Yeast Starter for 20 gal

I am going to make my first 20 gallon batch of Porter and I am looking for the best way to build up my starter. I typically split batch into 4-5 gallon using different strains and building starters with 4 different strains but this time I want to do one beer of 20 gallons. I have 2 stir plates. Is it best to build 2 starters and put them in a 1 gallon jar and build 2 more? Will the yeast be fine in the jar for 5 days while the other starters are growing? Does anyone have a better method? Any help is much appreciated.

Thanks everyone.

Man, I think if I was going to do a 20 gallon batch, I'd just brew a 5 gallon batch and use that as the starter. Mr Malty says a 20 gallon batch of 1.050 wort requires about 340 of slurry. Since it's so much starter wort, I'd like to have something I could drink rather than pour down the drain. I suppose you would want to brew something pretty light weight though, to not stress out the yeast too much. Just a thought.

Hello pjkutah welcome to HBT. I wouldn't worry to much about the batch size. If you can do 4 separate yeasts for a 20 gallon batch then a single yeast should be easy. Take a look at Yeast calc

http://yeastcalc.com/

It should help you to get to your goal. You can play around with any size step ups depending on your equipment and time.

The bigger the batch, the easier it is to step up. Pro step up rates are 10:1.

Haven't done a starter for 20 gallons yet but I am working on stepping up two starters. Making a 10 gallon batch of Imperial stout and I used yeastcalc.com to figure out the best steps needed. I am using 1 gallon glass jugs cause I only have a 2000ml flask..

Thanks for the responses. It sounds like if I just build 2 normal 5 gallon starters in my 2L flasks and put the slurry in a 1 gallon jug, add 500ml or so of starter and crash them, then create 2 more and do the same thing I should be ok. Does this sound ok? Will crashing the yeast from the first 2 starters be a problem? I plan on buying a 5L flask in the near future so I won't have any problems but I am hoping this will work in the meantime. I think doing a 5 gallon batch might be overkill but maybe not. Mr. Malty calculates me at 989B cells at 4.9L but I probably won't achieve the 96% viability if I crash the original 2 starters. I would plan on waking them up before pitching to get them going again. Any more thoughts? Again, thanks for all of the responses.

You are making a 1.071 beer? You have (2) stir plates and (2) 2l flasks? What is the production date on the yeast? What I would do if the yeast is 3 months old.
1) make a 1.5l starter, cold crash, decant
2) pour half of that batch of slurry into your other flask and make 2 1.5l starters, cold crash, decant
3) With the slurry in both flasks make two more 2l starters

You should end up with a trillion cells.

Thanks for all of the responses. Just made 2 1.5L starters and will step those up during the week. I thought I was on the right track but confirmation and to see how others do it is nice..

As PJKutah's cobrewer, I can tell you that he nailed the starters on this one. They started to krausen around 14 hours in and they are were at full krausen at 36 hours. Thanks for the support, guys!