Sourdough Starter

One US cup of sourdough starter weighs between 172 and 240 grams, depending on how the cup is filled and which source measured it. That range is the answer; a single number would not be. Every figure behind it is below, with the method each source used.

Also called sourdough discard, sourdough culture, wild yeast starter, levain, wheat sourdough starter, sourdough.

Sourdough starter weighs 172-280 g/cup depending on three independent variables. King Arthur's chart lists 227-241 g/cup, which covers standard 100% hydration wheat starter mid-feeding-cycle, but a freshly-fed active starter can be as light as 172 g/cup and a stiff or deflated discard can reach 280 g/cup.

Sourdough starter is a fermented paste of flour and water, colonized by wild yeasts and lactic acid bacteria. The three physical variables that determine density:

**1. Hydration ratio.** Hydration is the weight of water divided by the weight of flour, expressed as a percentage. A 100% hydration starter has equal parts flour and water by weight and forms a pourable batter. A stiff 65% hydration starter has proportionally more flour and forms a putty-like paste. A liquid 125% hydration starter is thin enough to pour like heavy cream. Stiffer starters weigh more per cup (roughly 260 g at 65%) because they contain proportionally more dense flour per unit water. Liquid starters weigh slightly less (roughly 215 g at 125%) because water alone at 237 g/cup is less dense than the flour-water paste.

**2. Activity state.** A freshly-fed starter at peak activity contains substantial trapped CO2 from yeast metabolism, and that gas inflates the volume without adding mass. A freshly-peaked 100% hydration starter can measure as low as 172 g/cup while chemically identical starter after collapsing (discard) can reach 280 g/cup — a 63% swing in the same starter at different times of day. Where you are in the feeding cycle matters more than most recipes acknowledge.

**3. Flour type.** Starters maintained on white flour, whole wheat, rye, or blended flours have different base densities. Whole wheat and rye starters are noticeably heavier per cup than white starters at the same hydration and activity, because the flour itself is denser.

King Arthur's chart entry at 227-241 g/cup is one of only a handful of range-published entries on their weight chart, and it covers the most common configuration: standard 100% hydration wheat starter measured a few hours after feeding when it is active but not at its aeration peak. This is the mental model most recipe developers assume when they write '1 cup of starter' without further specification. The KA range is honest and does not need correction.

Interpreting recipes that call for '1 cup of sourdough starter': the recipe developer almost certainly means 100% hydration wheat starter, mid-cycle, in the 220-240 g range. If your starter is at that configuration, measure by cup or weigh at 230 g. If your starter is stiffer, more liquid, freshly-peaked, or made with different flour, weigh instead and target 230 g total, adjusting the recipe's water and flour to compensate for the difference in your starter's flour-to-water ratio.

**Discard vs. active starter.** Discard is not chemically different from active starter — it is the same culture, just unfed and past its activity peak. Discard weighs more per cup (roughly 250-280 g) because the CO2 has escaped and the paste has settled. Recipes that specifically call for discard usually expect 240-250 g/cup and have been tested with older, denser starter. Substituting active starter for discard by cup measurement will give roughly 20% less mass of ingredient than the recipe expects, which affects bulk fermentation timing and final texture.

**Levain.** Some professional recipes use 'levain' to mean a purpose-built starter portion, often at a different hydration than the maintained mother starter. When a recipe calls for levain by weight, follow the weight. When it calls for levain by cup, treat it the same as generic 100% hydration active starter unless the recipe specifies otherwise.

Practical guidance: weigh sourdough starter. This is the ingredient in serious baking where volume measurement is least reliable, and where a 5-10% error compounds significantly across bulk fermentation, dough hydration, and final crumb. For volume-only recipes, 227 g/cup is the safest single-number assumption, and matches KA's lower bound. For any precision work, weigh.