Kefir

One US cup of kefir weighs 240 grams. Going the other way, 100 grams is about 0.42 cups.

Also called kefir, milk kefir, plain kefir, cultured milk kefir, drinkable kefir, kefir milk.

TL;DR: 240 g/cup (1.014 g/mL). Lifeway whole milk kefir label and USDA physical density figure converge at 240-243 g/cup within 1.25%. Publishing at 240 to match the dominant US brand's own labeling convention and align with the milk/buttermilk/cream cup weights already in the database. King Arthur is silent on kefir.

Kefir is fermented milk cultured with kefir grains — small cauliflower-like clumps of a symbiotic community of about 30 species of lactic-acid bacteria and yeasts held together in a polysaccharide matrix. The grains ferment lactose in raw or pasteurized milk over 12-24 hours at room temperature, producing a drinkable cultured milk with the tangy flavor of yogurt, the pourable body of buttermilk, and a slight natural effervescence from carbon dioxide produced by the yeast component. Traditional kefir is a Caucasus mountains product with origins in the North Ossetian and Karachay regions of the Russian Caucasus; commercial production in the US began in earnest when the Smolyansky family founded Lifeway Foods in 1986 and popularized the drinkable-yogurt-adjacent format.

Density data converges tightly around 240-243 g per US cup. Lifeway's Whole Milk Plain Kefir product label lists 1 cup = 240 g, following the standard dairy convention of treating 240 mL (approximately 1 US cup) and 240 g as interchangeable for water-density-adjacent liquids. USDA FoodData Central's entry for Lifeway lowfat kefir (NBD 1289) gives 257 g/metric cup or 243 g/US cup at 1.028 g/mL, which is the actual physical density measured under USDA reference conditions. Physical prediction from typical whole-milk-kefir composition (3.25% milkfat, 4.5% carbs, 3.5% protein, 88.75% water) gives 1.028 g/mL, matching the USDA figure exactly. Publishing at 240 g/cup keeps consistency with the milk, buttermilk, heavy-cream, and yogurt entries elsewhere in this database that all follow the same dairy-label convention.

The cultured dairy family lands in a tight density band because water dominates the composition. Kefir at 1.014-1.028 g/mL sits alongside whole milk (1.03 g/mL), buttermilk (1.03), plain yogurt (1.03), and half-and-half (1.02), and just below sour cream (1.03-1.04), Greek yogurt (1.03-1.05), and heavy cream (0.98-1.00 depending on whipping). The differences between these products at the volume-to-weight level are within 5% for practical baking purposes — where they differ meaningfully is in fat content, viscosity, and how the culture interacts with heat and acid in the pan.

Several disambiguations matter for search intent. Milk kefir (this entry) is not the same as water kefir, which is made from sugar water plus fruit fermented with water kefir grains (Tibicos or Ginger Beer Plant); water kefir has density essentially matching water (1.00-1.02 g/mL) and no dairy content, so it is a distinct product family. Kefir is not yogurt: yogurt is fermented by two specific bacterial strains (Streptococcus thermophilus and Lactobacillus bulgaricus) with no yeast component, produces no CO2, and sets to a firmer body; kefir's yeast-plus-bacteria consortium produces the pourable body and the slight fizz. Kefir is not buttermilk: cultured buttermilk uses a single-strain lactic-acid fermentation, has no yeast, no CO2, and a milder tang. All three are cultured dairy but the microbial communities and resulting products differ.

For sourdough baking, kefir feeds an active starter well — many bakers hydrate their starter refresh with kefir instead of water once a week for a burst of additional yeast and lactic-acid activity, which the starter's own microbes then metabolize alongside the wheat starches. In quick breads, kefir substitutes 1:1 by weight for buttermilk with slightly more tang and a slight effervescence that acts as a leavening boost when combined with baking soda. In smoothies, salad dressings, and marinades where buttermilk is called for, the substitution is direct at 1:1 by volume. For heat-set custards, cheesecakes, and pastry cream, kefir behaves like buttermilk: it curdles above 180°F and needs to be added off-heat or tempered gradually.