Calculating Boil Volumes

Isn't there software for this already?

Beersmith is great.  It's a really nice piece of software that has a ton of great features, and well worth the $30 price tag.  In an effort to learn what was going on inside Beersmith, and teach myself some fundamentals of AG brewing, I decided to make myself a spreadsheet in Excel instead.

Within the coming weeks, I am going to move to AG brewing.  I am setting myself up with a pot that I can use for BIAB, as well as converting a cooler I picked up for sale at Lowes into a MLT for more traditional AG brewing.

Before I embark on my AG brewing adventures, I wanted to figure out just where the hell Beersmith is getting the numbers it spits out.  I did myself some fancy googling, found the appropriate formulas and plugged everything into a spreadsheet.  For my purposes, this will suffice for the forseeable future but I wanted to share it so it makes more sense.

Total volume

The easiest way to start this is to work your way backwards - determine how much beer you intend to package, either in a keg or bottles, and work back from that.  For example, let's say you want to end up with 5G of finished beer to drink...

5G beer is the finished product

How much water to start out with will take the following into account:
 + .5G loss to fermentation trub
 + ~4% shrinkage from cooling
 + ~8% per hour loss to evaporation from the boil
 + 1/8G water per pound of grain will be absorbed from the mash

This is all taken into account to determine how much wort you need at the start of the boil, and how much water to mash and sparge with.  Some of these things will be different from system to system, such as the evaporation rate or grain absorbtion, but these are all the things that Beersmith tells you.

The Calculations

For this, I'll assume a final product volume of 5G
Add .5G for fermentation trub - this may be more or less depending on your system, whether you rack to a secondary, if you are moving to a bottling bucket or keg etc... but for the purposes of this example let's just say .5G

Total: 5.5G

Cooling loss: When the wort is cooled from the boil, you will generally lose ~4% volume.  This is a generally accepted value, and the default in Beersmith.

5.5 / (1 - CoolingLoss)
5.5 / (1 - .04)
5.5 / .96

Total: 5.85G

Evaporation Loss: As the wort is boiled, you will lose some to evaporation.  This certainly changes from system to system depending on a number of factors.  It will generally be between 5-10% per hour.  We'll assume 8%, and a 1 hour boil for this example:

5.85 / (1 - (EvaporationLoss * HrsBoiled))
5.85 / (1 - ( .08 * 1))
5.85 / .92

Total: 6.36G

Great!  So now, we know we will need 6.36G of wort before our boil to wind up with 5G of finished beer.  Next, we need to figure out the mash and sparge volumes, or if doing BIAB, just the additional water needed for the mash.  The easiest way to do this is to determine how much water will be absorbed by the grain, and add that to the wort volume we are looking to collect.

A generally accepted value for this is 1/8th gallon per pound, or .125G / lb
You may also see this as "1 gallon for every 8 pounds of grain".  Again, this may change depending on your system.  Your MLT may also have a false bottom with some deadspace - mine doesn't, so I didn't bother figuring that calculation out.

This is an easy one, just multiply the grain bill by this absorbtion rate.  If we assume 10lb grain, and losing .125G / lb then 10 * .125 = 1.25G

We already know we are looking to collect 6.36G of wort from our mash / sparge.  Given this absorbtion rate, if we were to added 6.36G water to the mash - we would lose 1.25G of that from being absorbed by the grain.  So we will need to use 6.36 + 1.25G of water total for our brew process.

Total: 7.61G

Now the only thing left to figure out is how much of that water will be used for the mash, and how much to use for the sparge.  The mash is generally completed using a water:grist ratio of somewhere between 1-2 quarts per pound.  For this example, we'll assume 1.25 qts / lb; in other words we will start our mash with 1.25 qts of water per pound of grain.  Again, assuming 10lb grain:

10 x 1.25 = 12.5 quarts

12.5 quarts = 3.125 gal

So that is the volume of water that will be added to the MLT at the appropriate strike temperature for your mash.  We know from our calculations above that we will need 7.61G total.  So, to figure out what to sparge with - just subtract the water we already used to mash.

7.61 - 3.125 = 4.49

This means we'll use 4.49G of water for the sparge.

What about for BIAB?

The idea is the same for BIAB, except we will assume that less water is going to be absorbed by the grain since we are going to be squeezing the shit out of it after the mash is completed.  Instead of .125 for BIAB, use something closer to .083.  The idea here is that since we are squeezing the wort out of the grain bed, some of that water that was initially absorbed is getting sqeezed right back out.

So 10lbs of grain, losing .083 gal/lb = 10 x .083 = .83 gal
Add this to our wort volume, 6.36 + .83 = 7.19G

Since we won't be doing a sparge with BIAB, we need a bit less water.  Ideally, after the mash and squeezing the shit out of our grain bed - we should be left with 6.36G of wort ready for the boil.

If Beersmith is confusing as far as BIAB is concerned (I found it to be), there is a cool website I found which has a neat calculator specifically for BIAB:

http://www.simplebiabcalculator.com

Putting it all together

Now we know we're going to use a total of 7.61G water for this batch, which is using 10lb grain.
We will mash with 3.125G water to get our water:grist ratio of 1.25 qts/lb
Out of this 3.125G water, 1.25G will be absorbed by the grain, leaving us with 1.875G sweet sweet wort
We want a total of 6.36G, however, so we add 4.49G water for our sparge
We'll lose ~8% of that volume due to evaporation from the boil, leaving us with about 5.85G
We'll lose ~4% of that volume from cooling the wort, leaving us with about 5.5G
We'll lose about .5G of that over the course of fermentation

Leaving us with 5G of delicious beer!