Calculating how much water you need at each step of the brewing process is fairly easy. So much so that you can do it in your head!

Terms and Definitions

Let's start with some quick definitions in order of when they appear in your brew day.

  • Strike Water - The starting amount of hot water (usually between 60C and 80C) that is used for the mash (adding grain to water).
  • Grain Absorption - The amount of water absorbed by the grain.
  • First Runnings - The amount of wort (sugary water) that is collected from the mash tun after your mash has finished.
  • Sparge Water - Water that is added to the mash tun after mashing to rinse the grain of any left over sugars.
  • Pre-Boil Volume - The amount of wort (sugary water) that is in your kettle before you start your boil.
  • Evaporation Rate - The amount of water that is boiled off. Usually in Litres per Hour (or Gal/hr).
  • Post-Boil Volume - The amount of wort left after you finish the boil.
  • Kettle Loss - The amount of wort (if any) left at the bottom of the kettle that was not transferred to the fermenter.
  • Fermenter Volume - The amount of wort you were able to get out of the kettle and put into the fermenter.
  • Trub Loss aka Fermenter loss - The amount of beer (if any) at the bottom of the fermenter that was not transferred into bottles or kegs.
  • Batch Size - The amount of beer that you were able to get out of the fermenter and put into bottles or kegs.

Calculations

Next let's step through the calculations needed to get all the volumes for your brew day.

To calculate all of the volumes of water, we actually start at the end and work our way backwards.

Firstly we need a batch size. This is the amount of beer you want to end up with. For our example we'll use 20L or 5 Gal.

Batch Size: 20L (5 gal)

Next we'll compensate for any Trub Loss, by adding that amount to the Batch Size. This will give us our Fermenter Volume. We'll use 1L of trub loss for this example.

Fermenter Volume = Batch Size + Trub Loss
Fermenter Volume = 20L (5 gal) + 1L (0.25 gal) = 21L (5.25 gal)

Next we'll calculate our post-boil volume by compensating for any kettle loss. It's the exact same deal here, but this time we'll say that we don't have any kettle loss. We use a pickup tube :)

Post-Boil Volume = Fermenter Volume + Kettle Loss
Post-Boil Volume = 21L (5.25 gal) + 0L (0.00 gal) = 21L (5.25 gal)

Next we calculate the Pre-Boil volume based on how much water will evaporate during the boil.
Evaporation rates vary quite wildly from system to system, so you'll only know your rate once you've tried. A nice guess would be 5L/hr (1.25 gal/hr), but remember to measure this yourself! We'll use 3L/hr (0.75 gal/hr) for this example.

Pre-Boil Volume = Post-Boil Volume + (Evaporation Rate x Boil Length in Hours)
Pre-Boil Volume = 21L (5.25 gal) + (3L/hr (0.75 gal/hr) x 1) = 24L (6 gal)

Now we can start to work forwards from the beginning of the brew day again, towards our target pre-boil volume.

First we need to figure out how much strike water we'll need. The amount of strike water you need depends on how much grain you have in your grain bill (aka grist). This one is more of a guide towards best practice mash thickness, but you can alter to suit your needs. In this example we'll have 6kg grain, or 14 pounds for imperial.

Metric
Strike Water (L) = Grain Weight (kg) x 3
Strike Water = 6 x 3 = 18L

Imperial
Strike Water (quarts) = Grain Weight (pounds) x 1.25
Strike Water = 14 x 1.25 = 17.5 quarts = ~4.5 gal

Notice that we don't end up with the same volumes, that's because both systems round a little bit to make the calculations simpler. If you were to use a water calculator, you'll find they use much more accurate values to arrive at the same volume for both metric and imperial.

Now that we've got our strike water volume, we can start figure out how much wort we'll extract for our first runnings. Firstly, we'll figure out how much water we'll lose to grain absorption. Remembering that we have 6kg (14 pounds) of grain.

Metric
Grain Absorption = 1 L/Kg
Grain Absorption = 6L

Imperial
Grain Absorption = 1 pint/pound
Grain Absorption = 14 pints = ~1.5 gal

To calculate our first runnings, simply remove the grain absorption volume from your strike volume.

First Runnings = Strike Volume - Grain Absorption
First Runnings = 18L (4.5 gal) - 6L (1.5 gal) = 12L (3 gal)

Alright, next we can calculate how much sparge water we'll need to reach our target pre-boil volume. This one's easy! Since the grain has already absorbed as much water as it can, any water we add to our mash tun (or BIAB pot) will be added to our total water volume. This means that the sparge water volume is simply the difference between your pre-boil target and your first runnings.

Sparge Water = Pre-Boil Volume - First Runnings
Sparge Water = 24L (6 gal) - 12L (3 gal) = 12L (3 gal)

This completes the calculations.

Figure: Example values for 20L (5 gal) batch size.