You brew your own beer? -BIAB (brew in a bag) water volume calculations

I’ve seen some increasing traffic drawn to my BIAB methodology post and I imagine most are those with a new interest in BIAB. My water calculations comes from my experience in doing a number of brews. I thought it might be useful to go over how one might go about their first attempt at BIAB without the benefit of previous attempts. We need a few things here.

First-Kettle Volumes
You need a way to know how much water you have in your kettle. The easiest way is to use a metal ruler and some math. First measure across the top of the pot from inside edge to inside edge. Look at your kettle, the bottom and top may be different widths but if the difference isn’t drastic, don’t worry about it. We want the area of a circle first. The formula is:
Area=(π)(d^2) / 4

()() means multiply
/ means divide
π = pi = 3.142
d = kettle diameter in feet = kettle diameter in inches divided by 12
^2 means squared or simply a number multiplied by itself so it could be (d)(d)

Easiest way is take the measurement across middle of the top of your pot in inches. Divide that number by 12. Multiply this number by itself. Multiply this number b 3.142. Divide this number by 4. Now we have the area of your pot cross section in square feet. Now on to volume. It will be easiest to know how many gallons are in 1 inch depth of your pot. So first we need cubic feet. We have square feet with the cross section area so we only need depth in feet to get cubic feet. Here we go:

Volume = (Area)(Depth)(7.48)

Area = our answer from above
Depth = 1/12 = 0.0833
7.48 = gallons per cubic foot

So take your number from Area and multiply it by 0.0833. Take this number and multiply b 7.48. Now you know how many gallons are in an inch of water in your pot. Check it by pouring a known quantity of water into the pot and measure the depth.

For example, say we found out a pot has .68 gallons per inch. I pour 2 gallons of water in. 2/.68=2.94 inches.
I keep my ruler in the entire boil and cooling so I can measure depth at the end.

Now you can measure volumes of water in your kettle.

Second-Boil off
To figure out boil off, fill your kettle up to brewing volumes like 5 or 6 gallons. Bring it to a boil, keep it boiling for 60 minutes, cool it and measure how much water you have left. My boil off is pretty consistent at all depths so I use gallons per hour, but you may want to try larger and smaller volumes to boil to come up with a percentage.

For volume per hour it is simply preboil volume minus postboil volume. For example, I start with 5.25 gallons preboil and I boil for 60 minues, chill, and measure to find I have 3.25 gallons. My boiloff rate is 5.25-3.25=2 gallons per hour.

For percentage start with the volume per hour calculation above. Take that number and divide it by the original volume and you have percent boil off. For example with the above numbers, 2/5.25=0.38 (multiply by 100 to call it percent, ie 38%). If you run a bunch of trials at various volumes, average them.

Wort will boil off slightly different than water, so keep track of all your volumes when brewing and you can fine tune your estimates if you really want. The truth is, we are making beer, not building a bridge. Close is good enough, just be consistent so you can be predictable.

Last-Mash Volumes….for now

For your first BIAB, you have done enough. Take your desired final volume (usually 5 or 5.25 gallons) and add in your boil off volume. Make note of this volume. Don’t worry about absorption for now. Nobody said you HAVE to use all your water. Do your mash. Wring, squeeze, hang your bag….whatever you choose. Make a note of your drain method. Measure how much wort you have. Absorption is just the amount of water you started with minus how much wort you have after mash. Top up your kettle with fresh water to your original premash volume and you should end up with a postboil volume you wanted.

Do a few brews and take notes. Each time take your absorption volume and divide it by the weight of your grains. This will be gallons per pound. Average a few batches and fine tune as you get more brews under your belt. You may find trends like over 12 pounds of grains absorbs more than 8-10 pounds of grains, so you may have different absorptions based on the amount of grains. But I bet the numbers won’t be significantly different.

Later-Mash Volumes

Once you done a few brews and taken notes you can mash with all your water. The formula is simply:

Mash volume = final wort volume + boil off + absorption

How’s that? Clear enough? Message me with errors/typos, I read what is in my head sometimes and not what I actually wrote.

11 responses to this post.

  1. […] Update – See here for first timers looking for a way to figure out mash water volume (click) […]


  2. Posted by Convictus on May 2, 2011 at 11:29 pm

    This stuff is pretty interesting and you count yourself as the reason that I decided to take the Brew in a Bag plunge for my first all grain brew. I found that I am about 1 gallon every half hour so for a 90 minute boil I loose around 3 gallons. I am aiming for around 5 gallons final, so I think I may need a bigger pot. I have a 7.5 and with 6 gallons of water for the mash, I am pretty close to full, I did about 1/2 gallon rinse at 170 degrees, but it left me with 3 gallons of 1.068 wort after boil off. I think I may need a 10 gallon pot to get 5 gallon batches, after adding water to get to 5 gallons I was at 1.050sg. If I do this beer again I think I will add about 2lbs more grain and get to work on finishing my keggle. So I would dough in with 8 gallons and sparge with slightly more than 1/2 gallon of water to come out to just over 5 gallons.


  3. Thanks for reading, Convictus!

    While you wait for your keggle to be complete, you can always brew with as much water as your kettle can hold and slowly add preboiled water into the kettle throughout the boil, hopefully using all the required water by the end, if not that’s ok, just mix the remaining into the fermenter.


    • Posted by Convictus on May 5, 2011 at 12:33 pm

      I also did a sour beer with the spent grains, I added 2lbs of base plus all of the specialty grains at dough in, and had a 3 gallon batch with 1.040osg. When you sour your grains do you leave them in the pot covered? Do you go more than 2 days? Where did you get the idea for souring? I would like to read more about it, but you are the only place I found so far talking about it. I am interested in Lambic style beers so your soured grains option was doable. I would do a Labmic style, but lack the facilities (space) for barrel aging.


  4. Thanks for visiting. Glad I can be of some interest to someone.

    I do my spent grain sour in my BIAB bag placed inside a cooler for 1-2 days depending on my schedule. I have not gone more than just a couple hours over 48 hours.

    How did I get the idea….a crash of a few different things created an AHAH moment.

    I read this thread and had planned on doing a Kentucky Common some time:

    I had been kicking around the idea of doing a small sour mash for a Guinness clone.

    My homebrew club did a partigyle and things just came together. My buddy pitched some fresh grains and some sour culture into his spent mashtun. I left my bag open to the elements and took it home then pitched some hot water and grains with it into a cooler and did a sour brew the next day….FABULOUS! This event was the “Partigyles Gone Wild” episode on Basic Brewing Radio if you want some more insight.

    What I love about this method is that you don’t infect your equipment since the souring is preboil. It is also great that you get bonus beers for a few dollars. The Mad Fermentationist is a great place to check out anything sour beers.

    Good luck in your efforts.



  5. Posted by Convictus on May 10, 2011 at 7:12 pm

    The cooler may well be the difference here, I had my bag in a canning pot with the lid on, so it was at room temperature here in the northwest (read cold) I don’t believe that I got much in the way of souring, but the low efficiency on the first batch (like 67% or so)let me get the fairly high second batch from that grain bill. The stuff I am reading about souring talks about keeping the mash at above 100 for 2 or 3 days to encourage the bacteria to grow. I don’t know of a place that I could pull that off. The part I missed on your sour beer posts is that you put hot water in a cooler with the grains and then brew that.


  6. Yeah, you need to keep the mash hot. Keep your eyes open at yard sales or fleamarkets for a cheap used cooler and you are in business. Or find a way to insulate your pot.


  7. Posted by Tony on March 29, 2013 at 10:43 pm

    Thank you, this is a AWESOME simple explanation.

    Really learnt a lot.


  8. isn’t the area off a circle PI * r^2 not d^2?


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