Homebrew Dad's Online Yeast Starter Calculator
Use the calculator below to help you determine the proper size yeast starter for your next homebrewed beer. The calculator takes into account the details of your wort (i.e. the batch size and the gravity of the beer), and also allows you to overbuild your starter (if you are the type of brewer who likes to harvest yeast from your starters for future brew dates).The calculator will help you to predict the viability of your yeast, and will also help you to build your starter of the proper gravity.
Finally, the calculator will recommend a multi step starter, if needed, and will calculate out to four steps of yeast starters to help ensure that you meet your desired cell count.
If you are unsure as to what any of the terms used in the yeast starter calculator actually mean, you can simply mouse over the label in question, and a tooltip will open to help you out.
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If you prefer, we now have a downloadable version of the calculator available (requires Microsoft Excel).
Get started with starters today!
Settings
Your Wort Details  
Target Pitch Rate  Batch Volume (US gallons) 
Original Gravity (SG) 
Overbuild Cell Count (billions)  Yeast Cells Needed (billions) 
Your Yeast Details  
Initial Cell Count (billions) 
Production Date  Viability %  Viable Cell Count (billions) 
Your Starter Details  
Flask Size (liters) 
Gravity  Volume (liters)  DME Needed (grams) 
1st Starter Step  
Method of Aeration  Starter Volume (liters)  Inoculation Rate (millions/ml)  New Cells Created (billions)  Total Cells (billions) 
Growth Factor 
2nd Starter Step  
Not needed!  
3rd Starter Step  
Not needed!  
4th Starter Step  
Not needed! 
Assumptions used by the calculator

Liquid yeast is assumed to lose approximately 20% of the total viable cell count per month. This figure comes from a White Labs statement that indicated that you could expect 75%85% viability after a month; 20% is used as a middling factor. I also assume that the max viability on any package of yeast is 97%; call me a cynic, I don't see you getting 100 billion live cells unless you bring the package directly home from the yeast lab
There are multiple models out there for calculating loss of viability. If you go with a linear model (i.e. you lose 20% of the original cells per month), you end up with 0% viability after five months. I know of far too many cases of people who have brewed with yeast older than that, so I don't trust that model. Instead, I do this as a "running" total  you lose 20% in one month, 20% of the remaining balance the next month, and so on. This model has it own flaws, of course; since you are always deducting a percentage, you never get to zero cells  which obviously cannot be true.
In the end, I feel like this model is the most useful for normal brewing applications  if your yeast is four or more months old, you need to make a starter, regardless. Also, you may freely substitute your own viability percentages in this calculator if you so desire.

All growth figures are based off of a 75% attenuation rate. Obviously, multiple factors can influence this, most notably the actual strain of the yeast used, but also the gravity of the starter wort, the temperature of the wort, and other factors. It stands to reason that attenuation rate will absolutely impact the final number of cells produced.

I am assuming 2.72715 grams of extract per point of gravity per liter of starter.

The calculator does not try to give precise starter volumes, as I feel that it would be rather difficult to work with recommended volumes like "1.15 liters" or "3.27 liters". Instead, it makes calculations based off of 500ml increments. If it cannot hit your target cell count at, say, 2 liters, the next try will be at 2.5 liters, and so on, until either your target cell count or your flask size is reached. If the calculator determines that you cannot build a large enough starter in a given step to reach your target cell count, it automatically calculates the next step up.
Note that you can customize any step of the process at any time, so if you want that 3.27 liter step, have at it!

All growth rate data comes from the work done by Kai Troester of Braukaiser fame. My original intent was to include both the Braukaiser and Mr. Malty calculations, and allow users to pick from both. However, the more I delved into the subject, the more convinced I became that the Troester figures were more reliable. Troester makes his formulae freely available, while the Mr. Malty formulae are treated like something of a trade secret. Troester has published data from multiple experiments that seems to indicate repeatable results to back up his assertions, whereas I can't really find any experiments to back up (or back into) the Mr. Malty figures.
Specifically, the growth equations used in this calculator are, as follows:
 For stirred starters, if the initial cell count is less than 1.4 billion per gram of extract, the growth rate is 1.4 billion cells per gram of extract.
If the initial cell count is between 1.4 billion and 3.5 billion per gram of extract, the growth rate is 2.33  (.67 * initial cells) billion cells per gram of extract.
If the initial cell count is greater than 3.5 billion per gram of extract, there is no growth.  For simple (non agitated starters), the growth rate is .4 billion cells per gram of extract, up to an inoculation rate of 3.5 billion cells per gram of extract. If you exceed this, there is no growth.
 For shaken starters, the growth rate is .62 billion cells per gram of extract, up to an inoculation rate of 3.5 billion cells per gram of extract. If you exceed this, there is no growth.
 For stirred starters, if the initial cell count is less than 1.4 billion per gram of extract, the growth rate is 1.4 billion cells per gram of extract.
 Finally, realize that due to all of the assumptions above in regards to yeast viability and growth rates, no calculator will be able to accurately predict how many cells you have or will grow. According to the Troester models, you should assume a variance of +/ 15% in all such calculations. That being said, this yeast calculator will absolutely get you into the ballpark, and will help you to make noticeably better beer.
Additional credit goes out to Beersmith's article on the subject, and to various internet blogs and websites that I used to check data against. Finally, I'd like to thank my charter members of HomebrewDad.com for helping me to test the calculator; their input was absolutely invaluable!