check this out...
"So how do you determine the number of yeast cells to pitch? Lets do some some simple math. Lets say you have a 5 gallons of 1.035 wort and we selected a pitching rate of 0.8 million cells/ml/º Plato. First convert gravity to Plato scale. Divide the last two gravity points by four to get that number:
º Plato = 1.035 gravity / 4
Plato = 8.8º
Next, figure out how many millimeters are in 5 gallons of wort. Since 1 gallon has around 3785 mls that would be 18,925 mls. Now multiply all the numbers together to get your total number of cells needed to ferment the batch of beer:
Cells needed = (pitch rate) * (mls of wort) * (º Plato)
=(800,000) * (18,925) * (8.8)
= 133 billion yeast cells"