Link to Mr Malty's calculator. Select repitching from slurry tab.

Let's start with 1.080 OG, 5.25 gal batch, uncheck "Calculate viability from date" and enter 90% in the box.

We should all have 287 billion cells needed, which I understand.

To get the number of ml of yeast slurry needed, what I would do is start with the yeast concentration. The default is 2.4 billion/ml. I would then multiply that by the % yeast. To get that, we have to take the "non-yeast %" and subtract it from 100%, so that's 100% - 15% = 85%.

So 2.4 billion/ml x 0.85 = 2.04 billion/ml. I would then multiply that by the viability, 90%.

So 2.04 billion/ml x 0.9 = 1.836 billions/ml.

If I need 287 billion cells, I would take the 287 and divide it by the 1.836, to get 156.3 ml of that slurry needed.

But Mr Malty says I need 147 ml. Now I know that's not that different considering we are using approximations on approximations, but I just don't see how he came up with his math so I wondering if I'm missing something simple.