I need to split a 15 gallon batch of beer three ways. The brew is going to be an Imperial Stout, and the 5 gallon recipe calls for 20lbs of grain (60lbs for 15 gallons). Between the three brewers we have a enough equipment to boil 10 and 5 gallons separately, but my concern is the mash.

We only have two 10 gallon mash tuns. 30lbs of grain in each plus the water won't fit. If we max out the water and hit our mash temp:

  1. Can we just fly sparge the rest in order to achieve our volume?
  2. Will our mash efficiency take a huge hit or will we get relatively close to our pre-boil gravity?
  3. If our mash efficiency does take a hit would mashing longer allow for better conversion?

I know equipment is a factor in efficiency but I'm just looking to see if this is a reasonable approach.

  • Can't you just build another mash tun? You can build one out of a cheap cooler.
    – Escoce
    Nov 5, 2015 at 20:09
  • 3
    Why not have each brewer do a 5 gallon batch? You could always do two mashes consecutively if you don't have 3 tuns.
    – Wyrmwood
    Nov 5, 2015 at 20:37
  • Why do you need mash tuns? For decoction, you can go with just plastic fermentation buckets and 5 or 10 gallons boil equipment - probably you already have all that.
    – Mołot
    Nov 5, 2015 at 21:02
  • I appreciate the alternatives mentioned. I'll see what I can whip up on short notice. Can anyone answer the question anyway though?
    – RoraΖ
    Nov 6, 2015 at 12:21

1 Answer 1


If you split a 15 gallon batch into two 7.5 gallon batches, the 30lb grain alone will take up 2.34gallons of volume (30 * 10 / 128), as according to sizing the tun from HowToBrew, grain volume is 10 fluid oz per pound. Plugging in a 7.5 gallon batch at brew365, at a mash thickness of 1.5qt/lb, the water alone is 11.25 gallons. In order to get the mash to fit, you'd have to use a mash thickness of 1.0qt/lb. Then the mash water would be 7.5g and should fit with the 2.34g of grain, 9.84g. However, you will likely not get great efficiency at that mash thickness. It will also be difficult to stir without spilling, and difficult to stir effectively.

As you haven't tried this before, it's probably easiest to measure your first runnings, and calculate your total sparge water as

Total volume of sparge water = Target pre-boil volume - volume of first runnings

According to the previous calculator, your target pre-boil wort is 9.26g.

The available volume in your tun is roughly

10g(tun vol) - 9.84g(mash vol) + Xg(first running)

If your sparge water needed exceeds that, you'll need to do more than one batch, or fly sparge. Going back to the calculator, it estimates you'll need 6.66g of sparge water.

For sake of an example, lets say your first running is 4 gallons. Then 10 - 9.84 + 4 = 4.16g. If you split the sparge in half, and ran 3.33g, then another 3.33g, you should come close to your target.

I'm guessing you may be able to get away with two batch sparges, maybe 3, depending on your first runnings volume. If you're setup on both systems to fly sparge, you could do that as well.

One last concern is that if you are going to split the batch into 10 and 5 gallons for boiling, and 5 gallons 3 ways for fermenting, you will still need to somehow mix all the pre-boil wort together, otherwise it will be parti-gyle, where the gravity is different for each boil, which will produce different results. I'm not really sure how to accomplish this without using a ~20 gallon container.

My recommendation is to brew on the systems you have. If you have three, 5-gallon boil/fermentation setups, and two 5 gallon mash setups, I would recommend two parallel and one consecutive mash, into three kettles.

  • Thanks for the answer! I had thought about that last concern. I figured I'd take the pre-boil gravity of both 10 and 5 gallong batches. Then either add some DME to compensate for the lower gravity.
    – RoraΖ
    Nov 9, 2015 at 12:52
  • Also, you can't do a 10 gallon and a 5 gallon, you'll need to do two 7.5g, then split the results 3 ways. The 10 Gallon mash simply won't fit in a 10g tun.
    – Wyrmwood
    Nov 9, 2015 at 18:37
  • Yeah I understand that, but we'll be boiling 10g and 5g separately. We'll just be mixing the two mash worts together for the 10g.
    – RoraΖ
    Nov 9, 2015 at 18:46
  • The other way that might help to avoid the parti-gyle - if you have a couple of large pails; combine your first runnings in one pail, then your sparges in another pail, the take 1/3 for the 5g and 2/3 for the 10g from each.
    – Wyrmwood
    Nov 9, 2015 at 18:53
  • That's not a bad idea. I'm brewing on Wednesday if there's a chatroom around I'll let you know how everything went.
    – RoraΖ
    Nov 9, 2015 at 18:55

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