How to calculate Pitch Rate

Key points:

  • Consistent pitch rate = consistent beer
  • Knowing the formulas means you don't wildly over or under pitch
  • Not all online cell counters are the same

If you've started becoming a Serious Homebrewer™ you've probably acquired a few flasks, a stir plate and possibly even started your own house yeast strain. You may even make a starter for each batch, not leaving your precious wort to the winds of fate. Well now it's time to bump it up a notch and get ready to clear one of the last hurdles to perfecting the art of homebrewing - consistency. 

Measuring grain, water and hops are relatively simple tasks. Even calculating alpha acids isn't horribly difficult (though IBUs are another story, see Hops Utilisation for more info.) However, yeast pitching is another story. You'll need at least a microscope and a hemocytometer - both of which can be acquired relatively cheaply as all you're doing with them is counting cells. For the purposes of this article, we'll assume that all cells are alive, but I'll discuss how to determine if they're dead in another post. 

On brew day, take a 1 mL sample of your yeast starter. Mix it with 9 mL of water. If your starter has settled and you're sampling slurry, you may need to up the dilution factor. At any rate, mix well and add a drop to your hemocytometer. Place under the microscope and adjust until you can clearly see the cells. On your hemocytometer, you'll see a 5x5 grid of squares with double lines separating them called a Neubauer chamber. All of the cells in this grid represent 1 mL, and so if you count all the cells in all 25 boxes, you'll have a decent estimate of how many cells are in 1 mL, with a maximum margin of error of about 11%. Most pro cell counters only count 5 boxes (the four corners and the middle) and then average it out, which gives a max margin of error of around 25%. 

If you notice your cells clumping together, you can add a tiny bit of phosphoric acid to unstick them, but some strains are just naturally clumpy, like my house strain. In that case, it's best to dilute more, so you can clearly count the clumps and count all the cells in the Neubauer chamber since they will vary wildly. Once counted, you can then calculate the cell density. 

This, admittedly filthy lens, shows a cell count of 144.5 million cells per mL as there are 289 cells, diluted by a factor of 50, that is 1 mL of yeast starter to 49 mL of water. You can see that there are 25 boxes, each separated by a double line. Yeast cells are the tiny round dots and the hazy flecks are garbage on my lens because I didn't bother to cover up the microscope when I left for Europe for two weeks. 

What you won't see in this microscope is bacteria (it's too small) but if you harvest directly from wort, rather than from a starter, you'll notice hop and wort debris. Testing for infection and diasticus strains require more equipment and another article. 

Moving on - we now have to decide how much yeast we need. For a low gravity ale, 0.75 million x mL in 23 litres of beer (a standard 5 gallon batch) x 9 Plato (1.035 SG), you'll need to do the following:

750 000 cells x 23 000 mL = 17 250 000 000 cells

17 250 000 000 x 9 Plato = 155 250 000 000 cells total - roughly 155 billion cells. 

To speed up the calculation and not make errors with all the zeros, you can just do cell density factor x L x °P x 1 billion. Cell density factors are as follows:

  • 0.75 million/mL for estery, british ales
  • 1 million/mL for clean, American ales
  • 1.5 million/mL for high gravity or hybrid ales
  • 1.75 million/mL for lagers
  • 2 million/mL for high gravity lagers

Your cell count at the homebrew level for a 5 gallon batch should never be below 100 billion and never above 1 trillion. Even if we were doing an Eisbock, with a cell density factor of 2 and a gravity of 20, we would still be around 950 billion. If you end up outside these parameters, you need to check your math. 

(For what it's worth, if you are doing an Eisbock, I'd recommend using only slurry. I'll show you how to do that in a separate article in the future, but for now, know that 950 billion cells at this starter density would equal a quarter of your wort.)  

Moving on, how many mL of starter do I need with a density of 144.5 million cells per mL? The formula is simple - 155 billion divided by 144.5 million, or cells needed ÷ cell density = mL of yeast to pitch. 

155 000 000 000 ÷ 144 500 000 = 1072 mL, or just over a litre

Now, isn't there an easier way to do this, say an online cell counter thingy? Absolutely - but you need to know the formulas because they approach things differently. They use the same math, but they take shortcuts that aren't immediately evident. If, lets say, you have a nice, even non-clumpy spread of cells and only want to count the four corners and the middle block of the Neubauer chamber, go right ahead, and then use this site here - just make sure you only add the cells from those five chambers, because the site's going to automatically multiply that number by 5. If you add up all 25 blocks, it'll recommend you vastly underpitch. 

If you counted all your blocks in the Neubauer chamber, use this site instead. 

In brief:

Cell density count = all 25 Neubauer Chamber blocks x dilution factor x 10 000

Cells needed = cell density factor  x °P  x L x 1 billion

mL of yeast to pitch = Cells needed ÷ Cell density count