How to Calculate Your Pitch Rate
In order to get the most out of your fermentation, it is important to pitch the correct amount of healthy yeast. Not too little, and not too much. So how do we calculate the yeast pitching rate (cell count) to properly ferment your batch? The calculation will depend on two factors, which you already know even before brewing your batch.
1. Is it an ale or lager?
In general, lagers require twice as much yeast as ales. There are many reasons for this, but the primary reason is that lagers use a different strain of yeast... Saccharomyces Pastorianus.... which, although it is a close relative of ale yeast (Saccharomyces Cereviciae) it behaves much differently. Lager yeasts ferment much slower than ale yeasts, and they are deployed at significantly lower temperatures. Unlike ale yeast, lager yeast ferment from the bottom of the fermenting vessel instead of the top. It is because of these properties that we must pitch a higher cell count to meet the demands of the fermentation. The characteristics that lager yeasts impart into our beer make it well worth the effort!
2. What is the intended starting gravity of your beer?
This is the most important factor when determining pitch rates. The higher the gravity, the more yeast cells are needed to eat through the wort sugars.
Use this simple formula to calculate the pitch rate:
Ales: 750,000 cells - per milliliter - per degree plato.
Lager pitch rate (twice the ale rate): 1,500,000 cells - per milliliter - per degree plato.
DEFINITION - "Degree Plato" is another way to describe the sugar density of your wort. Homebrewers generally use "specific gravity" to describe wort density. Plato is just another way to describe it. It's like inches vs centimeters. You just need to know the conversion, and converting from specific gravity to plato (and vice versa) is simple. For example, if you have a starting gravity of 1.040, the density in Plato would be 10 degrees. If you have a wort that measures 1.080 on your hydrometer, your density in Plato is 20 degrees. Starting to see a pattern? Just divide your gravity by 4... and it gives you Plato. 1.060 = 15 Plato. 1.010 = 2.5 Plato. You get the picture.
One last hint. There are approximately 21,000 milliliters in a typical fermenter with 5.5 gallons, pre-fermentation. (some start with 5.5 gallons to leave room for the trub)
Now you have all the tools to calculate your own pitch rates. Shall we give it a try?
Let's say we're doing a five-gallon batch of an American IPA, at 1.068 original gravity. Of course this is an ale. Here's the formula:
750,000 (cells per milliliter) times 17 (degrees Plato) times 21,000 (milliliters in a 5.5 gallon fermenter) = 267,750,000,000 cells.
That's right. 267.75 BILLION cells. We've found it's best to start counting your yeast cells a few days before brewing, so you have enough time to count all of them.
We kid. We're kidders.
Each liquid yeast vial or smack pack contains approximately 100 billion cells, as long as it is fresh. Therefore, our pitch rate calculation tells us we need to pitch a little over 2-1/2 smack packs or vials. This is fine if you are in a hurry and haven't had time to build a yeast starter. However, this can get really expensive... especially in bigger beers. A little planning can help you save money, because you can grow one packet of yeast into multiple packets by making a starter. See our How to Make a Yeast Starter section for more info on starters and how to grow cell counts.
Now that you know how to calculate your pitch rate, we have created a spreadsheet which you can consult for every beer, so you won't need to run the calculations yourself. Just scroll down to your starting gravity, then scroll across to your pitch rate. It's easy!