Sourdough Starter Hydration Explained

One of the most confusing moments for new sourdough bakers is realising that your starter has its own hydration — separate from your dough — and that this matters when you're following someone else's recipe. Here's everything you need to know.

What is starter hydration?

Your sourdough starter is a mixture of flour and water that contains wild yeast and bacteria. The hydration of your starter is simply the ratio of water to flour in that mixture, expressed as a baker's percentage — exactly the same way dough hydration works.

• 100% hydration starter — equal weights of flour and water (e.g. 50g flour + 50g water = 100g starter). This is the most common type. It has a thick batter consistency.
• 75% hydration starter — 75g water per 100g flour. Slightly stiffer, holds its shape a bit more when peaked.
• 50% hydration (stiff starter) — 50g water per 100g flour. Dough-like consistency, used in some Italian baking traditions (e.g. panettone). Often more sour.
• 125–166% hydration (liquid starter) — more water than flour. Very runny, fast-fermenting.

Why does starter hydration affect your recipe?

Because your starter contains both flour and water, it contributes both to your final dough. When a recipe says "add 100g of starter," those 100g include a certain amount of flour and a certain amount of water — and that changes the effective hydration of your dough.

The formula is straightforward: for a starter at hydration H%:

• Flour in starter = starter weight ÷ (1 + H/100)
• Water in starter = starter weight × (H/100) ÷ (1 + H/100)

So 100g of a 100% hydration starter = 50g flour + 50g water. But 100g of a 50% hydration starter = 67g flour + 33g water. If you swap one for the other without adjusting, your dough will be noticeably drier or wetter than intended.

Most recipes assume 100% hydration

The vast majority of English-language sourdough recipes — from popular blogs, YouTube channels, and books — assume you're using a 100% hydration starter (equal parts flour and water by weight). If your starter is a different hydration, the recipe's stated dough hydration won't match what you'll actually get.

How to convert a recipe to your starter's hydration

The goal is to use the same amount of flour from your starter as the recipe intends, while compensating for the difference in water.

Step-by-step example

Recipe calls for: 100g of 100% hydration starter. You have a 75% hydration starter.

1. Find the flour content of the recipe's starter: 100g ÷ (1 + 1.0) = 50g flour
2. Calculate your new starter amount (same flour, different hydration): 50g × (1 + 0.75) = 87.5g of your 75% starter
3. Find the water difference: original starter had 50g water; your starter has 87.5 × 0.75/1.75 = 37.5g water. Difference = 12.5g less water.
4. Add 12.5g of water to your free water (the water you add separately) to compensate.

This keeps the total flour, total water, and therefore overall dough hydration identical to what the recipe intended.

Just use the calculator

This is exactly what our sourdough calculator does automatically. Enter your recipe, set the recipe's assumed starter hydration, set your actual starter hydration, and it adjusts both the starter grams and the free water in real time. No manual calculation needed.

Does starter hydration affect flavour?

Yes — and this is one of the main reasons some bakers deliberately maintain a stiff starter rather than the standard 100%.

• Stiff starters (50–65%) tend to produce more lactic acid (milder, yoghurt-like sourness) and less acetic acid, leading to a less sharp flavour. Some bakers find them more predictable and less prone to over-fermentation.
• Liquid starters (100%+) tend to be more active and produce more acetic acid at cooler temperatures, which can contribute to a more pronounced tang.
• Neither is objectively better — it's a matter of preference and baking style.

Does starter hydration affect rise speed?

Generally, a wetter starter ferments faster because the more fluid environment makes it easier for microbes to move and access food. A stiff starter will peak later and stay active for longer before collapsing. If you're converting from a liquid to a stiff starter, expect to adjust your feeding schedule and timing.

Changing your starter's hydration

You can change your starter's hydration gradually over a few feedings. If you currently maintain a 100% hydration starter and want to convert to 75%:

1. Feed 1: use 100g flour + 75g water (instead of 100g + 100g)
2. Repeat for 2–3 feedings until the starter has stabilised at the new consistency and rise pattern
3. Adjust your feeding timing — a stiffer starter will peak more slowly