How to Use This Simple Sourdough Calculator
A simple sourdough calculator should do the arithmetic you would otherwise do on a scrap of paper, and it should show its work. This one has two panels. The first tells you how much flour and water to add when you feed a starter. The second tells you the dough’s true hydration after you mix that starter in. Both stay on the home page.
You can work in grams or ounces. Pick one unit and use it for every field. The tool does not convert for you mid-formula, because mixing units is how scale errors sneak in.
Feed a starter with a ratio
A feeding ratio is written as starter : water : flour, by weight. The first number is the starter you already have. The next two numbers are what you add.
A 1:2:2 feeding is a common daily or next-bake refresh. For every part of starter you keep, you add two parts water and two parts flour. If you keep 50 g of starter and choose 1:2:2, you add 100 g water and 100 g flour. The jar then holds 250 g of freshly fed starter.
The same pattern scales. 20 g of starter fed 1:5:5 takes 100 g water and 100 g flour, and finishes at 220 g. 40 g fed 1:1:1 takes 40 g water and 40 g flour, and finishes at 120 g. If you need a lopsided feed, choose Custom and enter three numbers, for example 1:2:3 if you want more flour than water.
The result box lists the water to add, the flour to add, and the total after the feeding. That total is useful when you want to know whether the jar will be too full, or how much ripe starter you will have for the dough.
Then check true hydration
Hydration, in baker’s terms, is the water in a dough as a percentage of the flour in that dough. The trap is to divide only the water you pour from the vessel by only the flour you scoop from the bag. The starter is also flour and water. If you leave it out, the percentage you write down is not the dough you are mixing.
The hydration panel asks for four things:
Flour in the dough — the flour you add, not including the starter.
Water in the dough — the water you add, not including the starter.
Starter added to the dough.
Starter hydration — the water in the starter as a percent of the flour in the starter. A starter kept with equal weights of water and flour is 100%. A stiffer starter might be 80% or 50%.
The tool splits the starter into its flour and water, adds those to your recipe flour and water, and divides. The headline number is true dough hydration.
Worked example
Use 500 g flour, 350 g water, and 100 g starter at 100% hydration. Those are the default numbers in the calculator.
The starter at 100% is half flour and half water: 50 g of each. The dough contains 550 g flour and 400 g water. True hydration is 400 ÷ 550, which is 72.7%.
If you had ignored the starter, 350 ÷ 500 would have read 70%. The two-and-a-half-point difference is enough to change how a dough feels, especially in a lean bread where a few percent of water is the difference between a slack mix and a dough you can handle comfortably.
Change the starter hydration to 80% and the same 100 g starter is no longer a 50/50 split. It contains 100 ÷ 1.8 grams of flour, and the rest water. The true hydration falls to 71.0%. The recipe on paper did not change. The starter did.
A short routine that keeps you out of trouble
Weigh the starter you will feed, or the starter you will mix, on a scale that can read in the unit you chose. Do not guess a “heaping spoon.” Enter the numbers, read the result, then add the flour and water the tool named. If the hydration comes out wetter or drier than you wanted, adjust the dough water — not the flour already in the starter, which you cannot take back out.
The calculator does not tell you when the starter is ready, how long a loaf should ferment, or what temperature to use. Those depend on your flour, your room, and the starter in front of you. It only does the weights and the percentage. Use it for that, then bake as you usually would.
If you want more detail on why the starter changes the number, read true sourdough hydration. For choosing 1:1:1 versus 1:5:5, see starter feeding ratios. For habits at the jar and the scale, see using the calculator with your starter.