For a dough made from flour and water, hydration is water mass divided by flour mass, multiplied by 100. A sourdough starter contributes both ingredients, so leaving it out can give a misleading percentage.
The Whatson Dough Hydration Calculator asks for the flour and water added directly, the starter mass and the starter’s hydration.
A 100% hydration starter is not all water
At 100% hydration, starter contains equal flour and water masses under this accounting model. A 100 g starter therefore contributes 50 g flour and 50 g water.
This flour based convention is explained in King Arthur Baking’s hydration guide. The calculator tracks the entered flour and water; it does not chemically analyse a starter.
The 500 g flour example is 68.18%, not 65%
Enter 500 g of fresh flour, 325 g of added water and 100 g of starter at 100% hydration. The checked totals are 550 g flour and 375 g water.
Total hydration is 375 ÷ 550 × 100 = 68.18%. Dividing only 325 by 500 gives 65%, which describes the fresh additions and misses the starter contribution.
The result table shows that contribution separately. It gives you a place to check which flour and water are already present before you change the recipe.
Different starter hydration needs a different split
The starter flour calculation is starter mass ÷ (1 + starter hydration/100). Its water is the remaining mass. A stiffer starter and a liquid starter of equal total mass therefore do not contribute identical amounts of water.
Enter the hydration that describes your starter rather than selecting 100 merely because it is the opening example. Sourdough Starter Feeding Calculator can help account for a feed and its seed contribution.
This percentage does not describe every liquid ingredient
Milk, yogurt, egg and oil are not entered as separate ingredients in this tool. Do not count their entire mass as water or assume the displayed number includes their composition.
The naan worksheet, Naan Dough Calculator, intentionally reports ingredient masses rather than calling its combined milk and yogurt content true water hydration.
If your aim is to reach a target overall hydration while preparing a sourdough formula, Sourdough Recipe Calculator allocates the starter and calculates the remaining flour and water. This hydration tool instead measures the percentage produced by amounts you already know.
Keep the starter hydration beside the recipe note. A total without that assumption is harder to reproduce when the starter feeding practice changes.