What pitch rate does this calculator use?

It uses the widely-cited industry guideline of about 0.75 million cells per millilitre per degree Plato for ales and 1.5 million for lagers. Multiplying that rate by volume in millilitres and gravity in Plato gives the total cells required for a healthy, clean fermentation.

How is original gravity converted to degrees Plato?

It uses the standard polynomial: °P ≈ −616.868 + 1111.14 × SG − 630.272 × SG² + 135.997 × SG³. Roughly, every 0.004 of SG equals about one degree Plato, so a 1.048 wort is close to 12 °P.

How many cells are in a packet of yeast?

A fresh liquid pack is assumed to hold about 100 billion cells; a gram of dry yeast holds roughly 10–20 billion (this tool uses 10 billion per gram as a conservative default). Slurry is assumed at about 1 billion cells per millilitre. Real counts vary with age and viability.

Why do lagers need more yeast?

Lagers ferment at low temperatures where yeast is sluggish and reproduces slowly, so a higher starting cell density is needed to ferment fully and avoid off-flavours and stuck fermentations. The standard guidance is roughly double the ale rate.

What happens if I under-pitch or over-pitch?

Under-pitching stresses the yeast, producing excess esters, fusels, and possible stuck fermentation. Severe over-pitching can mute desirable character and stress cells through too little growth. Pitching near the calculated rate gives the cleanest, most predictable results.

What is the Yeast Pitch Rate Calculator?

Determine the required yeast cell count and the number of dry or liquid packs, or slurry volume, from batch size, original gravity, and ale-versus-lager pitch rate. Uses standard cells-per-mL-per-degree-Plato guidelines. Runs in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Yeast Pitch Rate Calculator

Name: Yeast Pitch Rate Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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The Yeast Pitch Rate Calculator works out how many yeast cells your batch needs and converts that into practical units — grams of dry yeast, number of liquid packs, or millilitres of slurry. Pitching the right amount is one of the biggest levers on clean fermentation and finished-beer quality.

How it works

The industry pitch-rate guideline is expressed in million cells per millilitre per degree Plato:

cells = rate × volume(mL) × gravity(°P)

The standard rates are:

Ale: 0.75 million cells / mL / °P
Lager: 1.5 million cells / mL / °P (lagers ferment cold and need roughly double the density)

Original gravity is converted from specific gravity to degrees Plato with the standard cubic:

°P = −616.868 + 1111.14 × SG − 630.272 × SG² + 135.997 × SG³

The total cell count is then divided into your chosen yeast format using these common assumptions: a fresh liquid pack ≈ 100 billion cells, dry yeast ≈ 10 billion cells per gram, and slurry ≈ 1 billion cells per millilitre.

Worked example

A 20 L ale at 1.048 (about 11.9 °P) needs:

0.75 × 20,000 mL × 11.9 °P ≈ 178 billion cells

That is roughly 1.8 fresh liquid packs, 18 g of dry yeast, or 178 mL of slurry. Brew the same wort as a lager and the requirement doubles to about 357 billion cells, which is why lagers usually need a starter or several packs.

Ale vs lager: why the pitch rate doubles

The ale pitch rate (~0.75 M cells/mL/°P) assumes fermentation temperatures between roughly 18–22 °C (65–72 °F), where yeast is active and reproduces readily through its lag and growth phases. Lager fermentation at 8–12 °C (46–54 °F) puts the yeast under cold stress, slowing reproduction significantly. Starting with a higher cell count (~1.5 M/mL/°P) compensates so that fermentation starts promptly and completes cleanly.

Underpitching a lager is one of the most common causes of sulfur off-notes, excessive diacetyl, and prolonged lagering times — all of which point back to sluggish yeast with too much work per cell.

Yeast starter vs buying more packs

For batches requiring more than two liquid packs, building a starter is almost always cheaper and often produces healthier yeast. A starter is simply a small-volume wort (typically 1–2 litres at 1.030–1.040 gravity) pitched with one pack and stirred overnight. This grows the cell count to whatever your batch requires at a fraction of the cost of extra packs. The calculator’s output gives you the target cell count; whether you get there via multiple packs or a starter is a cost and freshness question.

Notes

Pack and slurry counts assume fresh, viable yeast; old yeast contains fewer live cells, so size up or build a starter.
Dry yeast cell density varies by strain (10–20 billion/g); this tool uses a conservative 10 billion/g as a baseline.
For large or high-gravity batches, a yeast starter is usually cheaper than buying many liquid packs.
Slurry viability depends heavily on age and storage conditions; freshly harvested slurry is far more reliable than slurry stored for several weeks.
High-gravity worts (above about 1.070) benefit from pitching at the upper end of the ale rate or even slightly higher, because osmotic stress from concentrated sugars reduces yeast efficiency.