Another note on the 200 billion cell count magic number....
The cell count needed depends on the OG....
Ales & Lagers
The general consensus on pitching rates is that you want to pitch around 1 million cells of viable yeast, for every milliliter of wort, for every degree plato. A little less for an ale, a little more for a lager. George Fix states about 1.5 million for a lager and 0.75 million for an ale in his book, An Analysis of Brewing Techniques. Other literature cites a slightly higher amount. I'm going with Fix's numbers and that is what the pitching calculator uses.
The Math
If you're curious, here is the simple math to calculate the number of cells needed. For an ale, you want to pitch around 0.75 million cells of viable yeast (0.75 million for an ale, 1.5 million for a lager), for every milliliter of wort, for every degree plato.
(0.75 million) X (milliliters of wort) X (degrees Plato of the wort)
There is about 3785 milliliters in a gallon. There are about 20,000 milliliters in 5.25 gallons.
A degree Plato is about 1.004 of original gravity. Just divide the OG by 4 to get Plato (e.g., 1.048 is 12 degrees Plato).
So, for a 1.048 wort pitching into 5.25 gallons you need about 180 billion cells.
(750,000) X (20,000) X (12) = 180,000,000,000
As an easy to remember rough estimate, you need about 15 billion cells for each degree Plato or about 4 billion cells for each point of OG when pitching into a little over 5 gallons of wort. If you want a quick way of doing a back of the envelope estimate, that is really close to 0.75 billion cells for each point of gravity per gallon of wort. Double that to 1.5 billion for a lager.
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Proper Yeast Pitching Rates