Yeast - is 1 pack enough?

Dry, liquid, starter or no starter? Is it really that important?

Recently I have had several discussions both in person and online about how much yeast to use in your brews. Over the last 18 months I have made a step up with yeast ensuring I have adequate cell numbers for my brews. I have been using 2 packs of dry yeast for ales and 3 packs dry for lagers; or good size starters if using liquid yeasts. I have been following the guidelines on the Brewers' Friend Pitching Calculator and have found that my beers are much better quality as a result.

It has made such a difference that I now will recommend 2 packs plus to anyone asking about yeast or having issues in general with their beers; though there are still those brewers who want to argue that 1 pack is fine and any more is a waste of money. This discussion reared up again this morning and I decided to have a real good look at the numbers; what I discovered shocked me.

Using a standard ale brewed to 1.046 SG, 23 litre in the fermenter; the recommended pitching rate is 189 billion cells. 1 pack of dry yeast is calculated as 110 billion cells (10 billion cells per gram). That means that 1 pack only has 58% of the recommended cells; assuming that your dry pack is 100 viable with no dead cells from age and storage and every cell survives pitching, which is very unlikely at best.

Further reading implies an even worse situation when Fermentis, the makers of US-05, probably the most commonly used yeast in home brewing; claim that it has 6 billion cells per gram, that puts you now at 66 billion cells in your pitch, only 35% of your target 189 billion. Therefore a pitch with 2 packs will give you somewhere between 132 and 220 billion cells, 70% - 116% of the recommended number, a much better range and I'm happy to work with that.

Of course using liquid yeast gives you a little more control over cell numbers with a much more accurate cell count starting point and growth rate,

The reality is; either way you are only getting a rough number of cells, but I certainly prefer to be in the ballpark with my pitch, then in the carpark with a long haul to the main game.