Start With the Numbers, Then Find the Hole in Them
A 1,000 g sourdough loaf at 75% hydration with 20% starter and 2% salt works out to 565 g of flour, 424 g of water, 113 g of starter, and 11.3 g of salt. Put those four figures in front of most bakers and the dough gets mixed exactly as written.
It also comes out wetter than 75%.
The reason is that the 113 g of starter is not a fifth ingredient. It is flour and water that already arrived in the bowl, and the formula above counts it twice: once as starter, and again because the 565 g and the 424 g were derived as if the starter did not exist. Learning how to calculate hydration in sourdough is mostly learning to move that 113 g to the right column.
One line does it. Hydration % = total water ÷ total flour × 100, where “total” means every gram of each that entered the bowl, starter included.
Split the Starter Before You Do Anything Else
A starter is flour and water in a fixed ratio, so any weight of it can be divided back into its two parts.
- Starter flour = starter weight ÷ (1 + starter hydration as a decimal)
- Starter water = starter flour × starter hydration as a decimal
A 150 g levain at 60% hydration, the stiff kind that feels like cold clay: 150 ÷ 1.6 = 93.8 g of flour, and 93.8 × 0.6 = 56.3 g of water. Those 150 g are 150 g of dough ingredients, not 150 g of magic.
At 100% hydration the split is the easy one. Half and half, every time. A 100 g scoop of starter is 50 g flour and 50 g water, and this is the case almost every sourdough recipe online assumes without saying so.
Read Your Feeding Ratio as a Hydration
You do not have to measure your starter’s hydration. It is set by how you feed it, and it stays set as long as you keep feeding the same way.
The ratio is written starter : flour : water. Hydration is the water part divided by the flour part.
| Feeding ratio | Starter hydration | Flour in 100 g | Water in 100 g |
|---|---|---|---|
| 1:2:1 | 50% | 66.7 g | 33.3 g |
| 1:2:2 | 100% | 50 g | 50 g |
| 1:3:3 | 100% | 50 g | 50 g |
| 1:5:5 | 100% | 50 g | 50 g |
| 1:1:2 | 200% | 33.3 g | 66.7 g |
Three of the four options in the Sourdough Calculator feeding tab are 100% starters, because doubling the flour and doubling the water leaves the ratio where it started. The 1:1:2 option is not. That one is a 200% starter, and 100 g of it carries 67 g of water and only 33 g of flour, which is a very different ingredient from the same 100 g of a 1:2:2 feed.
The starter portion in the ratio is the seed you carry over, not an ingredient you add, which is why 1:5:5 and 1:2:2 both land on 100%. Only the flour and water parts decide the hydration.
Build the Formula Backward From the Dough Weight
Working forward from scooped flour is how the double count happens. Working backward from the dough weight you want avoids it, because the starter has to come out of the totals rather than sit on top of them.
Total flour = dough weight ÷ (1 + hydration + salt percentage)
The salt belongs in the divisor because salt is weighed against flour and takes up room in the final weight. At 2% it moves the answer by about 11 g on a kilo of dough, small but not nothing.
That is the calculation running behind the bread formula tab on the Sourdough Calculator. Enter 1,000 g, 75%, 20% starter, 2% salt, and it prints the flour and water as totals, then subtracts what the starter brings and shows the two figures you actually weigh.
Here is the same loaf built on a 60% levain, written as a card you can tape to the jar:
| Line | Weight |
|---|---|
| Bread flour, total | 565 g |
| Water, total | 424 g |
| Starter at 60% (contains 70.6 g flour, 42.4 g water) | 113 g |
| Salt | 11.3 g |
| Flour to scoop (565 − 70.6) | 494 g |
| Water to pour (424 − 42.4) | 381 g |
Check the arithmetic before you mix: 494 + 70.6 = 564.6 g of flour, 381 + 42.4 = 423.4 g of water, and 423.4 ÷ 564.6 = 75.0%. The four things on the counter add to 999 g, which is the 1,000 g you asked for, minus rounding.
What the Shortcut Costs at Different Starter Hydrations
If you skip the split and divide only the flour you scoop into the water you pour, the answer drifts. Same 1,000 g target, same 75%, same 113 g of starter, five different starters:
| Starter hydration | Flour inside | Water inside | Flour to scoop | Water to pour | What the plain division reports |
|---|---|---|---|---|---|
| 50% | 75.3 g | 37.7 g | 489.6 g | 386.1 g | 78.8% |
| 60% | 70.6 g | 42.4 g | 494.4 g | 381.4 g | 77.1% |
| 100% | 56.5 g | 56.5 g | 508.5 g | 367.2 g | 72.2% |
| 125% | 50.2 g | 62.8 g | 514.8 g | 361.0 g | 70.1% |
| 200% | 37.7 g | 75.3 g | 527.3 g | 348.4 g | 66.1% |
Every column is a 75% dough. The last column spans 12.7 points, from 78.8% down to 66.1%, purely because of how the starter was fed.
There is a clean rule hidden in that table. When the starter’s hydration matches the dough’s hydration, the shortcut is exact, because the fraction of flour and water coming in is the same fraction you are aiming for. A 75% starter in a 75% dough needs no correction at all. Since most home starters run at 100%, the doughs that get misread are the 65% to 80% range, which is where nearly all sourdough baking happens.
The 90-Second Check After Mixing
Weigh the bowl empty, then weigh it with the dough in it and subtract. That number should match your formula. If it is more than 5 g off, something was not tared or a scoop was short.
If the dough is mixed and reading wrong, correct it in small steps:
- Too wet: add flour a tablespoon at a time, 8 to 9 g each, resting 15 minutes between additions. Taking a 565 g flour batch from 75% down to 72% means removing about 17 g of water, or adding roughly 24 g of flour, which is 3 level tablespoons.
- Too dry: add water a tablespoon at a time, 15 g each. A single tablespoon moves a 565 g batch by about 2.6 points.
Add flour to the dough, never to the bench, or you undo the correction at shaping.
Where the Count Goes Wrong
Starter measured by volume. Half a cup of 100% starter is roughly 120 g, but a bubbly starter and a deflated one differ by 10 to 15 g in the same cup, and that gap lands directly in your flour total. Weigh the starter.
Pouring off the hooch. The grey liquid on a neglected starter is water that separated out. Drain it and you have silently changed the starter’s hydration while the ratio on the jar still says 100%. Stir it back in.
A whole wheat or rye starter. Those flours absorb more water than white flour at the same percentage, so a formula that is arithmetically 75% will feel like 70%. The number is right. The flour is different.
Cold starter straight from the fridge. Temperature does not change hydration, but a 38°F starter drops the dough temperature by several degrees and pushes bulk fermentation from 3 hours to 5. Warm the water, not the starter, and use a dough temperature of 76°F as the target.
Salt left out of the divisor. Solving backward without the +0.02 gives 571 g of flour instead of 565 g, and the loaf comes out 6 g light on flour with 11 g of salt unaccounted for.
Try Our Free Tools
The Dough Hydration Calculator takes flour and water for a percentage, or flour and a target for the water you need in grams, ounces, pounds, or milliliters. The Sourdough Calculator does the full bread formula with the starter split out, and the Yeast Calculator sizes dry yeast when you swap the starter for commercial yeast. The Recipe Servings Calculator rescales the whole card when you want two loaves instead of one. Free, instant, no signup.
Hydration in sourdough is total water divided by total flour, and both totals include whatever the starter carried in. The exception is a starter fed at exactly the hydration you are targeting, where the worked formula and the lazy division give the same answer and you can skip the split entirely.