Sourdough hydration math: the only formulas a home baker needs

Sourdough is a hobby that has accumulated more jargon than it needs. The math underneath the jargon is genuinely simple — three formulas that you'll use over and over, and once you've internalized them, you can read any sourdough recipe and reverse-engineer it inside a minute.

Formula 1: dough hydration

Dough hydration is the water-to-flour percentage, by weight. Add up all the water in the dough. Add up all the flour in the dough. Divide water by flour. Multiply by 100. That number is the hydration.

For a country loaf with 500 g flour and 375 g water, hydration is 375 ÷ 500 = 0.75, or 75 percent. This tells you what kind of bread you're making before you do anything else. 65–70 percent is a stiff dough that kneads into a smooth ball. 72–78 percent is a standard country loaf — tacky, holds shape after the bench rest but slumps if it sits. 80+ percent is the territory of ciabatta, focaccia, and high-hydration sourdoughs that need a Dutch oven to hold their shape during the bake.

A common confusion: many sourdough recipes report hydration excluding the levain (more on that in a moment). Some report it including. The difference can be 5 percentage points. When a recipe says "75 percent hydration," ask whether that includes the starter.

Formula 2: starter is half flour, half water

A 100-percent-hydration starter (the default for nearly every sourdough recipe published in the last decade) is, by weight, exactly half flour and half water. So 200 g of starter contributes 100 g of flour and 100 g of water to your dough's totals.

This matters because most recipes specify the levain (the active starter you mix into the dough) separately from the dough's flour and water. If you want to calculate the dough's true hydration, you have to add the levain's flour and water to the dough's flour and water, then redo the percentage.

Example: a recipe calls for 500 g flour, 350 g water, and 100 g levain (100 percent hydration).

• True flour total: 500 + 50 = 550 g (the levain contributed 50 g of flour)
• True water total: 350 + 50 = 400 g (the levain contributed 50 g of water)
• True hydration: 400 ÷ 550 = 72.7 percent

The recipe's headline "70 percent hydration" figure was calculated excluding the levain. The actual dough behavior corresponds to 72.7 percent. Both are valid. Knowing the convention matters because if you swap starter weights or change starter hydration, the dough behaves differently and you need to know which way.

Formula 3: leaven percentage

The amount of levain in a dough is usually expressed as a percentage of the total flour. A 20 percent leaven means 20 g of starter per 100 g of total flour. That's a relatively high inoculation — the dough ferments fast (3–4 hours of bulk at 75 °F). A 10 percent leaven is moderate (5–6 hours of bulk). A 5 percent leaven is what you'd use for an overnight retard — the slow ferment develops more sour notes.

To swap inoculation levels, scale the levain weight and slightly reduce the flour and water in the rest of the dough to keep the hydration constant. The arithmetic is the trickiest of the three formulas because changing the levain changes both the flour and water totals.

Putting it together: building a recipe from scratch

You want to make a 900 g country loaf at 75 percent hydration with a 15 percent leaven inoculation.

A baked loaf is about 90 percent of the unbaked dough weight (the rest is oven moisture loss). So unbaked dough weight is about 1000 g.

Total dough is roughly 1000 g, composed of:

• Flour (100 percent)
• Water (75 percent of flour)
• Salt (2 percent of flour)
• Levain (15 percent of flour)

The percentages sum to 100 + 75 + 2 + 15 = 192. Total dough = flour × 1.92. So flour = 1000 ÷ 1.92 = 521 g.

From flour, every other line falls out: - Flour: 521 g - Water: 391 g (521 × 0.75) - Salt: 10 g (521 × 0.02) - Levain: 78 g (521 × 0.15)

Total: 1000 g. The loaf will bake to roughly 900 g. The math checks.

But that's hydration calculated excluding the levain. Including the levain (which is 39 g of flour and 39 g of water): - True flour: 521 + 39 = 560 g - True water: 391 + 39 = 430 g - True hydration: 430 ÷ 560 = 76.8 percent

So your "75 percent" loaf is really a 76.8 percent loaf. For most home bakers that's well inside the margin of error — your kitchen humidity has more effect on the dough than 1.8 percent of water does. But knowing the math means you can adjust precisely if you ever need a specific target hydration.

Why this matters past the math

Sourdough's reputation for being finicky comes mostly from variables that are not in the math: kitchen temperature, starter strength, flour brand, fermentation time, shaping technique. The math itself is solid. If you can calculate the three formulas above, you can take any sourdough recipe — from a magazine, from a YouTube video, from a friend's hand-written notebook — and read it cleanly. You'll know what kind of dough you're being asked to make, how active the inoculation is, and where the levain fits.

That clarity is the actual benefit. Most "failed" sourdoughs aren't failed because the math was wrong. They're failed because the math was opaque, and the baker followed the recipe without understanding what was supposed to happen — so when something looked off mid-bake, they couldn't tell whether to wait, fold, retard, or restart. With the math, you can read your own dough.

A starter note on hydration drift

Many home starters are not actually at 100 percent hydration. They drift over weeks of feeding — slightly stiff, slightly slack, depending on how the baker measures. If your starter looks more like pancake batter than thick yogurt, it's probably 110–120 percent. If it looks like dough, it's probably 80–90 percent. The exact figure doesn't matter much for daily baking, but if you ever calculate true dough hydration to two decimal places, knowing your starter's real hydration is the small extra honesty that closes the loop.