Calculating Mash Water Volume

Easy, really, it is.

When planning your all-grain recipe, all factors rest on the anticipated amount of water used, wasted, and boiled-off throughout your brewing, cooling, and transfering to fermenter. If you use too much water, the Original Gravity (OG) will be lower than expected. If too little water is used or factors such as lauter tun dead space and volume taken up by trub and hops, you will end up with either an OG that is too high or, worse, too little beer in your fermenter.

The most simple way to calculate these volumes is to plug all variables into a software program and let it calculate these things for you. I have a Mash & Sparge Water Calculator here on Brew365. I also have an Excel spreadsheet that you can save to do the same calculations offline.

But maybe you're the curious type, and just want to know how it works. Or, if you are like me, you might find that things just do not turn out quite as expected with the program and you want to do it yourself with numbers based on your brewing system and all of its nuiances.

Well, it's not as difficult as you think. To get your mash water volumes consistent, you simply need to do a couple of experiments to find some constant values or information about your particular brewing setup, then plug these into a simple equation to find your values.


In order to find out how much water we need, the first thing we need to figure out is how much we lose along the way during brewing. More importantly, YOU need to find out how much YOUR system loses along the way as everyone's brewing system is different.

Grain Absorbtion -

The amount of water absorbed by the grain during the mash can vary from system to system due to crushing methods, stirring practices, etc. Ray Daniels1 provides a constant of roughly 0.2 gallons of water absorbed per pound of grain. Denny Conn, in his great article on Cheap and Easy Batch Sparging2 provides an estimate of 0.1 gallons of water absorbed per pound based on his brewing system. As you can see, these vary greatly and can make a big difference in the total water added.
The best thing to do is for you to note the volume for your particular system over several brew sessions, and keep a table of pounds of grain to water absorbed for your system. When taking these measurements, remember that some of the liquid that did not make it into the boil pot may just be in the 'dead spaces' of your mash tun and not necessarily absorbed by grain (see below for example.)

Loss to Mash Tun -

This is the volume of wort left in your mash tun's 'dead space' after you have drained your mash tun in your normal manner. If you have a false bottom, this value may be a bit higher than if you have a manifold. To find this figure simply measure the liquid left in the tun next time you brew and empty the grains from your tun.

Remember this value can interact with our Grain Absorbtion calculation and you should not count this twice.

EXAMPLE : I conduct a mash using 14 pounds of grain and use 4.75 gal. of sparge water.
I find that after draining my mash tun (and before sparging) that I have collected 2.15 gallons of waterin my first runnings. I finish my sparge, drain the tun completely, and measure the volume of wort left in the bottom of the mash tun and find that there is 0.5 gallons left.

So, I can now calculate my grain absorbtion rate as :
Sparge Water Added - First Runnings Collected - Loss To Tun.
In this case : 4.75 - 2.15 - 0.5 = 2.1.

So, I know that for my system 14 pounds of grain absorbs 2.1 gallons of liquid. I also know from this experiment that my mash tun retains 0.5 gallons of wort.

Evaporation During Boiling -

Loss of liquid vapor to the air is inevitable during a good vigorous boil and is essential to carry off unwanted compounds in our final wort such as DME precursors. So, we must factor this amount into our equations as gallons boiled off per hour. A generally accepted rule is about 5 to 10% per hour loss1,4.
However, you may need to conduct your own experiment to find out exactly what this value is for your system based on the strength of your average boil. Simply observe your kettle volume at the beginning of boil and again at 60 minutes then plug it into the following formula :

Evaporation Pct. = 100 - (postBoil volume * 100 / preBoil volume)4

Shrinkage Loss -

When cooling from boiling to around 60 Degrees, liquid will lose density and, therefore, volume. This loss is known to be about 4%1. No experiment needed here unless you just really want to.

Loss To Trub -

All of the cold break, hop matter, and whatever else that settles at the bottom of your boil kettle after cooling is stuff that is better left in the kettle and not transfered to our fermenter. This is one factor that individual brewers will differ on. Some prefer to make more wort in anticipation of leaving plenty of this stuff behind. Some brewers don't worry so much and allow a bit to make it into the fermenter.
Either way, you will need to calculate this volume by getting an average volume of the trub you leave behind once you transfer to your primary fermenter. I usually leave behind about 0.5 to 0.75 gallons if you need a place to start.

Total Water Needed

Now that you have everything you need, you can figure out how much Mash and Sparge water are required for your batch. I think the best way to approach this is to break it into two steps.

1. Calculate Boil Kettle Related Losses [Kettle Loss] - This is anticipated Ending Batch Volume with the Trub Loss and Shrinkage and Evaporation Loss factored in. Kettle Loss is calculated by : ((Ending Volume + Loss to Trub) / Shrinkage Factor) / Evaporation Loss Factor)

This gives you a good start. Now you will need to factor in the water lost in the mash tun to get the total water needed.

2. Calculate Mash Tun Loss to get Total Water Needed - Take the Kettle Loss volume (from above) and add in the volume that will be lost to your mash tun deadspace, then add in the volume that will be absorbed by the grains. This will give you the total water needed for the batch.

Having these values, you can now calculate the Mash Tun Loss and total water needed by first calculating Kettle Loss:

((Batch Vol. + Loss to Trub) / ShrinkageFactor) /Evaporation Loss Factor)
((5 + 1) / 0.96) / 0.95) = 6.58

Then Calculate total Water Needed For the Entire Batch:

Kettle Loss + Mash Tun Loss + Grain Absorption Loss = Total Water Needed
6.58 + 0.5 + 2.1 = 9.18 gallons

Calculate Mash/Sparge Water Volumes

Now that you know the total water required for your batch, you can figure out which portion of this total to use for mashing and the rest will be used for sparging. To know this, you will need to know what mash thickness ratio to use. For most purposes, a good ratio is to use 1.25 Qts. per pound of grain (0.3125 gal/lb.)3.

Calculating the water is then straight-forward. The formula for mash water needed is :

If using qts/lb. : (MashThickness(qts./lb) * Pounds of Grain) / 4 = Gal. of Mash Water Needed
If using gal./lb.: MashThickness(gal./lb) * Pounds of Grain = Gal. of Mash Water Needed

EXAMPLE : Using the numbers from above, we have 14 lbs. of grain and are going to use a mash thickness fo 1.25 qts./lb. We know from calculations that the total water needed is 8.63 gallons.
So, to figure out how much of that water is mash water we use the following math :
(1.25 * 14 lbs.) / 4 = 4.37 gallons of Mash water

The sparge water volume can then easily be figured out by what is left over from the total water needed once the mash water is subtacted :
9.18 - 4.37 = 4.81 gallons of Sparge water


1. Designing Great Beers by Ray Daniels
2. by Denny Conn
3. BYO Magazine - Managing Mash Thickness by Tom Flores
4. BYO Magazine - How do you account for evaporation loss during an aggressive boil?

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