sourdough hydration math

you may have seen the following phrase or a variation of the phrase floating around my blog posts.

“i have a stiff starter that’s 80% hydration. please adjust amount of flour in final dough to account for your starter’s hydration.”

you may have even wondered, “um *ahem* erica. more on this word adjust please.”

if that’s you, you’re definitely not alone. this post is for you, and i’m way more excited than i should be to talk about math and baking at the same time.

before i begin, a surprise

i know not everyone likes to nerd out on ratios. i’ve made a few cute calculators you can use to make things a bit easier and less manual at sourdoughcalculator.com

first things first

it’s important to know what hydration your sourdough starter is. an easy way to figure this out is to write down the amount of flour and water per feeding, and divide the water amount over the flour amount.

i usually feed my starter 30g of flour, and 24g of water. 24 / 30 = 0.8, meaning my starter is at 80% hydration. at 80% hydration, my stiff starter has a dough-like consistency.

many people feed their starter equal proportions of flour and water, meaning their starter is at 100% hydration. at 100% hydration, the liquid starter has a batter-like consistency.

let’s do some math

now the fun part (caveat: my fun might be much, much nerdier than your fun).

we can reverse calculate the amount of flour and water in our sourdough starter. because our sourdough starter is just two ingredients, and because the amount of water is just a proportion of our flour amount, our equation becomes:

amount of starter = amount of flour + hydration * amount of flour

this equation says, the weight of my starter is equal to the weight of the flour and water it’s made out of, and the weight of water is just a proportion of the weight of flour.

since we already know the amount of starter and our hydration, we can solve for the amount of flour.

a practical example calculation: if i have 100g of my stiff starter at 80% hydration, i can calculate the amount of flour by doing:

100 g = x + 0.8x

where x represents the amount of flour in my starter. when i solve for x, i calculate that my starter has 56g of flour, and 44g of water.

adapting recipes to your starter

so you have a 100% hydration liquid starter but want to make some of my recipes? i gotchu.

there are two cases to be aware of: one in which i’d recommend adjusting the amount of starter, as well as the amount of flour, and the second in which i’d recommend maintaining the amount of starter, and adjusting the flour and water.

let’s say you’re eyeing shortbread cookies, which calls for 100 g of starter discard.

100g of liquid starter means you have 50g of flour and 50g of water.

since the cookie recipe doesn’t call for water, i would first reduce the amount of starter to match the water content of my stiff starter, and then add additional flour to compensate.

we know that 100g of stiff sourdough starter has 44g of flour. therefore, i would probably use 88g of liquid starter (44g flour and 44g of water), and then add 12g extra AP flour (44 + 12 = 56g flour) to achieve the same flour:water ratio.

an example that has water are my fluffy scallion pancakes.

50g of stiff starter means 28g flour and 22g water.
50g of liquid starter means 25g flour and 25g water.

since this recipe includes flour, water, and starter, you can maintain the amount of discard, and directly adjust the flour and water content, by adding an additional 3g of flour, and subtracting 3g of water.

et voila

that’s all folks! if you want to see more of this kind of content, let me know.