Why your starter changes the hydration you think you have
Hydration in bread baking is one number: total water divided by total flour, expressed as a percentage of the flour. The trouble is that a sourdough starter is already a mix of both, so it lands on both sides of that fraction at once. Weigh out 800 g of flour and 520 g of water and you might call the dough 65% hydration. Add 200 g of a 100% starter and the real figure is 68.9%, because the starter quietly delivered another 100 g of flour and another 100 g of water.
Four percentage points is not a rounding error. It is the difference between a dough you can shape on a bare bench and one that needs a bench scraper and a wet hand. It is also why the same formula, copied from a book, behaves differently in two kitchens: one baker keeps a 100% starter, the other keeps a stiff 55% levain, and neither of them corrected for it.
The correction is arithmetic, not judgement. Any starter at hydration h is h parts water for every 1 part flour, so a mass S of it contains S/(1+h) of flour and the rest is water. Once you have those two numbers you can add them into the totals and the whole formula behaves like a straight dough again.
The same split matters for a second figure professionals track: prefermented flour, the share of the formula's flour that has already been through a fermentation. That number, far more than the starter's raw weight, predicts how fast the bulk will move and how sour the crumb will taste.
The formula, and why it splits the starter in two
Start with the definition of hydration inside the starter itself. If the starter holds flour f and water w, then h = w/f, and its total mass is S = f + w = f(1 + h). Rearranging gives you the two pieces:
- Flour in the starter = S / (1 + h)
- Water in the starter = S − S/(1 + h) = S·h/(1 + h)
Note that h is a decimal here. A 100% starter is h = 1, so the divisor is 2 and the starter is exactly half flour. A 50% starter is h = 0.5, the divisor is 1.5, and two thirds of the mass is flour. A 125% starter is h = 1.25, the divisor is 2.25, and only 44.4% of the mass is flour.
Now push both pieces into the totals. Total flour is the flour you weighed plus the starter's flour; total water is the water you poured plus the starter's water. Divide the second by the first and multiply by 100, and you have the hydration you should quote.
Solving for a target works the same way in reverse, with one useful property: adding or removing water does not change the flour side of the fraction. So the flour total is fixed the moment you decide how much starter to use, and the water you need is simply target × total flour. Subtract the water already present and you have the adjustment. If that adjustment is larger in magnitude than the water you were going to pour, the target is unreachable at that starter weight, and the calculator says so rather than handing you a negative pour.
It is worth writing down how large the error is if you skip all this, because it comes out exact rather than as a rule of thumb. Call the naive figure W/F and the true one H. Then H − W/F = (starter flour ÷ total flour) × (h − W/F): the prefermented-flour fraction multiplied by how much wetter the starter is than the flour and water you weighed. Three consequences follow. The correction vanishes when the starter happens to sit at exactly the hydration of the weighed-out portion; it grows in proportion to how much starter you use; and it changes sign when the starter is stiffer than the dough, so a stiff levain makes the naive figure too high rather than too low.
Salt sits outside the fraction entirely. It is quoted as a percentage of total flour and it contributes to dough weight, but it never appears in the hydration calculation. Neither does oil, honey, or an egg — those are separate percentages, and bakers who fold them into hydration end up with formulas nobody else can reproduce.
Worked example: 800 g flour, 520 g water, 200 g of 100% starter
This is the calculator's default, and every figure below can be checked on paper.
- Convert the starter hydration. 100% becomes h = 1.00, so the divisor is 1 + 1.00 = 2.00.
- Split the starter. Flour = 200 ÷ 2.00 = 100 g. Water = 200 − 100 = 100 g.
- Total the flour. 800 + 100 = 900 g. This is the 100% baseline for every other percentage in the formula.
- Total the water. 520 + 100 = 620 g.
- Divide. 620 ÷ 900 = 0.68889, so the dough is at 68.9% hydration — not the 65.0% you get from 520 ÷ 800.
- Prefermented flour. 100 ÷ 900 = 11.1% of the formula flour has already fermented.
- Salt at 2%. 0.02 × 900 = 18 g. Note that this is 2% of 900, not of 800 — using the wrong baseline costs you 2 g of salt.
- Total dough. 900 + 620 + 18 = 1,538 g, which you can check against the ingredients you actually weighed: 200 + 800 + 520 + 18 = 1,538 g.
Now suppose you want 75%. The flour side is locked at 900 g, so you need 0.75 × 900 = 675 g of total water. You already have 620 g, so the change is 675 − 620 = +55 g, and the water you pour goes from 520 g to 575 g. Nothing else in the formula moves.
Run it the other way for a stiff levain. Swap the 200 g of 100% starter for 200 g of a 50% levain: flour = 200 ÷ 1.5 = 133.3 g, water = 66.7 g. Total flour becomes 933.3 g, total water 586.7 g, and hydration drops to 62.9%. Same starter weight, same recipe, six percentage points of difference.
How to read the number you get
Read overall hydration as a handling forecast first and a crumb forecast second. Below about 65% you have a dough that holds a shape on the bench and takes a firm hand; between 70% and 78% you are in the range most open-crumb country loaves live in, and you will want coil folds rather than kneading; above 82% the dough starts to behave like a batter unless the flour is strong and the bulk is well managed. Those bands are rules of thumb, and they shift with flour protein, milling, and how much whole grain is in the mix — whole wheat and rye absorb far more water than white flour at the same stated hydration, which is why an 80% whole-wheat dough can feel drier than a 72% white one.
Read prefermented flour as a clock. At 10–15% you have a conventional overnight bulk. At 20–30% the dough moves noticeably faster and the flavour leans more sour. Above 40% you are effectively baking a very large levain, and unless the room is cool the dough can overproof before you have finished shaping. When you change starter weight, the prefermented flour figure moves and your timings must move with it.
Read the water adjustment as an instruction about the pour, not about the dough. It never changes total flour, salt weight, or prefermented flour percentage — those depend on the starter and flour you already committed to. If you want to hold hydration constant while changing starter weight, adjust the added water using this figure and leave everything else alone.
One caution about comparing your number to a published one: many recipe writers quote hydration excluding the starter, exactly the way this calculator's information note reports it. When a formula from a book seems to produce a wetter dough than expected, check which convention the author used before you blame the flour. If you are also building the levain from a feeding ratio, the sourdough levain build calculator gives you the seed, flour and water for the build and hands back the same prefermented-flour figure.
Flour and water in 100 g of starter at common hydrations
| Starter hydration | Flour per 100 g | Water per 100 g | Flour share |
|---|---|---|---|
| 50% (stiff levain) | 66.67 g | 33.33 g | 66.7% |
| 60% | 62.50 g | 37.50 g | 62.5% |
| 75% | 57.14 g | 42.86 g | 57.1% |
| 80% | 55.56 g | 44.44 g | 55.6% |
| 100% (equal weights) | 50.00 g | 50.00 g | 50.0% |
| 125% | 44.44 g | 55.56 g | 44.4% |
| 150% (liquid levain) | 40.00 g | 60.00 g | 40.0% |
Each row is 100 ÷ (1 + h) for flour and the remainder for water, with h the hydration as a decimal.
Mistakes that put hydration off by five points
- Dividing water by flour and stopping there. The single most common error, and the size of it is exact rather than approximate: the gap between the true figure and W÷F equals the prefermented-flour fraction multiplied by (starter hydration − the naive figure). In the worked example that is 0.111 × (100% − 65%) = 3.9 points. Watch the sign — a starter wetter than the dough means you were understating hydration, a stiffer one means you were overstating it.
- Treating the starter as pure water. The opposite mistake, and just as wrong. Half of a 100% starter is flour, and that flour raises the denominator.
- Quoting starter hydration as water divided by total starter weight. A 100% starter is 50% water by total mass. If you enter 50 here you have described a stiff levain and every downstream number shifts.
- Taking salt as a percentage of the flour you weighed out. Salt is a percentage of total flour. In the worked example that is 900 g, not 800 g.
- Forgetting that whole grain drinks more. Hydration is a weight ratio, not a measure of how wet a dough feels. A 75% whole-rye dough and a 75% white dough are not comparable at the bench.
- Ignoring bench and shaping flour. Dust flour is not in the formula, but a heavy hand at shaping can add a percent or two of flour to the finished loaf.
- Changing starter weight without re-running the numbers. Doubling the levain raises hydration if the starter is wetter than the dough, and lowers it if the starter is stiffer. Check the direction rather than assuming.
Key terms
- Baker's percentage
- A formula convention in which every ingredient is expressed as a percentage of total flour weight, and total flour is always 100%. Percentages therefore sum to well over 100.
- Hydration
- Total water divided by total flour, as a percentage. Milk, eggs and other wet ingredients are quoted separately unless you deliberately account for their water content.
- Levain
- The specific build of starter made for one bake, as distinct from the mother starter you maintain. Its hydration need not match the mother's.
- Prefermented flour
- The share of the formula's total flour that spent time in a preferment (starter, levain, poolish or biga) before mixing. It is the clearest single predictor of fermentation speed.
- Stiff levain
- A levain at roughly 50-60% hydration. It ferments more slowly, produces more acetic acid, and keeps its strength longer than a liquid one.
Where this fits with the rest of your formula work
Hydration is one of four numbers that decide how a sourdough behaves; the others are prefermented flour, dough temperature, and time. This calculator gives you the first two. For the third, the desired dough temperature calculator tells you what temperature water to mix with so the dough leaves the mixer at the number your schedule assumes — and since that calculation needs your total water weight, run the hydration first and feed the result in.
If you are converting a yeasted formula to sourdough, or the reverse, you also have to deal with the leavening itself. The yeast conversion calculator handles swaps between fresh, active dry and instant yeast and reports the dose as a baker's percentage, which is the form you need to compare it against a levain.
Pizza is the one case where hydration is not the number that decides portioning. There you scale by dough loading per unit of pan area instead, which the pizza dough thickness factor calculator works out; hydration still governs how the dough handles, but ball weight comes from area.
Two limits worth stating plainly. First, this calculator assumes your starter is at the hydration you say it is. Starters drift — a jar fed by eye rather than by scale can be twenty points off. If you want the number to be trustworthy, feed by weight for two or three cycles before you rely on it. Second, hydration says nothing about absorption. Two flours at the same protein can differ by five percentage points in how much water they will take, and the only way to find your flour's ceiling is to bake at increasing hydration until the dough stops holding. Use the calculated figure as a repeatable reference point, not as a target handed down from someone else's kitchen.