Estimated by doing a cell count of a representative sample with a hemocytometer. The cell concentration on my last batch was 1.4 trillion per liter, and it filled three quart size mason jars. Viability was 97% on that batch. Sometimes it's as low as 60% post fermentation.
Wow I didn't know over pitching could even be an issue. I plan on brewing an Irish Red in a few weeks (est OG 1.056), then racking a Blue Moon clone (est OG 1.055) right onto that yeast cake. By this method I'd be grossly over pitching?
Maybe I'd be better off taking a liter of slurry from the cake and repitching into the blue moon clone?
Yes, you need very little, but keep in mind that pitch rate calculators are only providing a rough estimate. They can be off by a factor of ten pretty easily. IME the viability estimations and cell counts are much lower than the actual, so if you use the Mr. Malty calculator you will probably be over pitching, but that's generally better than under pitching.
See this for a detailed explanation:
http://woodlandbrew.blogspot.com/2012/11/counting-cells.html
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