Soy Milk

One US cup of soy milk weighs 243 grams. Going the other way, 100 grams is about 0.41 cups.

Also called soy milk, soymilk, unsweetened soy milk, unsweetened soymilk, plain soy milk, soy beverage, soy drink.

TL;DR: Unsweetened soy milk runs 243 grams per cup by USDA measurement across major brands, denser than almond or oat milk because of its higher protein content; producer labels state 240 mL per cup under the FDA nutrition-labeling convention that treats a milliliter as a gram, understating true density by about 1.2%.

Soy milk is made by soaking, grinding, and straining whole soybeans, producing a stable emulsion of soy protein, oil, and water. Commercial unsweetened varieties typically contain 6-7% total solids by mass — roughly twice the solids content of unsweetened almond milk and comparable to whole cow's milk — with protein as the dominant contributor at 2.9-3.5% of mass, oil at 1.5-2%, and residual carbohydrates at 1-2%. Fortified versions add trace calcium, vitamin D, vitamin B12, and riboflavin that do not measurably affect density.

USDA Standard Reference for Silk Unsweetened Soymilk (FDC 175223, the market-leading US brand) lists 243 grams per US cup, a direct measurement rather than a label-derived conversion. USDA's Global Branded Food Products Database converges tightly at 243-244 g/cup across every soy milk variant tested — original, vanilla, unsweetened, fortified, unfortified, lowfat, nonfat, chocolate. The generic USDA soymilk record sits at 244 g/cup. This convergence across sweetening levels and fortification profiles indicates that density is dominated by the soy protein content, not by added sugars or fats.

Producer nutrition labels use the FDA nutrition-labeling convention of treating 240 mL as the reference amount for a 1-cup serving, with mass implicitly assumed equal to volume (240 g). Silk, Westsoy, Edensoy, and other major brands all publish 240 mL per cup on their labels. This convention systematically understates the true density of protein-rich plant milks by 1-2%, the same pattern documented in the eggnog entry for dairy beverages. The correct physical density is 243 g/cup, not 240 — the discrepancy is not measurement error but a regulatory-convention artifact.

Physical density prediction from composition confirms the USDA number. At 3% soy protein (partial specific volume around 0.73 mL/g, contributing about +0.02 g/mL above pure water), 2% soybean oil (partial specific volume around 1.09 mL/g, contributing about -0.002 g/mL), and 1.5% carbohydrate (contributing about +0.005 g/mL), the predicted density is 1.026-1.030 g/mL, giving 243-244 g/cup. The observed USDA figure matches the prediction to within measurement precision; the convergence is not coincidental but reflects the physics of a soy-protein-in-water emulsion.

King Arthur Baking does not publish soy milk specifically. Its chart lists "Milk (fresh) 1 cup, 8 oz, 227 g" using the same FDA fluid-ounce-as-mass-ounce convention, which is legitimate for skim or lowfat cow's milk but understates whole cow's milk (~245 g/cup) and most plant milks. There is no direct correction opportunity for soy milk because it isn't listed, but users should not extrapolate KA's generic milk figure to soy — the correct number is 243, not 227.

Compared with other plant milks in this database, soy milk sits at the denser end: unsweetened almond milk at 237 g/cup (low protein, low solids), unsweetened oat milk at 237 g/cup (moderate carbs, low protein). Soy milk's higher protein content produces the higher density. Sweetened varieties add 2-4% dissolved sugar, which pushes density slightly higher (typically 245-248 g/cup) but rarely crosses the 5% liquid tolerance from the 243 baseline.

For practical baking use, treat unsweetened soy milk as 243 g/cup. When substituting for cow's milk in a recipe, the 243 vs 245 (whole milk) difference is within tolerance for muffins, quick breads, pancakes, and most cake batters. In precision applications like custards or roux-thickened sauces, use the actual carton figure plus a 1.2% correction if the label states 240 mL per serving.