Buttermilk

One US cup of buttermilk weighs 245 grams. Going the other way, 100 grams is about 0.41 cups.

Buttermilk in an American supermarket in 2026 is not the liquid drained from butter churning that the name suggests. It is cultured lowfat milk — skimmed or lowfat milk fermented with lactic acid bacteria until it thickens and turns tangy. The traditional churned buttermilk still exists in some rural and craft dairies, but the mass-market carton labelled 'buttermilk' is a cultured product, and both USDA's data and this entry describe that version.

King Arthur publishes 1 cup at 227 grams, the same figure they publish for milk, yogurt, heavy cream, Greek yogurt, pumpkin purée and vanilla extract. As with those entries, 227 is what you get from taking 8 US fluid ounces and multiplying by 28.35 grams per ounce — the conversion factor for weight ounces rather than fluid ounces. The number implies a density of 0.9595 grams per millilitre.

Buttermilk cannot have that density. It is roughly 90 percent water plus dissolved milk solids (protein at about 3.3 percent, lactose at about 4.8 percent, small amounts of fat and minerals). All of those solids are denser than water, so the mixture must be denser than water. We reject King Arthur's figure on the same physical grounds as the other 227-per-cup dairy rejections.

USDA FoodData Central publishes 1 cup of cultured lowfat buttermilk at 245 grams (FDC 170874, Milk, buttermilk, fluid, cultured, lowfat). That gives a density of 1.0355 grams per millilitre, essentially identical to plain milk and yogurt at 244-245 grams per cup. The consistency across cultured and non-cultured dairy is expected — culturing changes the flavour and acidity of the liquid but not the composition significantly enough to shift its density.

The mismatch between the King Arthur convention and physical reality matters in recipes that use buttermilk in meaningful amounts. A biscuit dough calling for 1 cup of buttermilk gets 245 grams by USDA measurement and 227 by the King Arthur figure — 7.4 percent less liquid on the volume side, which in a hydration-sensitive dough is enough to change the texture perceptibly. Baking soda-based leavening in biscuits also depends on the acidity that comes with the buttermilk, so 'a little less liquid' is not neutral.

Related products this entry does not cover: cultured whole milk (higher fat, slightly lower density around 1.03), traditional churned buttermilk (thinner, closer to skim milk at 1.03), and powdered buttermilk (a separate ingredient, effectively dry milk with cultured flavour, weighs about 120 grams per cup dry and is measured very differently). Sour milk made at home by adding vinegar or lemon juice to regular milk is an emergency substitute but not chemically the same product; it has the acidity of cultured buttermilk but not the same protein structure.

FDA identity standard for cultured buttermilk products is at 21 CFR 131.112, requiring the product to be cultured with specific bacteria and to meet minimum milk solids content. No density is specified. The Codex Alimentarius standard for fermented milks (CXS 243-2003) covers cultured buttermilk internationally, with the same focus on culture identity and composition rather than physical measurement.

The category here is 'liquid' rather than 'sugar' or 'milled' because cultured buttermilk pours and self-levels in a measuring cup exactly like plain milk, with density set by its water content rather than by scooping method. The 5 percent liquid tolerance applies. The 7.4 percent gap between the accepted USDA figure and the rejected King Arthur figure exceeds that tolerance, which is one of several reasons King Arthur's figure was rejected rather than range-published: the underlying method (fluid-oz-as-weight) is not a valid measurement of the same product, so the two figures are not comparable measurements to be averaged.