Yeast Pitch Rate & Starter Calculator

How this yeast pitch rate calculator works

This tool estimates how many yeast cells you need for a healthy fermentation and helps you decide whether to pitch more packs or build a starter. It is inspired by professional guidelines (Wyeast, White Labs, Lallemand) and popular homebrew calculators, but exposes the assumptions so you can adjust them for your system.

1. Pitch rate formulas

Pitch rate is usually expressed in million cells per milliliter per degree Plato:

Typical targets:

• Ales: 0.75 M cells/mL/°P
• Lagers: 1.5 M cells/mL/°P
• Hybrid / Kölsch: around 1.0 M cells/mL/°P

2. Converting OG to °Plato

We convert specific gravity (OG) to degrees Plato using a standard polynomial approximation:

For example, OG 1.050 is roughly 12.3 °P.

3. Liquid yeast viability model

Liquid yeast loses viability over time. Manufacturers publish typical curves; here we use a simple exponential model:

You can override this indirectly by changing the “cells per pack” or by adjusting the age if you know your yeast was stored especially well or poorly.

4. Dry yeast assumptions

Dry yeast is more stable and usually has very high viability. Manufacturers often quote around 18–20 billion cells per gram. This calculator lets you set:

• Cells per gram (default 20 B/g)
• Pack size (default 11.5 g)

For dry yeast, most experts recommend not making a starter. Instead, rehydrate (if recommended) and pitch enough packs to hit your target rate.

5. Harvested slurry

For harvested yeast, we estimate cells based on slurry density and viability:

Typical density ranges:

• Thin slurry: ~0.75 B cells/mL
• Medium slurry: ~1.0 B cells/mL
• Thick slurry: ~1.5 B cells/mL

6. Starter growth model

Yeast growth in a starter depends on the ratio of cells to wort volume, oxygenation, nutrients, and agitation. Instead of a single fixed factor, we use a simple empirical model:

This yields growth factors in the same ballpark as popular calculators: a 1–2 L stir-plate starter from a single pack often reaches 200–300 B cells.

Practical brewing tips

When you can skip a starter

• Small ales (≤ 1.050 OG) with fresh liquid yeast packs (200 B cells) in 19–23 L batches.
• Most standard ales when using enough dry yeast packs at manufacturer-recommended rates.

When you should use a starter

• Lagers, especially above 1.050 OG.
• High-gravity beers (1.065+).
• Older liquid yeast packs or harvested slurry with uncertain viability.

Common underpitching symptoms

• Lag time > 24 hours before visible fermentation.
• Stalled or incomplete attenuation.
• Solvent-like fusel alcohols, harsh heat, or excessive fruity esters (when not desired).

Frequently asked questions

Is it possible to overpitch?

Yes, but it is less common in homebrewing. Very high pitch rates can reduce ester formation and lead to bland beers, and in extreme cases may affect flocculation. For most homebrew scenarios, being slightly above the target rate is safer than being far below it.

How accurate are these numbers?

All pitch rate calculators rely on assumptions about cells per pack, viability, and growth. This tool makes those assumptions visible and editable. For critical production batches, lab cell counts (e.g., methylene blue or flow cytometry) are recommended.

Can I use this for mixed cultures or kveik?

You can, but recommended pitch rates may be very different. Many kveik strains are intentionally underpitched to encourage ester production. For mixed cultures and sour beers, follow the lab’s specific guidance rather than generic ale/lager targets.