Density

Density is mass per unit volume, typically expressed in g/cm³ or kg/L (which are numerically identical). Formula: ρ = m / V.

Reference densities at room temperature:

• Water: 1.00 g/cm³ (by definition at 4°C)
• Whole milk: 1.03
• Honey: 1.42
• Olive oil: 0.92
• All-purpose flour: 0.53 (uncompressed)
• Granulated sugar: 0.85
• Brown sugar (packed): 0.93
• Powdered sugar (sifted): 0.56
• Cocoa powder: 0.43
• Salt (table): 1.20

The density difference is why volume measurements are unreliable for dry ingredients: a cup of flour is between 120 g and 150 g depending on how you scoop it. A cup of water is always 240 g (within rounding). This is the core reason serious baking has moved to weight measurements over the last 20 years.

Our cup-to-grams converter applies ingredient-specific densities to give accurate weight conversions for 36 common ingredients. See cooking methodology for the data sources.

Worked example

A recipe calls for “2 cups all-purpose flour” (US cup = 237 ml). At a density of 0.53 g/cm³ for uncompressed AP flour: 2 × 237 × 0.53 = 251 g. But the same volume of scooped-and-packed flour can hit 0.65 g/cm³, giving 308 g — 23% more flour for the same nominal “2 cups.” That difference is the gap between a tender cake and a brick. Compare to honey: 2 cups × 237 ml × 1.42 g/ml = 673 g; honey’s high density makes volume-to-weight conversion forgiving because it pours and self-levels. Now compute a substitution: a brioche recipe asks for 100 g whole milk; you have only skim. Whole milk density 1.030, skim 1.034 — the volumes differ by 0.4%, well below kitchen-scale precision. The substitution is fine. The same recipe asking for 100 g of olive oil substituted with melted butter (0.92 vs 0.91 g/ml) is similarly forgiving by mass but very different in flavour and fat composition.

Flour density is also affected by humidity and how long the bag has been open. Freshly milled flour is fluffier (lower density); flour that has settled in storage for months packs denser. This is one source of recipe variability that even careful weight-measurement doesn’t solve — the same 250 g of flour can hydrate differently depending on its history. Professional bakeries store flour at controlled humidity and use it within a defined turnover window for exactly this reason.

When and why it matters

Density matters in three contexts most people don’t think about: baking (where a 10% flour discrepancy from volume measurement ruins the recipe), shipping (where a freight quote depends on volumetric vs actual weight — “dim weight” pricing means low-density cargo is charged by volume), and chemistry (where solute concentrations are reported as either molarity, molality, or weight percent, all of which require density to interconvert). The home cook who switches from cups to grams discovers immediately how variable their previous baking was; professional bakeries quote every recipe in baker’s percentages by weight precisely so density variation does not enter the system. The shipping mistake — sending a large lightweight box and paying dim-weight rates — is common with small e-commerce sellers who quote shipping based on actual weight only. Reference: NIST SP 811 — Guide for the Use of the SI.

Why density changes with temperature: almost every material expands when heated, so its density drops slightly. Water is the famous counter-example below 4°C — it expands as it cools toward freezing, which is why ice floats and lakes freeze top-down. For cooking, temperature effects on density are negligible: water’s density at 20°C (0.998 g/cm³) is within 0.2% of its 4°C reference. For metrology, calibrated mass standards require temperature-corrected density tables, and refineries gauge oil cargo in temperature-compensated litres so a tankerful in Texas heat doesn’t shrink when delivered to a cooler port.

Specific gravity vs density: specific gravity is the dimensionless ratio of a substance’s density to water’s density (at a reference temperature). A vodka is “SG 0.94” meaning 94% as dense as water. Brewers use specific gravity for both wort (sugary unfermented beer, SG ~1.05) and finished beer (SG ~1.01) to compute alcohol content via the SG drop. Petroleum is graded by API gravity, a transformed SG specific to crude-oil markets where the relationship to density is inverted (high API = low density = light crude). For most non-specialist work, density and specific gravity are interchangeable shorthand. Related: cooking converter, hydration. Reference: NIST SP 811.