A sourdough starter is a living culture of yeast and bacteria, and feeding it is the one routine every sourdough baker must get right. Each feed adds fresh flour and water so the microbes have food, and the amounts matter: too little food and the starter grows weak and acidic, too much and you waste flour on a jar that grows out of control. This calculator removes the arithmetic: enter the starter you keep and your feeding ratio, and it gives exact grams of flour and water per feed, per day, and per week.
Feeding ratios are written as three numbers, like 1:1:1 or 1:2:2, and they mean parts by weight. The first number is the starter you keep, the second is the flour, and the third is the water. A 1:1:1 feed on 50 grams of starter means 50 grams of flour and 50 grams of water, giving 150 grams total. A 1:2:2 feed on the same 50 grams means 100 grams of flour and 100 grams of water, giving 250 grams total. The bigger ratio feeds the culture more and stretches the time until the next feed.
Why ratios matter comes down to how fast the culture eats. At warm room temperature, a 1:1:1 feed peaks in roughly 4 to 8 hours, which suits a baker who feeds morning and evening. A 1:2:2 feed peaks later, roughly 8 to 12 hours, which suits one feed a day. In a cold kitchen everything slows down, so the same ratio lasts longer, while in summer heat it peaks faster and may need a bigger ratio or a second feed. The ratio is your main tool for matching the starter to your schedule and your kitchen.
Here is a worked example. You keep 50 grams of starter, feed 1:1:1, once a day. Each feed needs 50 grams of flour and 50 grams of water, and the jar holds 150 grams after feeding. Per day that is the same 50 and 50, and per week it is 350 grams of flour and 350 grams of water. If you switch to 1:2:2 once a day, each feed needs 100 grams of each, the jar holds 250 grams, and the week needs 700 grams of each. Seeing the weekly total surprises many bakers, and it explains why a small starter is cheaper to keep.
Discard is the part beginners misunderstand most. Before each feed, you remove all but the starter amount you plan to keep, which is why the calculator starts from kept starter rather than jar total. The removed portion, called discard, can go into pancakes, crackers or pizza dough, or into the compost if you prefer. Skipping discard and just adding flour doubles the jar every feed, and within a week you would be feeding a bucket. Keeping a small starter, 20 to 50 grams, keeps discard and flour use manageable.
Weighing in grams is not optional for a starter. A cup of flour can weigh anywhere from 100 to 150 grams depending on how it was scooped, which changes the hydration and the ratio without you noticing. A starter fed by cups drifts over days: sometimes too dry, sometimes too wet, and the rise becomes unpredictable. A cheap kitchen scale fixes this completely, and it also makes the calculator exact, since every figure it gives is a gram weight you can measure directly.
Use this calculator when you change your routine: a new ratio, a bigger bake that needs more starter, or a holiday when the starter goes into the fridge. For a bake, work backwards from the starter your recipe needs, choose a ratio that reaches it in one or two feeds, and let the tool give you the flour and water. For the fridge, feed once, let it rise a little, then refrigerate, and feed weekly. A fed starter is forgiving, and exact numbers make it easier to keep that way.