Sourdough Starter Feeding Calculator

What is Wild Yeast Fermentation and Feeding Ratios?

Maintaining a sourdough starter is essentially managing a living ecosystem of wild yeast and lactic acid bacteria. The feeding ratio — expressed as Starter : Flour : Water — directly dictates how quickly the yeast consumes the available food and reaches its peak activity window, which is the optimal time to bake with it.

Mathematical Foundation

The Feeding Ratio Multiplier

\text{New Feed Amount} = \text{Retained Starter} \times \text{Ratio}

\text{Retained Starter}

= The weight in grams of the active culture you choose to keep in your jar after discarding the rest.

\text{Ratio}

= The scale integer. In a 1:2:2 ratio, the multiplier is 2. In a 1:5:5 ratio, the multiplier is 5.

The 100% Hydration Standard

\text{Added Water} = \text{Added Flour}

\text{Hydration}

= Almost all traditional sourdough starters are maintained at exactly 100% hydration, meaning the weight of the water equals the weight of the flour.

Laws & Principles

The Peak Timeline Rule: A 1:1:1 ratio provides a small amount of food, causing the yeast to consume it rapidly and peak within 4 to 6 hours at 75°F (24°C). A 1:5:5 ratio provides massive amounts of food, taking 12 to 18 hours to peak. You control the timeline solely by changing the ratio.
The Discard Mandate: You must remove (discard) a portion of your starter before every feeding. If you don't, the exponential growth mathematics dictate you would need industrial quantities of flour to feed the colony after just a few days.
The Metric Weight Law: You cannot feed a starter accurately using volume (cups/spoons). One cup of flour weighs ~120g, but one cup of water weighs ~240g. Mixing them 1:1 by volume yields a severely over-hydrated, watery starter. You must use a gram scale.

Step-by-Step Example Walkthrough

" A home baker wants to mix dough tomorrow morning at 8:00 AM. It is currently 8:00 PM. She needs an overnight feed and targets a 1:5:5 ratio to slow down the fermentation. She requires 150g of total peaked starter for her morning dough. "

Set the ratio: 1 part starter, 5 parts flour, 5 parts water (11 parts total).
Calculate part size: 150g target ÷ 11 parts = ~13.6g (round to 14g for safety).
Calculate retained starter: 14g × 1 = 14g of starter.
Calculate flour to add: 14g × 5 = 70g of flour.
Calculate water to add: 14g × 5 = 70g of water.

Final Result: The baker discards everything except 14g of her mature starter. She strongly mixes in 70g of flour and 70g of room temperature water. Over the next 12 hours, the 1:5:5 ratio ensures the yeast slowly ferments, peaking perfectly at 154g right when she wakes up.

Quick Answer: How much flour and water should I feed my sourdough starter?

For a standard daily maintenance feed, you should use a 1:1:1 ratio by weight. This means you keep 25 grams of your old starter, and thoroughly blend it with exactly 25 grams of fresh flour and 25 grams of water. Always measure in grams, never in cups or tablespoons. If you want the starter to rise slower (e.g., overnight), feed it a 1:3:3 or 1:5:5 ratio, which gives the yeast more food to process. Use the Sourdough Feeding Calculator to instantly determine exact gram weights for any size jar or feeding schedule.

The Sourdough Feeding Mathematics

Because a starter must remain at 100% hydration (equal parts water and flour by weight), bakers calculate feeds entirely based on the retained seed weight:

1:2:2 Feeding Ratio Formula Target Feed Weight = Retained Starter (g) × 2 (Flour) + Retained Starter (g) × 2 (Water)

Total Jar Weight Post-Feed Total = Retained Starter + Added Flour + Added Water

Baker Feeding Scenarios

Scenario: The Fridge Maintenance Feed

A baker only bakes on weekends and keeps their starter in the fridge. On Thursday night, they pull it out to reactivate it using a standard 1:2:2 feed.

Retained Starter: 30g
Ratio Target: 1:2:2
Flour Added: 60g
Water Added: 60g
Final Jar Weight: 150g

Why: 30g of starter fed at 1:2:2 yields 150g total. This ensures a strong, healthy culture that will double or triple overnight, ready to be immediately used for an autolyse on Friday morning.

Scenario: The Volume Measurement Disaster

A beginner tries to feed their starter a "1:1:1" ratio using measuring cups: 1/4 cup starter, 1/4 cup water, 1/4 cup flour.

1/4 Cup Starter: ~60g weight
1/4 Cup Water: ~60g weight
1/4 Cup Flour: ~30g weight
Actual Ratio Achieved: 1 : 0.5 : 1

Context: Because flour is mostly air, 1/4 cup of flour weighs half as much as water. The baker accidentally starved their yeast (only providing half the required flour) and created a runny, overhydrated soup that will not properly rise or trap gas.

Feeding Ratios against Peak Time

Feeding Ratio (S:F:W)	Flour Multiplier	Est. Peak Time at 75°F (24°C)	Best Use Case
1 : 1 : 1	1.0×	4 to 6 Hours	Fast reactivation / Same-day baking
1 : 2 : 2	2.0×	6 to 8 Hours	Standard daily maintenance
1 : 3 : 3	3.0×	8 to 12 Hours	Overnight feed / Warm climates
1 : 5 : 5	5.0×	12 to 16 Hours	Vacation mode / Extreme hot weather

*Note: A 10 degree increase in room temperature will roughly cut peak times in half. A 10 degree decrease will roughly double them.

Pro Tips & Fermentation Control

Do This

Use a digital kitchen scale. Always place your empty glass jar on a scale, tare it to zero, add your retained starter, tare again, add water, tare again, and finally add flour. This guarantees absolute 100% hydration precision.
Change ratios to fit your schedule. You don't serve the starter; the starter serves you. If you sleep 10 hours, use a 1:4:4 ratio before bed so the peak aligns exactly with when you wake up to mix your dough.

Avoid This

Refusal to discard. If you keep 100g of starter, and feed it 1:1:1, you now have 300g. The next day, you feed 300g and have 900g. By day 4, you will need 17 pounds of flour just to feed your monster. You must throw most of it away before feeding.
Using bleached flour. Wild yeast and bacteria live on the outer bran of wheat berries. Heavily bleached, industrial all-purpose flour has been chemically stripped of these microbiomes. Use unbleached bread flour, whole wheat, or rye.

Frequently Asked Questions

What does 100% hydration mean in sourdough?

In baking math (baker's percentage), flour is always considered the 100% baseline. Therefore, a 100% hydration starter means that the weight of the water equals 100% of the weight of the flour. If you add 50g of flour, you must add 50g of water. If you used 40g of water and 50g of flour, you would have an 80% hydration starter (which is a stiff *lievito madre* style).

Why does my starter only smell like acetone or nail polish remover?

The acetone smell means your starter is intensely starving and producing acetic acid. When the yeast completely consumes all available flour sugars, the colony panics and alters its metabolic output. You need to immediately feed it a higher ratio (like 1:3:3 or 1:4:4) to give it enough food to survive the timeframe between your feedings.

Can I feed my starter straight from the fridge?

Yes. However, the yeast is completely dormant at 38°F (3°C). Even if you feed it a 1:1:1 ratio, it will not peak in 4 hours because the entire jar must first slowly warm up to room temperature before the yeast even wakes up to start eating. It may take 10-12 hours for a cold-fed starter to peak.

Do I have to use filtered water?

It depends on your city's municipal water treatment. Many cities treat water with chlorine, which will evaporate if you leave a glass of water on the counter overnight. However, some cities use chloramine, which does not evaporate and actively kills wild yeast and bacteria. If your starter refuses to rise despite proper feeding, switch to cheap bottled spring water to eliminate chemical death as a variable.

Sourdough Starter Feeding Calculator

Sourdough Starter Feeding Calculator