Sourdough hydration math, step by step

Hydration is the weight of water divided by the weight of flour, expressed as a percentage.

Hydration = water weight ÷ flour weight × 100

A dough with 500 grams of flour and 350 grams of water has 350 ÷ 500 × 100 = 70 percent hydration. Because the calculation uses a ratio, a larger dough with 1,000 grams flour and 700 grams water has the same hydration.

Main-dough hydration

Some recipes calculate hydration from only the flour and water listed in the main dough. That is straightforward, but it can omit flour and water contributed by a starter or preferment.

If the recipe states a headline hydration, check whether its definition includes those prefermented ingredients. Two correct calculations can produce different percentages if they use different boundaries.

Total-formula hydration

To calculate total-formula hydration, add every flour contribution and every water contribution before dividing.

Consider this example:

• Main-dough flour: 500 g
• Main-dough water: 350 g
• Starter: 100 g, maintained with equal weights flour and water

The starter contributes 50 grams flour and 50 grams water. Total flour is 550 grams. Total water is 400 grams. Total hydration is 400 ÷ 550 × 100 = about 72.7 percent.

The main-dough-only calculation is 350 ÷ 500 × 100 = 70 percent. The total-formula calculation is about 72.7 percent. The difference comes from what the calculation includes, not from faulty arithmetic.

Starter hydration

Starter hydration uses the same formula. A starter containing 60 grams water and 50 grams flour is 60 ÷ 50 × 100 = 120 percent hydration. One containing 40 grams water and 50 grams flour is 80 percent hydration.

Do not assume that every starter divides into equal flour and water. Use the actual feeding ratio or measured composition. If that information is unavailable, label the assumption rather than presenting the total hydration as exact.

Solving for water

Hydration can also be used to calculate a target water weight:

Water weight = flour weight × target hydration as a decimal

For 600 grams of total flour at 75 percent hydration, target total water is 600 × 0.75 = 450 grams. If a starter already contributes 60 grams of water, the remaining water added elsewhere in the formula would be 390 grams.

This calculation preserves the target ratio while making the starter contribution explicit.

Interpreting the number carefully

Published baking guidance uses hydration ranges to describe broad differences in dough consistency, but the same percentage can behave differently with different flours and conditions. Whole-grain flour, high-protein flour, temperature, and mixing can all change what the baker observes.

Treat hydration as a precise description of the formula, not a complete prediction of the dough. Adjustments should be small, measured, and recorded so the next bake can reproduce or reverse them.

A short audit checklist

• Did the calculation include all flour?
• Did it include all water?
• Was the starter’s hydration known or assumed?
• Are percentages calculated by weight rather than volume?
• Is the recipe’s stated hydration using the same boundary?

Answering those questions prevents most apparent hydration disagreements. The formula is simple; the important part is declaring exactly which ingredients entered it.