Baking guides

Preferment maths: poolish, biga and levain

A preferment is part of the dough, mixed hours ahead. Flavour is the reason to use one, but the maths is where most formulas go wrong: the flour and water inside the preferment still belong to the dough. This guide shows how to keep the numbers straight.

Last reviewed: 2026-10-25

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What a preferment is

King Arthur Baking describes a preferment as a mix of flour, water, a leavening agent and time, usually fermented for 12 to 16 hours before the final dough is mixed. It sorts preferments into four styles using two questions: is it liquid or stiff, and is it leavened with a sourdough culture or with commercial yeast?

Poolish

Equal weights of flour and water, so 100% hydration, with a small amount of yeast and no salt. King Arthur notes that how much yeast to use depends on the time and temperature available. A poolish ripening for about 8 hours needs more yeast than one left for 14 to 16 hours, and one in a 65 °F room needs more than one in a 75 °F room.

Biga

The Italian preferment. It can be stiff or loose (the loose version is 100% hydration), it contains no salt, and it can use as little as 0.1% yeast on the biga flour: 1 g of yeast for 1,000 g of biga flour. It ripens for at least several hours and up to 12 to 16 hours.

Pâte fermentée

Literally "fermented dough": a piece taken from a fully mixed batch, so it already contains flour, water, salt and yeast in that dough's proportions. It is kept for several hours or overnight and added to the next batch.

Levain and sourdough

A culture of wild yeasts and bacteria, with no commercial yeast. King Arthur gives a hydration range of 50% to 125% for levains. Unlike a poolish or biga, a sourdough culture can be kept going indefinitely: part of each ripe build is held back, before any salt is added, to seed the next one.

Prefermented flour: the number that describes a preferment

The most useful single figure is the share of the total flour that goes into the preferment, often called prefermented flour. 300 g of flour in a poolish, in a dough with 1,000 g of flour in all, is 30% prefermented flour. The amount to preferment is something the baker chooses for flavour, timing and dough strength. It is not a fixed standard, so a good formula states it rather than leaving you to assume it.

The other thing to state is which basis the percentages use. In the total (overall) formula, every flour in the recipe adds up to 100%, including the flour inside the preferment, and every other ingredient is a percentage of that total. The total formula is what tells you the bread's real hydration and salt level. The final dough is what actually goes into the mixer on the day: the ripe preferment plus the remaining flour, water, salt and yeast.

Worked example: turning a straight dough into a poolish dough

Start from a simple straight dough: 1,000 g flour, 700 g water (70%), 20 g salt (2%) and 5 g instant yeast (0.5%). We decide to preferment 30% of the flour as a poolish. The 30% and the 0.5% yeast are choices for this example. The method is the same for any numbers.

Step 1, the total formula. Nothing changes: the bread still contains 1,000 g of flour and 700 g of water. Step 2, the poolish. 30% of 1,000 g is 300 g of flour, and a poolish is 100% hydration, so it also gets 300 g of water and a small amount of yeast, here 0.3 g (0.1% of the poolish flour). Step 3, the final dough. Subtract what is already in the poolish from the total.

Check: (300 + 400) g water ÷ (300 + 700) g flour = 700 ÷ 1,000 = 70%. The overall hydration has not changed, which is exactly what you want.

If you list the poolish as a single ingredient against the final-dough flour only, it comes to 600.3 ÷ 700 = 85.8%. That number is correct but says little about the bread, which is why the total formula is the one to keep. King Arthur Baking shows the same idea at bakery scale: a formula whose poolish is 96% of the final-dough flour works out, overall, to 59.44 lb flour (100%), 38.64 lb water (65%), 1.19 lb salt (2%) and 0.74 lb yeast (1.25%).

Many bakers lower the final-dough yeast when a ripe preferment goes in, because the preferment brings active fermentation with it. The right amount comes from trial batches, so note the dough temperature and proof times and adjust the next batch.

Worked example: adding a levain

Take the same target bread, 1,000 g flour at 70% hydration and 2% salt, but leaven it with a 100% hydration levain made from 20% of the flour. The levain holds 200 g flour and 200 g water, 400 g in all. The final dough is the levain plus 800 g flour, 500 g water and 20 g salt.

The common mistake is to add a 400 g levain on top of a recipe that already has 1,000 g flour and 700 g water. The bread then really contains 1,200 g flour and 900 g water, which is 75% hydration, and the salt drops from 2% to 20 ÷ 1,200 = 1.7%. If your sourdough comes out slacker and blander than the formula promised, check this first.

A levain at a different hydration splits the same way. A 500 g levain at 60% hydration contains 500 ÷ 1.6 = 312.5 g flour and 187.5 g water.

Water temperature with a preferment

A ripe preferment brings its own temperature into the mix, so the desired dough temperature method gains a fourth factor: water = desired dough temperature × 4 − flour − room − preferment − friction. King Arthur Baking gives this example. For a 75 °F dough with 68 °F flour, a 68 °F room, a 71 °F preferment and a 26 °F friction factor, the water should be 300 − 68 − 68 − 71 − 26 = 67 °F.

The Celsius version: for a 25 °C target with 22 °C flour, a 23 °C room, a 21 °C poolish and a stand mixer that adds about 13 °C, the water should be 25 × 4 − 22 − 23 − 21 − 13 = 21 °C. The dough temperature calculator does this sum when you tick the preferment box, and the desired dough temperature guide explains how to measure your own friction factor.

Putting it in the calculator

To check a formula with the baker's percentage calculator, enter the total formula: all the flour as the flour weight, and all the water, salt and yeast as ingredients. The hydration it shows is then the real one. For the basics of the notation, see baker's percentages explained.

Frequently asked questions

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