The other day, I made a big batch of Biga Naturale which is a biga made from a sourdough starter, at 75% hydration. Once it was mostly risen – nicely domed with bubbles forming on the surface as pictured above – I divided it into 4 X 200 gram portions (for pizza/flatbread starter that I’d store in the fridge for the coming week) and a single 360 gram portion that I’d use to make a double recipe of Pane di Como Antico.

All was well. I used two of the pizza portions to make some pizza dough for dinner last night, and early this morning, I took the 360 gram biga out of the fridge to bake my Pane di Como Antico for a community service event this afternoon.

Normally, I would’ve followed the recipe exactly, but this time, I was doubling the recipe and changing the hyrdration a bit so I needed to figure out the hydration of the starter. Had my starter been a 100% starter, it would’ve been a no-brainer to figure out the formula percentage of the flour and water – they’d both be 50%. But because I was using a 75% starter, I needed to figure out the actual formula percentage that each ingredient contributed to the overall mass of the dough.

For any given hydration rate, to figure out the flour and water percentages, it’s simple math:

Flour Percentage = (1 / (1 + Hydration Rate)) * 100
Water Percentage = 100 – (Flour Percentage)

So where did I get the 1? When calculating decimal fractions, we always use 1 as the “whole.” In this case, 1 represents the total amount of flour. So when we add 1 and the hydration rate, what we’re looking at is the representation of the flour and the water together.

Then to get the mass of the flour and water, it’s again just simple math:

Flour Mass = Dough Mass * Flour Formula Percentage
Water Mass = Dough Mass * Water Formula Percentage

Here’s a simple table where I’ve done the calculations:

Hydration Rate	Flour Formula %	Water Formula %
60%	63%	38%
61%	62%	38%
62%	62%	38%
63%	61%	39%
64%	61%	39%
65%	61%	39%
66%	60%	40%
67%	60%	40%
68%	60%	40%
69%	59%	41%
70%	59%	41%
71%	58%	42%
72%	58%	42%
73%	58%	42%
74%	57%	43%
75%	57%	43%
76%	57%	43%
78%	56%	44%
79%	56%	44%
80%	56%	44%

Mind you, this is not totally accurate. We’re not taking into account the weight of the microbes (which will be minuscule anyway) and we’re also assuming that there’s no water loss due to evaporation. But it’ll get us close enough to get the job done in calculating an overall contribution to the final dough.

Also, you’d think that you could apply this technique to figure out the weight of any ingredient in the final dough. I suppose you could if you built the dough under a tightly controlled and consistent environment. But the problem with that is that during mixing you may add a bit of flour or water, to adjust for temperature and humidity. So it throws off the actual flour and water you may have used. In this case, the best you can do is get an estimate of an ingredient. Personally, I’ve found that the margin for error is about 10%.

And even with the starter calculation, it’s not totally accurate. But we’re only dealing with a two-ingredient dough, so there’s not going to be much else to take into consideration.