Almost every sourdough recipe quotes a hydration figure, and almost every one of them quotes it wrong. A formula reading 400 g flour, 300 g water and 100 g starter gets described as 75 percent hydration, because 300 divided by 400 is 0.75. But that starter is not a seasoning — it is flour and water. At 100 percent starter hydration it carries 50 g of flour and 50 g of water, which makes the real totals 450 g of flour and 350 g of water, and the real hydration 77.8 percent. This sourdough hydration calculator pulls that hidden flour and water back out and reports the dough you are actually mixing.
Arb Digital built this as the sourdough-specific companion to a more general tool. Our baker's percentage calculator converts any recipe into flour-relative percentages and scales it up or down; that is the ratio system every bakery uses and it stays the right tool for a straight yeasted dough. This page owns the one thing that system does not handle on its own: a preferment that is itself part flour and part water, and therefore changes both sides of the hydration fraction at once. It also sizes the levain build and rescales bulk time for dough temperature.
What This Sourdough Hydration Calculator Does
It splits the starter into its two components using the starter hydration you give it, adds each component to the correct side of the recipe, and reports six things: true total flour, true total water, true hydration, the salt weight on total flour, the finished dough weight, and the naive hydration figure the recipe would have claimed. It then works backwards from the starter weight to a levain build — how much seed, flour and water you need to mix the night before to end up with exactly that much ripe starter and no waste.
Finally it rescales bulk fermentation time for temperature. It does not tell you how long to bulk your dough. It asks for a bulk time you have already observed in your own kitchen at a temperature you measured, then adjusts that figure for today's dough temperature using a standard rate-versus-temperature relationship. That keeps the number anchored to your flour, your starter and your kitchen rather than to a stranger's.
For a straight commercial-yeast dough the arithmetic is simpler and our pizza dough calculator handles it, including ball weights and pan sizing. If you are scaling a tin or a tray rather than a formula, the cake pan converter does area-based scaling, which is a different problem again.
How to Use It
- Enter the flour and water you actually weigh into the bowl. Exclude the starter from both — the tool adds its contribution itself. Double-counting here is the most common input error.
- Enter the starter weight and its hydration. If you feed equal weights of flour and water, that is 100 percent. If you feed a stiff levain at half water, it is 50.
- Set salt as a percentage of total flour. The tool applies it to total flour including the starter's flour, which is the convention, and returns a weight in grams.
- Choose a levain build ratio. Five means 1:5:5. A larger ratio means less seed and a longer ripening window; a smaller one ripens faster and carries more of the previous build's character.
- Give your own bulk reference, then today's dough temperature. The rescaled time is a planning estimate. Read the dough, not the clock.
The Formula and How It's Calculated
Starter hydration h is water as a fraction of the starter's flour. So a starter of weight S at hydration h contains flour equal to S ÷ (1 + h) and water equal to S − that flour. At 100 percent hydration the divisor is 2 and the split is exactly half and half; at 50 percent the divisor is 1.5, so two thirds of the starter is flour.
From there everything follows. Total flour is the flour you added plus the starter's flour. Total water is the water you added plus the starter's water. True hydration is total water divided by total flour, expressed as a percentage. Salt is total flour times the salt percentage. Final dough weight is total flour plus total water plus salt. Inoculation — the figure bakers use to describe how much starter a formula carries — is the starter's flour as a percentage of total flour, not the starter weight as a percentage of the flour you added.
Run the defaults. A starter of 100 g at 100 percent hydration is 100 ÷ 2 = 50 g flour and 50 g water. Total flour is 400 + 50 = 450 g; total water is 300 + 50 = 350 g. True hydration is 350 ÷ 450 = 77.8 percent, against a naive 300 ÷ 400 = 75 percent. Salt at 2 percent of 450 is 9 g. The dough weighs 450 + 350 + 9 = 809 g. Inoculation is 50 ÷ 450 = 11.1 percent.
The levain build works backwards. For a 1:5:5 build the total is 1 + 5 + 5 = 11 parts, so 100 g of ripe levain needs 100 ÷ 11 = 9.1 g of seed, 45.5 g of flour and 45.5 g of water. Change the starter hydration and the water part changes with it, so a stiff 60 percent levain built at a ratio of 5 uses five parts flour and three parts water.
Why the Error Grows With Inoculation
At 100 g of starter in a 400 g flour recipe, the gap between naive and true hydration is under three percentage points — annoying but survivable. The gap is not fixed, though. It scales with how much starter the formula carries, and it moves in a direction that surprises people.
Take the same 400 g of flour and 300 g of water but push the starter to 200 g at 100 percent hydration. Now the starter brings 100 g of flour and 100 g of water. Total flour is 500, total water is 400, and true hydration is 80 percent against the same naive 75. Double the starter again to 400 g and true hydration reaches 83.3 percent. The recipe never changed its stated hydration, but the dough got measurably wetter each time.
The direction flips if your starter is stiff. A 50 percent starter is two thirds flour, so adding more of it drags true hydration down, not up. This is exactly why two bakers can follow the same written formula, both keep a healthy starter, and end up with doughs that handle completely differently. One keeps a liquid levain and one keeps a stiff one, and the recipe never asked.
Bulk Time and Temperature
Fermentation is a biological rate process, and rate processes speed up as temperature rises. The rule of thumb bakers borrow from chemistry is the Q10 relationship: rate roughly doubles for each 10 degrees Celsius of warming, so time roughly halves. The tool applies exactly that, scaling your reference bulk time by 2 raised to the power of the temperature difference divided by ten.
With the defaults, a 4 hour bulk at 26 °C rescales for a 22 °C dough to 4 × 2^(4 ÷ 10) = 4 × 1.32 = 5.3 hours. That four-degree drop cost more than an hour, which is why the same recipe that finishes before dinner in August is still slack at bedtime in February.
Two honest caveats. First, the doubling rule is an approximation that holds reasonably across the temperatures a kitchen actually sees and breaks down at the extremes, where yeast and bacterial activity diverge and the dough sours faster than it rises. Second, the microbial community in a starter is not one organism. A review of sourdough starter practice published in the peer-reviewed literature, A review of sourdough starters: ecology, practices, and sensory quality with applications for baking, notes that hydration is defined as the percent of water to flour and that published work frequently fails to record even the refreshment ratio used — a sign of how much variation sits behind any single timing figure.
What Your Starter's Flour Actually Does
The flour in the starter is not inert filler that happens to be pre-wetted. It has been fermenting, sometimes for many hours, so its starch and protein are partly broken down by enzymes and its pH is low. Adding 100 g of ripe starter to a dough is not the same as adding 50 g of flour, 50 g of water and a pinch of culture.
The flour type matters too. Research tracking starters raised on ten different flours, published as Sourdough starters exhibit similar succession patterns but develop flour-specific climax communities, found that starters follow comparable succession stages regardless of flour but settle into different mature bacterial communities depending on what they were fed, producing different aromas. A rye starter and a white-flour starter at identical hydration will not behave identically in the same formula, and no arithmetic on this page captures that.
Practically, this means the hydration number is a description of the water in the dough, not a promise about how the dough will feel. Whole grain and high-extraction flours absorb far more water than white bread flour, so an 80 percent whole-wheat dough can handle more easily than a 72 percent white one. Use the true hydration figure to compare your own bakes with each other over time, which is what it is genuinely good for.
Salt, and Why It Sits on Total Flour
Salt percentage is quoted on total flour by convention, and the tool follows that. It matters because the starter's flour is often ten to twenty percent of the total. Calculating 2 percent salt on the added flour alone gives 8 g in our worked example instead of 9 — a twelve percent shortfall in salt, which is enough to notice in both flavour and dough strength.
Salt slows fermentation as well as seasoning the loaf, so an under-salted dough ferments faster than the schedule expects and can be past its best before the timer goes. If you want to convert a salt weight into a sodium figure for a nutrition panel, our salt to sodium converter does that conversion and cites the factor. Ratio problems of the same family turn up outside baking too — the rice water ratio calculator is the same kind of arithmetic applied to a grain and its absorbed liquid.
Arb Digital designs and builds calculators that people actually return to — fast, self-contained and search-visible.
See Web Design Services Talk To Our TeamCommon Mistakes to Avoid
- Counting the starter's flour twice — enter only what you weigh into the bowl, and let the tool add the starter's share once.
- Guessing your starter hydration — if you have never measured your feed, weigh one refreshment. Everything on this page hangs on that percentage.
- Taking salt on added flour instead of total flour — it under-salts by roughly the inoculation percentage, which also speeds fermentation.
- Comparing hydration figures across recipes without normalising — two recipes both labelled 75 percent can differ by several real points once the starters are resolved.
- Treating the rescaled bulk time as a rule — it rescales your own observation, and dough readiness is judged by the dough.
Related Free Tools From Arb Digital
Convert a whole formula to flour-relative ratios with the baker's percentage calculator, size and portion a yeasted dough with the pizza dough calculator, scale a recipe between tins with the cake pan converter, turn a salt weight into a sodium figure with the salt to sodium converter, or handle grain and liquid ratios with the rice water ratio calculator. The free online tools hub lists every food and baking tool we publish.
Frequently Asked Questions
Split the starter into flour and water using its own hydration, add each part to the recipe's flour and water, then divide total water by total flour. A 100 g starter at 100 percent hydration contributes 50 g of flour and 50 g of water.
Because most recipes divide added water by added flour and ignore the starter entirely. A liquid starter carries as much water as flour, so including it raises the true figure, and the gap grows as the formula carries more starter.
It is the water in the starter expressed as a percentage of the flour in the starter. Feeding equal weights of flour and water gives 100 percent. A stiff levain fed half as much water as flour is 50 percent.
On total flour, including the flour inside the starter. Using added flour alone under-salts the dough by roughly the inoculation percentage, which affects both flavour and the speed of fermentation.
One part seed starter to five parts flour and five parts water by weight, eleven parts in total. To produce 100 g of ripe levain you would use about 9 g of seed, 45 g of flour and 45 g of water.
As an approximation, the rate roughly doubles for every 10 degrees Celsius of warming, so the time roughly halves. A four-degree drop from 26 to 22 stretches a four-hour bulk to about five and a quarter hours.
It lowers it. A 50 percent starter is two thirds flour by weight, so adding more of it puts more flour than water into the dough and pulls the true hydration figure down.
The arithmetic is identical, but the handling is not. Whole grain and high-extraction flours absorb considerably more water than white bread flour, so the same hydration percentage produces a much firmer dough.
This page performs recipe arithmetic for general information. Fermentation times depend on your own flour, starter and kitchen, and the timing figure here only rescales an observation you supply. Follow standard food-hygiene practice for the ingredients and equipment you use.
