How much yeast should you pitch?
Enough cells for the sugar and the volume: the standard is written as million cells per milliliter of wort per degree Plato — 0.75 for ales, 1.5 for lagers (which ferment cold and need the head start), 1.0 for hybrids, with 1.0 and 2.0 as the "professional" rates. Five gallons of 1.050 ale works out to about 176 billion cells; the same wort as a lager, 352 billion.
cells = rate × volume (mL) × °Plato · 0.75 × 18,927 mL × 12.4 °P ≈ 176 billion
A fresh liquid pack holds about 100 billion, so that ale wants 1.8 packs and the lager three and a half. Most people pitch one — and most ales turn out fine, which tells you something about how wide the safe range really is. A single fresh pack into 5 gallons of 1.060 is a 0.36 M/mL/°P pitch: half the "standard", and exactly what the manufacturer's "one pack per 5 gallons" instruction works out to. They're not wrong; they're quoting a lower rate than the textbooks.
How many yeast cells do I need? (chart)
Billions of cells, ale and lager, by gravity and batch. Divide by what your pack actually delivers.
| OG | 5 gal · 19 L | 5.5 gal · 21 L | 6 gal · 23 L | 10 gal · 38 L | ||||
|---|---|---|---|---|---|---|---|---|
| ale | lager | ale | lager | ale | lager | ale | lager | |
| 1.040 | 142 | 284 | 156 | 312 | 170 | 340 | 284 | 567 |
| 1.045 | 159 | 318 | 175 | 350 | 191 | 381 | 318 | 636 |
| 1.050 | 176 | 352 | 193 | 387 | 211 | 422 | 352 | 703 |
| 1.055 | 193 | 385 | 212 | 424 | 231 | 462 | 385 | 770 |
| 1.060 | 209 | 419 | 230 | 460 | 251 | 502 | 419 | 837 |
| 1.065 | 226 | 452 | 248 | 497 | 271 | 542 | 452 | 903 |
| 1.070 | 242 | 484 | 266 | 533 | 291 | 581 | 484 | 968 |
| 1.080 | 274 | 549 | 302 | 604 | 329 | 659 | 549 | 1,098 |
| 1.090 | 306 | 612 | 337 | 674 | 367 | 735 | 612 | 1,225 |
| 1.100 | 337 | 675 | 371 | 742 | 405 | 810 | 675 | 1,350 |
Fresh liquid pack ≈ 100 B (×0.79 at 30 days, ×0.37 at 90); dry sachet ≈ 11 g at 6–20 B/g. From this page's engine.
Liquid yeast: count the days, not the packs
Liquid yeast is alive and dying from the day it's packaged — about 0.7% of it a day, 21% a month, by the rule of thumb every calculator uses. A pack that's 30 days old is 79% viable; at 90 days it's 37%, and you're pitching a third of a pack. So a metric brewer with 21 L of 1.052 ale (203 billion needed) and a two-month-old pack is looking at 3.5 packs — or, more sensibly, a starter. Read the date, not just the number of packs.
When the packs you have don't cover the cells you need and buying more is silly — which in practice means: any lager, anything over about 1.060, or any liquid pack more than a month old. A 1–2 liter starter on a stir plate grows a tired pack back to 150–250 billion. Under about 1.050 with a fresh pack, skip it; the difference between 0.5 and 0.75 M/mL/°P on a session ale is not one you'll taste.
Dry yeast: grams, not packs — and an honest cell count
Dry yeast sidesteps the viability problem (kept cool it's good for years) and reframes the question as grams. The catch is the cells-per-gram figure. The number most calculators carry — "about 200 billion per 11 g sachet" — is roughly 18–20 billion per gram; Fermentis guarantees at least 6 billion per gram; independent counts mostly land around 10. That's a three-fold spread: our 176-billion ale needs 18 g at 10 B/g, 9 g if the optimistic figure is right, 29 g at the guaranteed minimum. The calculator shows the range rather than pretend. In practice one 11 g sachet per 5 gallons of ordinary ale works because dry yeast is pitched in huge, healthy numbers even at 6 B/g; for lagers and big beers, two sachets is cheap insurance.
Too few cells means a long lag, stressed yeast, more esters and fusels, and beers that stall high. Too many — which on a homebrew scale takes a deliberate effort — gives a fast, clean, slightly thinner fermentation with less yeast character. If you're going to miss, miss high: a 1.5× pitch costs you a little fruitiness; a 0.5× pitch can cost you the beer. The exception is styles that want the yeast's voice — hefeweizens, Belgians, English ales — where pitching at the low end of the range on purpose is a legitimate choice.