Sourdough Hydration Calculator ·
Starter percentage measures the starter as an ingredient
If a dough uses 100 g starter and 500 g added flour, one common shorthand is to call the starter 20% of the added flour. That can be useful for following a recipe, but it does not tell you how much flour in the final dough was already fermented inside the starter. A 100 g liquid starter and a 100 g stiff starter weigh the same while containing different amounts of flour.
Prefermented flour measures flour, not starter weight
Prefermented flour percentage = flour inside the starter ÷ total flour × 100. With 500 g added flour and 100 g starter at 100% hydration, the starter contains 50 g flour. Total flour is 550 g, so prefermented flour is 50 ÷ 550 × 100 = 9.09%. This percentage describes how much of the flour was already in the levain.
A stiff starter changes the answer
Keep the starter at 100 g but change it to 50% hydration. It now contains about 66.67 g flour and 33.33 g water. Total flour becomes 566.67 g, so prefermented flour is about 11.76%. The starter still weighs 100 g, but the amount of prefermented flour has increased.
Why the distinction is useful
Prefermented flour can help compare formulas that use starters of different hydrations because it focuses on the flour already fermented. It still does not predict fermentation time by itself: starter maturity, temperature, flour, acidity and dough handling matter. Treat the number as one part of the formula, not a schedule.
How this calculator reports it
The calculator splits the starter into flour and water, adds starter flour to total flour, and reports prefermented flour from those totals. The result panel also shows starter weight as a percentage of total flour so you can see both ideas without treating them as interchangeable.