I'm planning on brewing a double batch of "OktoberFAST" this weekend. I've used a few online brewing calculators and they seem to be giving me the wrong answer.

I've always read that if more then 50% of your grain is Pilsner malt you should boil for 90 minutes. Normally every hour of boiling you lose a gallon of water. So if you boil your wort for 90 minutes you'll lose 1.5 gallons of water.

So if I'm planning on making a 12 gallon batch. My pre-boil volume should be 13.5 gallons. The online calculator I always use Brew365 keeps one telling me I need 15 Gallons.


11 # Pilsner

6 # Dark Munich

6 # Vienna

1 # Caramunich

I'm thinking about mashing @ 157.

I know the MAX amount of grain a 10 gallon cooler can hold is 24 pounds.

Could someone help me calculate my Mash/Sparge water for a Single Infusion Batch Sparge?

  • 13.5 gallons is far too low. Just based on evaporation of 10%/hour you need more like 14 gallons. But don't forget that you're going to lose some volume from trub and from equipment losses. Why not trust the online calculators? Sep 12, 2014 at 22:06
  • @Tobias, I am not questioning your underlying point, but why do we always shoot for a 10% per hour evaporation rate? A better rate would be 6-8% (the same rate that the pros shoot for), and it is possible to get an adequate boil at that rate from my experience. Sep 13, 2014 at 2:58
  • 1
    10% per hour is just a rough guideline. The actual rate is dependent on many things, such as the shape of your kettle and the vigour of the boil. You'll want to characterize your setup by measuring the volume pre- and post-boil. Once you know the boil-off rate for your equipment, use that number next time. (I'm suspicious that % per hour is the wrong way to measure evaporation. It suggests that a smaller volume evaporates less than a larger one, which can't be right. Gallons per hour makes more sense, but all the calculators use % per hour...) Sep 13, 2014 at 15:12
  • 1
    Wouldn't the relative humidity of your climate affect boil off rate as well?
    – GHP
    Sep 15, 2014 at 12:41
  • Tobias is correct. My boil-off rate (not evaporation) is almost 15%.
    – Nimrod
    Dec 17, 2014 at 1:06

3 Answers 3


Forget the calculators...it's easier to do it empirically. And a couple of misconceptions...you don't necessarily need to boil pils malt for 90 min. I never do more than 70 (10 min, for hot break, then 60 after that) and have no problems. Also, figuring boil off in % is not accurate. You don't boil off twice as much for a 10 gal. batch as a 5 gal. batch. So, start with figuring out what you boil volume needs to be. For me, a 12 gal. batch would need 13.5-14 gal. pre boil. Mash at 1.65 qt./lb., rounding up to the nearest qt. When you run off your mash, measure how much wort you have in your kettle. Subtract that from your pre boil volume. The answer you get will be how much sparge water to use.


Quickly put this into brew smith see if I didn't screw something up. Disclaimer. I'm not a batch sparge brewer.

Mash in with 33.20 qt water at 168.2 degrees F 45 minutes.

3 step batch sparge. 1 drain. 2 add 15.9 qt at 168. 3 add another 15.9 qt water at 168.


I like to double check the calculators, but often if something looks off, it's because I didn't input it right. I usually use an online calculator and software (Brewtarget), but I sometimes use a spread sheet with formulas derived from http://byo.com/brown-ale/item/410-calculating-water-usage-advanced-brewing and http://www.howtobrew.com/section3/chapter16-3.html

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