True Sourdough Hydration: Why the Starter Counts
Hydration is one of the few numbers in bread baking that looks more precise than it is. Bakers use it to mean the water in a dough, expressed as a percentage of the flour in that dough. That definition is borrowed from baker’s percentage, where every ingredient is a percent of the total flour. Water at 70% of the flour is a 70% hydration dough — but only if you have counted all of the flour and all of the water.
In a yeasted dough with no preferment, that is easy. In a sourdough dough, part of both the flour and the water arrived inside the starter. A hydration sourdough calculator that ignores the starter will understate or overstate the dough you actually mixed.
This page explains the true figure, then shows how this site’s sourdough calculator arrives at it.
What “true hydration” means here
True dough hydration, as used on this site, is:
(water you add + water in the starter) ÷ (flour you add + flour in the starter) × 100
It is not a marketing term. It is the ordinary baker’s-percentage hydration, applied honestly to a dough that includes a preferment.
The starter’s hydration is a separate number. It is the water in the starter as a percent of the flour in the starter. If you feed with equal weights of water and flour, and you have done that consistently, the ripe starter is at 100% hydration. A firm starter fed with less water than flour might sit at 80% or 50%. You need that figure to split a jar of starter into its flour and water.
For a starter of weight S at hydration H% (written as a decimal h = H/100):
Flour in the starter = S ÷ (1 + h)
Water in the starter = S − flour in the starter
At 100% hydration, h is 1, so a 100 g starter contains 50 g flour and 50 g water. At 50% hydration, the same 150 g of starter contains 100 g flour and 50 g water.
Why the shortcut misleads you
The shortcut is to divide recipe water by recipe flour and stop. That number is not wrong as a description of what you poured and scooped. It is wrong as a description of the dough.
Take 500 g flour, 350 g water, and 100 g starter at 100% hydration. The shortcut is 350 ÷ 500 = 70%. After you include the starter, you have 550 g flour and 400 g water. True hydration is 400 ÷ 550 = 72.7%.
The starter added the same amount of flour and water, so it pulled the dough toward 100% — slightly wetter than 70%. If you had used a large amount of a wet starter, the gap would be wider. If you had used a stiff starter, the dough could come out drier than the shortcut suggested.
This matters because hydration is how bakers compare doughs, scale recipes, and decide whether to hold back water. A dough that is “70%” on paper and 73% in the bowl will feel different, extend differently, and may need a different mix or fold. You do not have to chase a perfect tenth of a percent. You do have to know which number you are looking at.
Another example, with a stiffer starter
Keep the same 500 g flour, 350 g water, and 100 g starter, but set the starter hydration to 80%.
Flour in the starter is 100 ÷ 1.8, about 55.6 g. Water in the starter is the rest, about 44.4 g. Total flour is about 555.6 g. Total water is about 394.4 g. True hydration is 71.0%.
Same recipe weights, different starter, different dough. If your starter is kept stiff, entering 100% in the calculator will overstate the water in the jar.
If you are not sure of the starter hydration, look at how you feed it. Equal weights of water and flour, repeated, is 100%. A feeding that regularly uses less water than flour is a lower percentage. The calculator will not know unless you tell it.
What this number does not tell you
True hydration does not tell you how sticky the dough will feel. Whole-grain flours absorb more water than a white flour at the same percentage, and temperature or mixing can change the feel more than a point of hydration. Salt, fat, and inclusions sit outside this percentage. Use the number to compare formulas fairly, or to decide whether yesterday’s “wet” dough was actually wetter, or just warmer. When you want to change hydration, change the water you add. The flour and water already in the starter are already in the bowl.
How to get the number on this site
Open the sourdough calculator and use the true dough hydration panel. Enter the flour and water you are adding, the starter going into the dough, and the starter hydration (the default is 100%). The result shows the true hydration, the flour and water inside the starter, and the totals.
If you are also refreshing the starter for a later mix, the feeding-ratio panel on the same page will tell you how much flour and water to add to the jar. For a walkthrough with the default numbers already filled in, see how to use this simple sourdough calculator.