.00 |
| Milk | 240 g | ≈ 1.01 |
| Vegetable oil | 198 g | ≈ 0.84 |
| Granulated sugar | 198 g | ≈ 0.84 |
| Brown sugar (packed) | 213 g | ≈ 0.90 |
| All-purpose flour | 120 g | ≈ 0.51 |
| Butter | 227 g | ≈ 0.96 |
| Honey | 340 g | ≈ 1.44 |
Gram-per-cup values from King Arthur Baking’s ingredient weight chart; density = grams per cup ÷ 236.588 mL (exact US cup volume, NIST SP 330). Values vary slightly with measuring method.
How to convert volume to mass
Step 1 — volume to mL: 1 cup = 236.588 mL, 1 tbsp = 14.7868 mL, 1 tsp = 4.92892 mL (exact US customary).
Step 2 — multiply by density: mass = mL × (grams per cup ÷ 236.588).
Example: 2 cups of flour = 473.176 mL × 0.507 ≈ 240 g. UnitFig does this per ingredient automatically.
Frequently Asked Questions
Why isn’t 1 cup of flour equal to 1 cup of water in grams?
A US cup is a fixed volume (236.588 mL), but flour and water have different densities. Water is about 1.00 g/mL, so 1 cup ≈ 237 g. All-purpose flour is about 0.53 g/mL, so 1 cup (spooned and leveled) ≈ 120 g. Same cup, very different mass.
What is density and how is it used in cooking conversions?
Density = mass ÷ volume (g/mL). To convert a volume to a mass you multiply: mass = volume × density. UnitFig applies each ingredient’s typical gram weight per US cup (its density × 236.588 mL) from King Arthur Baking’s chart, so cups become accurate grams.
Do I really need to weigh ingredients?
For baking and anything sticky or variable (honey, brown sugar, flour), weighing in grams is far more reliable than cup measures because cup packing varies. Use a scale; treat cup values as convenient approximations.
Where do the density numbers come from?
US cup volume is an exact definition (236.588 mL, NIST SP 330). Ingredient gram weights per cup come from King Arthur Baking’s published ingredient weight chart; density is simply that weight ÷ 236.588 mL.
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Sources & Accuracy
US cup volume (236.588 mL) is an exact international definition (NIST SP 330). Ingredient gram weights per cup are from King Arthur Baking’s published ingredient weight chart; densities are derived from those weights.
Volume definitions: NIST SP 330 (SI). Ingredient weights: King Arthur Baking — Ingredient Weight Chart.
Last Updated 2026 | Cup volume verified against NIST SP 330; ingredient weights per King Arthur Baking