Convert a Yeast Recipe to Sourdough: Arithmetic and Timing

You have a bread recipe you actually like. Maybe it is your grandmother’s sandwich loaf, or the focaccia from a magazine clipping, or the cinnamon rolls that come out right every Christmas. It calls for a 7 g packet of instant yeast, and you have a jar of bubbling starter on the counter that you feel faintly guilty about.

So you want to convert a yeast recipe to sourdough, and the internet gives you a one-liner: use 100 g of starter per 500 g of flour. That rule is fine. It is also about a third of the job, and the two-thirds nobody mentions are exactly where people’s first conversion goes wrong.

Here is the problem in one sentence. Starter is not a leavening powder, it is dough. Drop 100 g of it into a recipe and you have just added 50 g of flour and 50 g of water that the recipe did not ask for, which pushes your hydration up, dilutes your salt, and gives you more dough than your tin can hold.

The other missing piece is time. A yeasted bulk of 90 minutes becomes four to eight hours, and the correct number depends on your kitchen temperature far more than on your recipe.

This post covers both, arithmetic shown in full. Steal the table.

The short answer

To convert a yeasted recipe to sourdough, replace the yeast with 100 g of 100 percent hydration starter for every 500 g of flour in the recipe, then subtract that starter’s own contents from the recipe: take 50 g off the flour and 50 g off the water. Salt stays exactly where it was. Everything else stays put.

That keeps your hydration and your salt percentage identical to the original, which means the dough handles the way the recipe author intended. Then rewrite the timing: a 60 to 90 minute yeasted bulk becomes roughly four to eight hours depending on dough temperature, and the final proof becomes 1.5 to 3 hours at room temperature or 8 to 16 hours in the fridge.

Judge by dough state, not the original clock times. For a bulk ferment, that means a 50 to 75 percent rise in volume.

The rule of thumb, and what it leaves out

The standard substitution is 20 percent of the recipe’s total flour weight, as 100 percent hydration starter. On 500 g of flour that is 100 g of starter. On 800 g it is 160 g.

Notice what that rule is scaled to. Flour, not yeast. This trips people constantly, because they look at the 7 g packet and try to find an equivalent weight of starter, and there is no such thing: that same 7 g packet might be raising 400 g of flour in one recipe and 1,200 g in another. The yeast quantity in the original tells you almost nothing except how impatient the author was.

You will see the same figure written three different ways, and they are all the same number:

• 20 percent starter on flour weight
• 100 g starter per 500 g flour
• 10 percent prefermented flour, since half of that 100 g is flour

The last one is what professional bakers usually mean, and it is the version that survives a change of starter hydration. Hold onto it, because we come back to it.

Around that 20 percent figure there is a workable range of roughly 10 to 30 percent. Less starter means a slower ferment and, in most people’s experience, a more acidic loaf. More means faster and milder. Neither is correct. Pizza dough heading into the fridge for two days wants 5 to 10 percent; cinnamon rolls you need by tea time want 25 to 30.

The arithmetic, worked end to end

Say Ade has a sandwich loaf recipe he has made for years. Here it is, with baker’s percentages added.

Hydration is water divided by flour: 325 ÷ 500 = 0.65, so 65 percent.

Step one, the starter quantity

Twenty percent of the flour weight: 0.20 × 500 = 100 g of starter, maintained at 100 percent hydration.

Step two, split the starter into flour and water

A 100 percent hydration starter holds equal weights of flour and water. So 100 g of it is 50 g flour plus 50 g water. Nothing else. It is dough that has been fermenting, and every gram of it came from one of those two ingredients.

Step three, subtract

Take the starter’s flour out of the recipe’s flour, and the starter’s water out of the recipe’s water.

• Flour to weigh out: 500 − 50 = 450 g
• Water to weigh out: 325 − 50 = 275 g
• Salt: unchanged at 10 g
• Instant yeast: gone

The whole thing side by side

This is the table worth bookmarking. The middle column is what most people do on their first attempt.

Look at what the naive swap did. Hydration went up 3.2 points, which on a sandwich dough separates a loaf that shapes tidily from one that sags. Salt dropped from 2.0 to 1.8 percent, which you can taste and which slightly speeds fermentation, since salt restrains yeast. And the dough got 93 g heavier, enough to have a 2 lb (900 g) tin loaf mushrooming over the rim.

Checking your work

Always run the check. Total flour should be 450 + 50 = 500 g, total water 275 + 50 = 325 g, giving 325 ÷ 500 = 65 percent. The finished dough should come out 7 g lighter than the original, because that is the yeast you removed. If the numbers do not close like that, there is a slip somewhere. The percentage calculator will do the divisions.

The general formula, for any starter hydration

Not everyone keeps a 100 percent starter. Stiff levains at 50 to 60 percent hydration are common, particularly for enriched doughs, and some people run 125 percent because they like a pourable starter.

For a starter weight S at hydration h, written as a decimal (100 percent is 1.0, 50 percent is 0.5):

• flour in the starter = S ÷ (1 + h)
• water in the starter = S × h ÷ (1 + h)

At 100 percent hydration that gives you S ÷ 2 for each, which is the shortcut everyone uses. At 50 percent it gives you two-thirds flour and one-third water.

Take Ade’s loaf again, but with a stiff 50 percent levain. To keep the same 10 percent prefermented flour, he needs 50 g of flour to come from the levain, so:

• S ÷ 1.5 = 50, therefore S = 75 g of stiff levain
• Flour in it: 75 ÷ 1.5 = 50 g
• Water in it: 75 × 0.5 ÷ 1.5 = 25 g
• Flour to weigh out: 500 − 50 = 450 g
• Water to weigh out: 325 − 25 = 300 g

Check: total flour 500 g, total water 300 + 25 = 325 g, hydration 65 percent. Same dough, same handling, different levain. This is why the prefermented-flour framing is more useful than “20 percent starter”, because it survives a change of maintenance routine.

If the hydration side of this feels shaky, the hydration walkthrough takes baker’s percentage apart from first principles and shows why the levain’s contents count.

Rewriting the timing, which is the harder half

The arithmetic takes five minutes. The schedule takes a few bakes to learn, because a yeasted recipe’s timings are useless to you now.

Commercial yeast at 1 to 2 percent gives you a bulk of 60 to 90 minutes in a warm kitchen. A 20 percent levain does the same job in four to eight hours, and the spread between those numbers is almost entirely temperature.

Those are starting windows for a white-flour dough at 20 percent inoculation, not guarantees. Halve the starter to 10 percent and every row stretches by half again. Add 20 percent wholemeal and it speeds up, because whole grain ferments faster.

The rule of thumb behind the table is that fermentation rate roughly doubles for every 15-17°F (8-9°C) of extra dough temperature. It is an approximation borrowed from general reaction kinetics, it breaks down at the extremes, and I would use it to sanity-check a schedule rather than trust it to the minute. Nobody has a clean equation here, because your culture, your flour and your fridge all differ from mine.

The final proof

Yeasted recipes typically say “until doubled”, which for sourdough is too far. In a tin, stop when the dough crowns about a centimetre above the rim. For a free-standing loaf, the poke test still works: a floured fingertip pressed half an inch in should spring back slowly and not quite completely.

Room temperature proof runs 1.5 to 3 hours. A cold retard at 38-40°F (3-4°C) runs 8 to 16 hours and buys you flavour plus a dough that scores cleanly.

Dough temperature is a lever, not a fact

You can control the temperature you start at. The standard formula for a hand-mixed dough containing a levain is:

water temperature = (4 × desired dough temperature) − (flour temp + room temp + levain temp)

Say Ade wants a dough temperature of 78°F (25.5°C). His flour and kitchen are both at 70°F (21°C) and his levain is at 74°F (23.5°C). Then his water should be 4 × 78 = 312, minus (70 + 70 + 74 = 214), which is 98°F (37°C).

Two honest caveats. Different books use three or four factors depending on whether there is a preferment, and stand mixers add a friction factor of a few degrees that hand mixing does not. And if the formula ever tells you to use water above about 110°F (43°C), warm your flour instead, because pouring genuinely hot water onto your levain is a good way to damage it.

Working all this out on paper for every bake is tedious, which is precisely why the Sourdough Proofing Scheduler exists. Give it your dough temperature and your starter percentage and it returns the bulk window and the timings around it, so a converted recipe stops being an experiment and becomes something you can plan a Saturday around. If you are converting several recipes and want the oven slots laid out too, the Bread Bake Timeline Scheduler handles the overlapping bakes.

Convert any recipe in nine steps

1. Write the recipe out in grams. If it is in cups, weigh a cup of your own flour first, because a cup of all-purpose lands anywhere between about 120 g and 150 g depending on how you scoop, and that 20 percent swing wrecks every percentage that follows.
2. Add up the total flour, including any whole wheat, rye or semolina.
3. Multiply that total by 0.20 to get your starter weight. Round to something you can actually weigh.
4. Split the starter into flour and water using S ÷ (1 + h) and S × h ÷ (1 + h).
5. Subtract the starter’s flour from the recipe’s flour, and the starter’s water from the recipe’s water. If the recipe uses milk rather than water, subtract from the milk.
6. Delete the yeast. Leave salt, sugar, fat, eggs and everything else untouched.
7. Check: total flour and total water should match the original, and the dough weight should be lighter by exactly the weight of yeast you removed.
8. Replace the rise times using the temperature table above, or let the proofing scheduler work them out, and pick your bulk end point by rise rather than by clock.
9. Bake exactly as the original said, unless the original was baked in under 20 minutes, in which case check colour early.

Enriched doughs: butter, egg and sugar all slow it down

Fat coats gluten strands and slows both development and gas retention. Sugar competes with the yeast for water and, above a certain concentration, pulls water out of the cells by osmosis. Eggs bring fat, water and emulsifiers at once. Put all three in a dough and a culture that raises a lean loaf in five hours may need nine.

Rough thresholds, from experience rather than any published limit: below about 10 percent sugar and 10 percent fat on flour weight, treat it as a normal dough that runs slow. Between 10 and 25 percent, raise the inoculation to 25 to 30 percent, ferment at 78-82°F (26-28°C), and expect two to three times the lean-dough timing. Above 25 percent of either, you are in brioche territory and sourdough alone gets genuinely difficult.

Three things help. Develop the gluten before the butter goes in, adding it in pieces once the dough is smooth. Use a stiff levain at 50 to 60 percent hydration, which tends milder and less acetic. And bulk somewhere warm, because enriched doughs punish cold kitchens hardest.

A cinnamon roll conversion

Original: 500 g flour, 240 g whole milk, one large egg at 50 g, 60 g butter, 60 g sugar, 9 g salt, 7 g instant yeast.

Starter at 25 percent of flour, because it is enriched: 125 g at 100 percent hydration, holding 62.5 g flour and 62.5 g water.

• Flour to weigh out: 500 − 62.5 = 437.5 g, call it 438 g
• Milk: this is where bakers argue. Milk is roughly 87 percent water, so replacing 62.5 g of water strictly means removing 62.5 ÷ 0.87 = 71.8 g of milk, call it 72 g. Milk goes from 240 g to 168 g. The simpler approach subtracts 62.5 g straight, giving 178 g. The difference is 10 g on a 900 g dough, and honestly, in an enriched dough you will not feel it.
• Everything else unchanged, yeast removed.

Bulk at 78°F (26°C) for around six hours, or bulk for three and retard the shaped rolls overnight, which is what I would do if you want them for breakfast.

What converts well and what fights you

That last row matters more than it looks. A recipe leavened by bicarbonate is not a fermentation recipe, so starter goes in as an acidic flavouring rather than a raising agent, and the arithmetic you want there is the discard swap rather than this one.

On hybrids, since it comes up in every forum thread: around 0.5 percent instant yeast alongside your levain gives you most of the flavour with a rise you can schedule. A vocal part of the community calls this cheating. That position is aesthetic rather than technical, and a hybrid that gets cinnamon rolls on the table on a Tuesday beats a purist recipe you never make.

Flavour, and how to dial the sourness

Converted recipes usually taste less sour than people expect, which disappoints some bakers and relieves others.

The broad picture, which is well established even if the details are messy: warmer and faster fermentation favours lactic acid, which reads as mild, creamy, yoghurt-like. Cooler, longer, stiffer conditions favour acetic acid, which reads as sharp and vinegary. So a long cold retard sharpens a loaf, a warm five-hour bulk keeps it mellow, and whole grain or rye pushes acidity up in both directions because it brings more minerals and more microbial food.

Where I would be careful is in treating any of that as a dial with numbers on it. Two bakers running identical schedules with identical flour can get noticeably different results, because their jars hold different populations. The mechanisms are real. The precision people claim for them is not.

Want it sourer? Retard longer, build the levain from a starter you have not fed for a day or two, and add 10 to 20 percent rye. Want it milder? Build the levain that morning, keep it warm, bake the same day.

What goes wrong

The biggest one is doing the swap and not the subtraction, which you now know about. Second is scaling the starter up to speed things along: doubling to 40 percent inoculation does shorten the bulk, but it also brings a lot more acid into the dough at the start, and acidic dough degrades gluten faster. Slack, sticky, hard to shape, collapses in the oven. If you need it faster, raise the temperature rather than the starter.

Then there is the “until doubled” instruction inherited from the original recipe. A yeasted tin loaf can double happily. A sourdough one that has doubled in the tin usually has nothing left and bakes flat with a pale crust. Stop earlier than feels right. If you have already made this mistake and want to work out which direction you overshot in, the under-proofed versus over-proofed diagnosis reads the crumb backwards for you.

Assuming your starter’s hydration is a fact when you actually feed it by eye is another quiet one. If you tip in flour until it looks right, your “100 percent” starter might be 90 or 115 percent, and your careful arithmetic is built on a guess. Weigh the feed for a couple of cycles first.

Two smaller traps. Salt and sugar dumped straight onto the levain sit against it undissolved and stress the culture, so disperse them in the water. And a recipe given in volumes fails on the flour alone, so weigh a cup of your own flour before anything else, or use the unit converter and accept it is approximate.

Finally, the culture itself. A starter that manages a sluggish 40 percent rise in its jar will not raise a converted enriched dough, and no arithmetic fixes that.

Frequently asked questions

How much sourdough starter replaces one packet of instant yeast?

Do not scale from the yeast weight, scale from the flour. Use roughly 20 percent of the recipe’s flour weight as 100 percent hydration starter, which is 100 g of starter per 500 g of flour. A 7 g packet of instant yeast could be raising 500 g of flour or 1 kg, so it tells you nothing useful about how much starter you need.

Do I need to change the flour and water when I add starter?

Yes, and skipping it is the most common mistake. Starter is roughly half flour and half water by weight at 100 percent hydration, so 100 g of it brings 50 g of each. Subtract those from the recipe’s flour and water, otherwise your hydration and your dough weight both drift upwards and your salt percentage drops.

How long does a converted sourdough recipe take to rise?

Expect the bulk ferment to run four to eight hours at typical kitchen temperatures instead of the 60 to 90 minutes a yeasted recipe wants. At 68°F (20°C) it can stretch past ten hours. Judge by rise, aiming for 50 to 75 percent growth in a straight-sided container, rather than by the clock in the original recipe.

Can I convert enriched doughs like cinnamon rolls and brioche?

Cinnamon rolls convert well. Heavily buttered brioche fights you, because fat coats the gluten and sugar competes for water, so fermentation crawls. For enriched doughs, raise the starter to 25 to 30 percent of flour weight, ferment warmer at around 78-82°F (26-28°C), and accept that it takes two to three times as long as the lean equivalent.

Can I use sourdough starter and a little yeast together?

You can, and it is a genuinely useful technique for enriched or time-pressured bakes. Around 0.5 percent instant yeast on flour weight alongside your levain gives you sourdough flavour with a predictable rise. Purists dislike it and will say so. I use it for cinnamon rolls on a weeknight and lose no sleep over it.

Will my converted recipe taste sour?

Less than you expect, unless you push it. A warm, fairly quick fermentation with a healthy starter tastes mildly tangy and wheaty rather than sharp. Sourness comes mainly from long cold retards, stiffer starters, cooler temperatures and whole grain flours, so you can dial it up or down deliberately rather than accepting whatever you get.

Your first conversion

Pick the easiest recipe you own, not the one you care about most. A plain sandwich loaf or a focaccia will show you the timing shift without punishing you for it, and enriched dough on your first attempt will just teach you that sourdough is slow.

Do the arithmetic before you touch anything, check that total flour and total water match the original, and write the corrected recipe onto the card next to the old one. Bake it on a day when you are home, because the first attempt is really a temperature measurement dressed up as a loaf.

The second bake is where conversion actually happens. You will know how your kitchen behaved, and you can move the bulk end point by an hour in whichever direction the crumb asked for.