Mass versus volume in cooking: which cup is meant, and how to record the conversion actually used

What it is

Recipes state ingredients by mass (grams, ounces) or by volume (millilitres, cups, spoons). Converting between the two needs two things that are often left implicit: which volume unit is meant, and how much mass of this ingredient that volume holds.

The first is a matter of definition. NIST SP 811, Appendix B.8, lists the US customary cup as 2.365 882 × 10⁻⁴ m³ (236.588 mL), the tablespoon as 1.478 676 × 10⁻⁵ m³ (14.787 mL) and the teaspoon as 4.928 922 × 10⁻⁶ m³ (4.929 mL). The US nutrition-labelling regulation, 21 CFR 101.9(b)(5)(viii), uses rounded values for labelling purposes: a teaspoon means 5 mL, a tablespoon 15 mL, a cup 240 mL, a fluid ounce 30 mL, and an ounce in weight 28 g. A household may also own a metric measuring cup marked 250 mL. One word, at least three volumes.

The second is a property of the ingredient as filled: a cup of flour spooned loosely and a cup pressed and levelled do not hold the same mass, and no published table knows how a given household fills its cup.

Why it matters

A recipe written by volume and reproduced by mass (or the reverse) changes silently if the converter used a different cup or filling habit. A log that stores only the converted number cannot be checked later.

How to apply

• In the recipe or log header, name the cup definition used (236.6 mL, 240 mL or 250 mL) and the spoon definitions, with the source.
• For each dry ingredient the household converts, weigh the household's own cup as it is normally filled, three times, on the household scale, and record the three readings and the scale's display resolution. Use the median as the household's factor for that ingredient and package, and note the date and the package (brand, type) it came from.
• For liquids, record whether the volume was read in a measuring jug or by weight on a scale set to millilitres, and which density the scale assumes in that mode.
• Write conversions as a separate column beside the original quantity, never over it: "1 cup flour (240 mL cup) → N g, household factor of 2026-03-02".
• When a recipe is shared, ship the factor table with it.

Pitfalls

Rounding a labelling value (240 mL) and a NIST value (236.6 mL) into one recipe. Applying a flour factor to a different flour or to a new bag without re-weighing. Using a scale's millilitre mode for liquids other than water without noting it. Treating any published grams-per-cup number as a fact about this kitchen rather than a starting point to be measured.