How much yeast should you pitch?

Why the pitch rate matters

Yeast makes the beer, and the number of cells you add decides how fast and how cleanly it starts. Pitch too few and the yeast has to multiply a great deal before it can ferment the wort; Palmer lists "weak fermentations due to under-pitching" as the reason to build up the yeast first. The usual way to set the dose is a rate in million cells per milliliter of wort for each degree Plato: a stronger wort needs more cells.

The calculation

Cells needed = rate × milliliters of wort × degrees Plato. Degrees Plato (°P) are the grams of extract in 100 grams of wort, found here from the specific gravity with the same polynomial as our ABV calculator; roughly, a gravity of 1.048 is 12 °P. The rate for an ale is 0.75 and for a lager 1.5 million cells per mL per °P (George Fix, as reported by Mr. Malty). White Labs uses a flat 7.5 million per mL instead and does not scale with gravity.

Worked example. 5 gallons (19 L) at 1.050 (12.4 °P). Ale: 0.75 million × 18927 mL × 12.4 °P = 176 billion cells. Lager: 351 billion. White Labs' flat rate: 142 billion. At 100 billion cells per fresh pack, the ale needs 1.8 packs; at 10 billion per gram of dry yeast, 18 g.

Limits of the method

Three makers, three answers. The rate comes from different brewing authorities. Mr. Malty uses Fix's figures and says other writers recommend somewhat more; White Labs uses a flat rate that is close to the ale rule at about 10 °P (0.75 × 10 = 7.5) and below it for a stronger wort; Fermentis's dosing of 50 to 80 g per hectoliter implies 5 to 8 million cells per mL at its stated minimum count.

Cell counts differ. Dry yeast is quoted at more than 10 billion viable cells per gram by Fermentis, and about 20 billion by Mr. Malty from his own counts, so the gram figure may be off by a factor of two. Liquid yeast depends on the strain and the age of the pack.

Temperature and gravity adjustments. White Labs doubles its rate when fermentation starts below 61 °F (16 °C) or at 18 °P (about 1.074) and above, and triples it when both apply. The ale and lager rules here do not make those adjustments; enter your own rate if you need them.

No starter model. We do not size starters, because we found no published growth model we could verify. The Wyeast figures quoted by Mr. Malty are a rule of thumb; use the maker's own tool.

Frequently asked questions

How many yeast cells do I need to pitch?

The common rule of thumb is a number of cells per milliliter of wort for each degree Plato: 0.75 million for an ale and 1.5 million for a lager. Mr. Malty credits those figures to George Fix's book An Analysis of Brewing Techniques and notes that other literature cites slightly higher ones. The cells needed are the rate × the milliliters of wort × the degrees Plato. For 5 gallons (18927 mL) at a gravity of 1.050 (12.4 °P) that is 176 billion cells for an ale and 351 billion for a lager. Mr. Malty's own example, 20 liters at 1.048, comes to 178 billion, which he rounds to about 180.

How many packs or how much dry yeast is that?

It depends on what the package really holds. Mr. Malty puts a fresh White Labs tube at 70 to 120 billion cells depending on the strain, a large Wyeast Activator pack at about 100 billion and the small ones at 15 to 18 billion. For 176 billion cells and a 100 billion pack that is 1.8 packs when fresh. Dry yeast is counted by weight: Fermentis states "Viable yeast > 1.0 × 10^10 cfu/g" for SafAle US-05, i.e. more than 10 billion per gram, which means 18 g (1.5 sachets of 11.5 g); Mr. Malty counted about 20 billion per gram in his own tests, which halves that to 9 g. The two figures are shown side by side because they disagree.

Why do the yeast makers recommend less?

They often do. Fermentis recommends 50 to 80 grams of US-05 per hectoliter, which at its stated minimum of 10 billion cells per gram is 5 to 8 million cells per milliliter (our arithmetic), under the 9.3 million per milliliter that the 0.75 rule gives at 1.050. White Labs uses a flat 7.5 million cells per milliliter for standard wort, 142 billion for the same 5 gallons, and doubles it for fermentation that starts below 61 °F (16 °C) or at 18 °P and above. The rules were developed for different yeasts and goals; the calculator lets you choose.

Do I need a starter?

A starter is a small batch of wort used to grow more cells before pitching; it matters when a liquid pack has fewer cells than the recipe needs. Palmer notes that a starter gives the yeast a head start and prevents weak fermentations from under-pitching. Some publications give rough growth figures, for example Wyeast's statement, reported by Mr. Malty, that an Activator pack in a 1 liter starter reaches about 150 billion cells and in a 2 liter starter about 200 billion, but they are one maker's rule of thumb from 2005. We looked for a published, citable growth model (such as Kai Troester's) and could not open or verify one, so this calculator does not size a starter. Dry yeast needs no starter; Mr. Malty advises rehydrating it in tap water instead.

Does the age of the yeast matter?

Yes. Liquid yeast loses viable cells as it ages, and the viability field takes your estimate: 100% for a fresh pack, less for an older one. We did not find a published, verified aging rule, so we do not offer one. Mr. Malty suggests an 80 to 90% starting guess for yeast harvested from a recent batch and advises discarding yeast whose viability is much lower than 90%. Dry yeast is counted as fresh, as in the Fermentis data sheet.

Sources: Mr. Malty (Jamil Zainasheff), "Proper Yeast Pitching Rates" (mrmalty.com; vendor and community tool, copy at web.archive.org, read October 28, 2026); The Brewer Magazine, "White Labs Shares Science Behind New Pitch Rate Recommendations"; Fermentis, SafAle US-05 product page (fermentis.com); John Palmer, How to Brew, chapter 6 "Yeast" (first edition); Brew Your Own for the gravity-to-Plato polynomial. Fix's book, Troester's growth model and White Labs' own calculator were not opened. Neighbours: ABV calculator, priming sugar calculator, yeast converter. How we check our calculators.