About this tool
The Yeast Pitch Rate & Starter Calculator is a free, in-browser homebrewing tool that estimates how much healthy yeast you need for a clean, on-time fermentation, and how big a starter to build if your packs fall short. Under-pitching stresses the yeast and produces off-flavors, esters, and stuck ferments; over-pitching mutes character. Enter your batch volume, original gravity, beer style, number of yeast packs and their age, and starter method, and it reports the cells needed, cells available, and recommended starter size. Everything runs locally in your browser.
The method converts gravity to degrees Plato with P = −463.37 + 668.72 × SG − 205.35 × SG², then applies a pitch rate M in million cells per milliliter per °Plato: 0.75 for ales, 1.5 for lagers, and about 1.0 for high-gravity beers. Cells needed (billion) = M × volume (mL) × P / 1000. A fresh liquid pack holds roughly 100 billion cells, declining about 0.7% per day, so viability = 1 − 0.007 × age and available cells = packs × 100 × viability. If a deficit remains, the tool estimates a stir-plate, intermittent-shake, or unstirred starter using an approximate growth of about 100, 50, and 25 billion new cells per liter respectively — a practical planning estimate rather than an exact laboratory value.
As a worked example, 20 L of a 1.050 ale is about 12.4 °Plato and needs roughly 186 billion cells. A single 30-day-old liquid pack retains about 79% viability, so it supplies only about 79 billion, leaving a deficit near 107 billion — roughly a 1 L stir-plate starter. Pitch into well-oxygenated wort at the correct temperature, and remember that dry yeast packs contain far more cells (around 200 billion) and rarely need a starter at all.