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.
Pitching From Tubes, Packs, or Dry Yeast
Both White Labs and Wyeast make fantastic products and you can't go wrong with either one. There are differences between their strains and each brand has pluses and minuses yet neither is better than the other across the board. Use the brand your local homebrew shop carries, if you need a way to decide.
A White Labs tube has between 70 and 120 billion cells of 100% viable yeast, depending on the yeast strain. Some cells are much larger than others and there are more or less per ml based on size. (The information on the White Labs web site stating 30 to 50 billion cells is out of date.) We can just assume there are around 100 billion very healthy yeast. You would need 2 tubes if you were pitching directly into 5.5 gallons of 1.048 wort to get the proper cell counts.
A Wyeast Activator pack (the really big ones) and the pitchable tubes have an average of 100 billion cells of 100% viable yeast. The smaller packs are around 15-18 billion cells. You would need 2 of the large packs if you were pitching directly into 5.5 gallons of 1.048 wort to get the proper cell counts. For the small packs, you'd need eleven of them!
Some exciting work has been done on dry yeast lately. Reports are coming in of better quality, cleaner dry yeast. Personally, I really prefer the liquid yeasts, but the lure of dry yeast is strong. The biggest benefit is that it is cheap and does not require a starter. In fact, with most dry yeasts, placing them in a starter would just deplete the reserves that the yeast manufacturer worked so hard to build into the yeast. Most dry yeast has an average cell density of 20 billion cells per gram. You would need about 9.5 grams of dry yeast if you were pitching into 5.5 gallons of 1.048 wort to get the proper cell counts. (Recently there have been other numbers mentioned for cells/gram of dry yeast and folks have asked me why I believe there are 20 billion cells. I've actually done cell counts on dry yeast and they're always 20 billion per gram +/- less than a billion. Dr. Clayton Cone has also stated that there are 20 billion per gram, and other folks I trust tell me that 20 billion is correct. Until I see something different, practical experience tells me this number is correct.) For dry yeasts, just do a proper rehydration in tap water, do not do a starter.
