Sizing a Starter

Same lesson, with the figures in motion.

Why does the size matter?

The pitch calculator answers in litres, not in grams of yeast — and that is no accident. How many cells a starter gives is set almost entirely by how much wort it has, not by how much yeast you put in. This lesson shows why, and what it means when you choose a size.

The numbers under the flasks are billions of cells. They rest on rules of thumb, not exact measurements — the principle is the point.

1. Wort Sets the Ceiling

Yeast grows on the sugar and oxygen in the wort, and when they run out it stops dividing. A litre of wort on a stir plate is good for about 100 billion cells — that is the ceiling. Put in more yeast and the culture reaches the ceiling sooner, but not higher: all three flasks end on the same number.

2. Volume Sets the Cell Count

Double the wort and you double the ceiling. That is why the pitch calculator works out the starter in litres: the target divided by about 100 billion per litre. If the beer needs 200 billion cells, that makes two litres, almost regardless of how much yeast the pack holds.

If the starter won’t fit in the flask, build it in two steps. Chill the flask, pour off the spent wort and give the yeast cake fresh wort: the cake’s cells stay, and the fresh wort builds its own ceiling on top of them. The calculator works out the steps for you.

Open the pitch calculator →

3. Too Much Yeast, Too Little Wort

Pour 180 billion cells into half a litre and the culture is already far above the ceiling. The yeast ferments out the sugar — it bubbles — but it barely has time to divide, so you get out roughly as many cells as you put in. The starter wakes the yeast, but it does not multiply it. If you want more cells, it is the volume you need to increase.

If the pack is already close to the target, the calculator says "Close enough" — then you can pitch directly.

4. Too Little Yeast, Too Much Wort

The opposite ditch: below about 15 billion cells per litre the culture is slow to get going, and the wort sits unprotected for a long time. Ten billion straight into two litres is only five per litre. Build it in two steps instead: first half a litre, then pour all of it into the big flask. The calculator suggests the steps for you.

5. Stirring Raises the Ceiling

The ceiling also depends on oxygen. A stir plate keeps the yeast in suspension, drives off carbon dioxide and keeps drawing in air — that gives roughly twice as many cells as a flask left standing. Shaking the flask a few times a day lands somewhere in between, around 70 %.

No tornado needed: a small dimple in the surface is enough to keep the yeast in suspension.