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.