When I bake certain bread, especially Italian bread, the recipes often call for a stiff preferment. Though they usually call for making a stiff biga from commercial yeast, I like going the traditional route and make my preferment from a natural starter. Unfortunately, all my starters have a hydration of 100%. So, I need to do some conversion.

Though the conversion involves a bit of arithmetic, it’s not as difficult as it may seem on the surface. In fact, you have all the information you need. But the trick to doing the conversion is that you must work backwards.

Okay… Let’s suppose the recipe you’re working with requires creating a stiff starter that’s 200g with a hydration of 50%. What we first must do is determine the total flour and the total water of the target starter. To do that, we first calculate the total flour. That’s easily achieved by the following equation:

Total Starter Weight / (1 + Starter Hydration) = Total Flour

Pluggin in the numbers, that’s:

200 / (1 + 50%) = 133

From there, the total water of your target starter will be 200 – 133 or 67.

Now that we have our target numbers, we need to specify how much mother we want to be part of the new starter as a percentage of the total flour in the new starter. For this discussion, let’s say we’ll use 15%. Since any ingredient that goes into a dough is a percentage of the total flour of the dough, the original starter we’ll need is:

Total New Starter Flour * % Amount of Mother = Mother Weight Needed -or-

133 * 15% = 20g

Since we know that the hydration of the mother starter is 100%, we can simply divide the amount of mother starter we need by 2 to get both the amounts of flour and water in the mother starter. So…

Mother Needed / 2 = Mother Flour and Mother Water -or- 10g

This calculation is important because the flour and water of the mother contribute to the total flour and water of the new starter, so we must factor that in. So, to get the amount of extra flour and water we need to create the 200g starter, we simply subtract the mother flour and water from the new starter total flour and water respectively:

Extra Flour we’ll need: 133 – 10 = 123

Extra Water we’ll need: 67 – 10 = 57

So, our new 50% hydration preferment will consist of the following:

20g Mother Starter
123g Flour
57g Water

And those added up will be 200g.

See? Not so hard. And the cool thing about this is that it also works in reverse when you want to convert from a stiff starter to a liquid starter!

But to make it easy, I created a simple calculator in a Google Sheet that you can use. You can use it there, but I recommend copying the sheet to one in your own account because others might use it at the same time and you’ll end up stepping on each other.