One of the best ways to improve the quality of your homebrew is to pitch the right amount of healthy yeast. And while it may be tempting to simply pour a single pack or vial of liquid yeast into the fermentation vessel, this isn’t usually enough. Under-pitching can cause stuck fermentation and produce off flavors, so it’s best to plan ahead and have enough yeast ready on brew day.
Unfortunately, when you search for information about how much yeast you need, you usually encounter complex equations involving billions of yeast cells, mixed systems of units, and competing schools of thought. This is fun for microbiologists and advanced homebrewers, but for beginners and weekend warriors, it doesn’t have to be that complicated.
Here’s an easy way to estimate a reasonable pitch rate for your next batch of homebrew.
How strong is your wort?
The denser the wort, the more yeast you need to ensure a healthy fermentation. Wort strength can be expressed in gravity points, which are just the three digits to the right of the decimal point of the original gravity:
- A 1.050 pale ale wort has 50 gravity points.
- A 1.080 double IPA wort has 80 gravity points.
- A 1.120 barleywine wort has 120 gravity points.
Ale or lager?
Much like people, yeast becomes sluggish when it’s cold. And since lager ferments at a much lower temperature than ale, you need more yeast to get the job done. A good rule of thumb is to pitch about twice as much yeast for a lager as for an ale:
- For ale, you need about 0.007 fresh liquid yeast vials or packs per gallon per gravity point.
- For lager, you need about 0.015 fresh liquid yeast vials or packs per gallon per gravity point.
How much yeast do I need?
Simply multiply the number of gravity points by the multiplier you selected above, and then multiply by your batch size in gallons.
Five gallons (19 liters) of 1.050 ale will need
(0.007 packs per gallon per gravity point) × (50 gravity points) × (5 gallons) = 1.75 liquid yeast packs.
Whereas 2.5 gallons of 1.080 lager would require
(0.015 packs per gallon per gravity point) × (80 gravity points) × (2.5 gallons)= 3 liquid yeast packs.
That’s a lot of yeast!
Yes, it is. That’s why seasoned homebrewers often make a starter, which is just a small batch of wort (usually a quart or two) that’s used to build up a yeast population from a single package. Those making lagers or high-gravity ales will probably want to learn to make a starter straight away. But if you’re making your first or second 1.050 pale ale, just use two liquid yeast packages and keep things simple.
Liquid or dry?
Dry yeast is sometimes overlooked, but it can be a great choice for its ease of use and reasonable cost. Plus, a packet of dry yeast contains up to twice as much yeast as a package of liquid yeast. Use one 11.5 gram sachet for ales up to about 1.065 - 1.070, and use two sachets for higher gravity ales. Lagers will require twice as many sachets for the same strength wort.
You won’t find as much variety in dry yeasts as with liquid cultures, but for basic styles like American pale ale and English porter, dry yeast offers a convenient, affordable alternative. And stored in the fridge, a packet can remain viable for at least a year.
A caveat
Finally, this method really only works if you can assure the freshness of your liquid yeast. Yeast that’s more than a couple of months old won’t have as many viable cells, and this simple method can’t account for that. Several good calculators are available online that can factor in yeast age and other variables.