How much yeast do I need?

Looking at some recipes in the BS Cloud, I see that as the grain bill goes up, so too does the yeast. Is there a direct correlation here? And is there a simple guideline to follow? (X grams of yeast per pound of grain (or extract), or...?)

I'm seeing some Imperial IPAs using 2 or even 3 packets of yeast but in the recipe there is little or no indication of packet size so it's a little confusing to me. If I have this right; if I use too little yeast, my fermentation could get stuck. Don't want that, so some guidance from the experts would be much appreciated.

Someone can stop me if I'm wrong, I am new to brewing but reading some books on it. I found the corolation was more with the OG. I found some good equations for it just by googling your exact subject. Or the beer smith book had good equations in it too

I use Wyeast, and use their on line calculator. I'm sure White Labs has one also. There is another on line calculator that a lot of folks here use, but I can't remember the name of it.
OG does have a lot to do with it, and as a rule of thumb anything 60 and above needs more than 1 pack or a starter.
But I strongly suggest checking with the on line calculators.

Ed

There are deferring opinions but here is what I can add

Cells to Pitch  =  1 Million  x  milliliters wort  x  degrees Plato
The 1 million can be between .75 Million to 1.25 Million depending on the source of information.
Now in useful US units that is
Cells to Pitch  =  0.5 to 0.7 Billion  x  Gallons  x  Specific Gravity Points  [Palmer suggests 0.5 and White suggests 0.70]
For a lager, fermented at colder temperatures the general wisdom is to double the pitch rate.
So in a 5.5 gallons of 1.050 sg Wort
Cells to Pitch  =  .6 B  x  5.5  x 50  =  165 Billion Cells.  The really problem is how long it takes count to even 1 Billion ! ! ! 

So continuing on to usefulness.
There are 100 Billion in a Wyeast Smack Pack and in the White Labs vial.  There is about 14 ml of very clean yeast slurry in the  bottom of the White Labs vial so there are about 8 Billion cells per milliliter of "pure" yeast slurry.

So we need 165 Billion cells for a 5.5 Gallon batch of wort and the largest size we can buy only has 100 Billion cells.
Don't worry, the yeast love to grow.  You can either pitch the 100 cells as is or create a starter batch a grow some more yeast.  The problems are the yeast begin to die off as soon as they leave the factory and by the time you pitch several weeks or months after you get the yeast the less there is.  And if you order online in the summer the yeast has to suffer through some intense heat.  And during the growth stage you get some pretty awful favors, just ask anyone who ever tasted a starter.
There are a lot of opinions (and in my opinion, misconceptions on starters).  The things that I think I know are yeast love to reproduce and they stop reproducing at about 200 million per ml of wort.  They love oxygen and agitation (stirring).  So to get 150 Billion cells I need a minimum 750 ml of starter.  It really doesn't matter much how much yeast you start with as long as you have thoroughly oxygenated the wort and you continuously stir it (magnetic stir plate); the yeast will grow to the maximum concentration.

Yeast will double about every 2-4 hours, so the number of cells is  Cell at Start  x  2^doublings.  (The ^ is the exponent or raised to)
Assume 50 Billion cells to start  x  2^2  =  200 Billion; so 2 doublings are required at 4 hours each so about 8 hours.
Assume 1 Billion cells to start  x 2^8  =  256 Billion; so 8 doublings x 4 hours  =  32 hours. 
In both cases the maximum yeast growth will be limited by the 200 million cells / ml concentration.

You can try your own experiment.  Start with one smack pack or one vial.  Get two starters at 750ml.  Split the contents of the yeast package into one with 90% and the other with 10% of the yeast slurry.  Let both starters ferment out for 3-4 days.  Let the yeast settle to the bottom and see if you about the same yeast in each.  I think you will be close.

All the best, D

All good stuff - thanks!

Being an engineer; I can really get into the calculations and experimentation, and with these guidelines I'm sure I'll be able to get it in the ball park every time.

I did a simple calculator based on the typical formula.  While MrMalty goes into fine detail and is far more granular and therefor accurate, mine is close enough (what's a couple billion cells here and there).  http://jomebrew.doesntexist.com/beercalc.htm

Really cool calculators - thanks!

Am I looking for trouble if, say, I wanted to get into harvesting/growing/storing my own yeast strains? Can I purchase a bunch of different strains from my LHBS and keep them (under strict temperature control) for extended periods? Or am I just as well off buying it when I need it?

Yes you can.  It all depends on how much hobby you want.  You will never save money, but it can be fun. 

What I will share is my opinion and there are many others.  I have been storing yeast cells in sterile, distilled water at room temperature.  The objective is to get the yeast into as dormant condition possible that will still allow you to "re-awaken" the yeast on demand.  The problem with other methods (with the exception of honest to god cryogenic conditions on the order of liquid nitrogen) is the yeast will continue to slowly reproduce and start to consume themselves and decompose.  Stored in water, there is nothing to stimulate any growth so the yeast go into a state of dormancy.  Most of the literature recommends regenerating you master stores every 6-12 months; but that is a relatively simple process.  With this process I can have yeast ready to pitch in about 7-10 days.

http://brewery.org/library/SterileDW1096.html

http://www.alsand.com/beer/yeast/index_E.html

Start with these two references to get you started.  You will need to understand petri dishes, slants, agar etc.  since you are dealing with individual cells, sterilization (not typical sanitation) is important.  I store what amounts to a couple of hundred thousand cells in 12 ml vials; when the yeast settles out there is a very small white dust of yeast on the bottom.  To re-awaken the yeast I agitate the vial and take a small drop of liquid and smear it onto an agar slant.  These will incubate for 2-4 days and result in a growth of several (500-1000) single cells into colonies with 1-2 Million cells per colony.  These colonies are scrapped off and placing into a starter batch.  I have used stepped starters, but lately I have just dropped the 1-2 Billion cells into 500-750 ml and let it go.  You need to make sure your starter is well aerated and continuously stirred. 

Good luck

I store my yeast in a 50% Glycerin solution frozen. I've had good luck out to 6 months... Haven't gone longer. Between yeast ranching like this and repitching from batch to batch, i do save money... $5 per batch, when the cost of dme is factored in.

My comment on saving money, factored in all of the equipment I have purchased to support my hobby including all of the false starts, failed attempts, research and my time.  It IS a hobby and it DOES keep me entertained !   

I started serious ranching with the glycerin solution in the freezer, but the I was getting concerned with the amount of handling of the yeast in the washing process and the concerns of the freeze / flaw cycle of the modern refrigerator / freezers.  When I found the article of storing in distilled sterile water I converted to that process.  Of course it has its problems and concerns also, but I feel more comfortable with this method.

Of course, its all good !

David

I've been luckier I guess.  My amortized costs (all equipment (capital and recurring), and all starter wort, etc) are below what I would have spent on fresh yeast for each batch.  At this point, yeast continues to cost me about 25 cents a batch.

Agreed....its all good.