Using the Calculator with Your Starter
A sourdough calculator is only as honest as the starter weight you give it. The tool on this site can do the feeding arithmetic and the true hydration math. It cannot see the jar. This page is about the habits that make the numbers match what you actually mix.
Start on the home page. Keep the unit toggle on grams or ounces and leave it there for the whole session.
Weigh the starter, don’t estimate it
Spoon the starter you will feed, or the starter you will add to the dough, onto a scale in a clean container or into the mixing bowl. Use the weight on the scale, not a memory of “about half a cup” or “two spoons.” Starter is airy at the peak and denser when it has fallen, so volume is a poor stand-in for weight.
Enter the scale weight in the feeding panel’s starter field, or in the hydration panel’s “starter added to the dough” field. You usually should not feed the entire jar. Take out the amount you need, and either discard the rest or use it in another dough. Feeding a large leftover with a wide ratio is how jars overflow.
Leave something behind on purpose
Plan a leftover. After you remove the starter for the dough, a small amount should remain to be fed for next time. If the dough needs 100 g and you want a 20 g seed for the next feeding, you needed 120 g ripe starter in the first place. Build that amount in the previous feeding, then divide it on purpose: 100 g into the dough, 20 g back into a clean jar with fresh flour and water.
If you dump the whole jar into the dough, you have no starter tomorrow. If you feed everything and then guess how much you scooped into the bowl, both the jar and the dough formula become fuzzy. The feeding panel’s “total after the feeding” tells you how much will be in the jar. If that number is more than you can use, start with a smaller seed or a tighter ratio.
Know the starter’s hydration
The hydration panel asks for starter hydration, defaulting to 100%. That default is right for a starter that is regularly fed with equal weights of water and flour. It is wrong for a stiff starter.
If you feed 1:1 water to flour (for example, 20 g starter, 20 g water, 20 g flour, which is a 1:1:1 feeding), the new mixture is 100% hydration. If you regularly feed with less water than flour, compute the starter hydration from that habit: water added ÷ flour added × 100. A feeding of 20 g starter, 16 g water, and 20 g flour is an 80% hydration feed.
A single unusual feeding does not instantly rewrite the whole jar, but after a consistent pattern the starter you weigh is close to the hydration of that pattern. Use that figure. If you leave the default at 100% for a stiff starter, the calculator will assign too much water to the starter and the true dough hydration will read high.
Grams or ounces, not both
The math is the same in grams or ounces. What does not work is mixing the two, or mixing a cup of water with gram weights of flour. Convert the recipe first — or switch the scale — and enter every field in one unit. Ounce results show two decimal places and gram results show one, which is precise enough for home baking.
Put the two panels in the right order
If you are refreshing the starter today for a dough tomorrow, use the feeding panel first. Weigh the seed, choose the ratio, add the flour and water it names, and let the starter ripen as you usually do. When it is ready, weigh the portion that goes into the dough and use the hydration panel with that weight, the recipe flour and water, and the starter hydration.
If you are mixing now, you may only need the hydration panel. The feeding panel can wait until you put a leftover back in the jar.
Either way, treat the calculator as a scale companion, not as a substitute for looking at the starter. A correct 1:2:2 feeding of a starter that is not yet active is still a feeding of a weak starter. The ratio cannot fix that.
Common slips
Entering the whole jar’s weight when only part of it is going into the dough.
Forgetting to subtract the leftover you intend to keep.
Using a 100% starter hydration for a starter that is kept stiff.
Reading a ratio as flour : water : starter instead of starter : water : flour.
Changing units on the scale and not in the calculator.
If a result looks off, check those five items before you change the recipe water.
For the meaning of 1:1:1, 1:2:2, and 1:5:5, see starter feeding ratios. For why the starter belongs in the hydration number, see true sourdough hydration. The FAQ collects shorter answers to the same problems.