Calculating Water Usage
Mashing
The volume of strike water for the mash is a function of the amount of grain and the desired mash thickness. The mash thickness can vary with the recipe, the mash tun configuration, the volume of any additional mash water infusions, the sparge water volume and individual brewer preferences, but
a value in the range of 1.0–1.5 quarts of water per pound of grain is typical for home-brewers.
Therefore the formula for calculating the strike water volume is:
Strike water volume = weight of grain * desired mash thickness
For example, for a mash thickness of 1.25 qts./lb. and a grain bill calling for 10 lbs. of grain, the calculated strike water volume is 12.5 quarts (3.25 gallons).
The water absorbed by the grain will vary with the specifics of the grain bill, the type of malt and adjuncts and their moisture content, but an average value of 0.50 quarts per pound has proven to be a very reasonable assumption in most cases. In the hypothetical recipe above (10 lbs. of grain), the volume of water absorbed is 5.0 quarts (1.25 gallons).
In order to ensure adequate capacity of vessels for mashing and sparging, it is useful to know the total volume of the mash. The following simple formula should be rather obvious:
Total mash volume = volume of water + volume of grain
It is necessary to know the volume that the grain displaces when mashed (which is different from its dry volume). Once again this depends on the specifics of the grain bill, but a value of 0.32 quarts per pound is a reasonable average. Therefore, in the example above, the mash volume is 12.5 + 3.2 = 15.7 quarts (@4 gallons).
Sparging
Calculating the correct volume of sparge water is of particular importance because it greatly determines the wort pre-boil volume, which is critical to achieving the target original specific gravity. An old very general rule of thumb is to use approximately two quarts of sparge water per pound of grain, but other factors such as the mash thickness and any additional water infusions can change this considerably.
It is becoming increasingly popular for home-brewers to batch sparge by adding the sparge water in one or more batches, followed by stirring the mash, allowing it to settle and briefly recirculating the runoff until it clears before draining it into the boiling kettle. This can simplify and shorten the brewing session somewhat.
A useful value to know is the volume of first runnings that are drained from the mash tun prior to adding the sparge water. This is calculated using the following formula: Volume of first runnings = Strike water volume + volume of any other water added to the mash – volume of water absorbed by the grain – volume of liquid remaining in the bottom of the mash tun – volume of liquid remaining in lines or pump
In our example from the section on mashing earlier, and also using values of 1 quart for the liquid remaining in the mash tun and 0.25 quart for line losses, the calculated volume of the first runnings is:
12.5 + 0 – 5.0 – 1.0 – 0.2 = 6.3 quarts (1.6 gallons)
Whether you employ continuous sparging or batch sparging, the total volume of sparge water is calculated from the target pre-boil volume by subtracting the volume of the first runnings. Therefore the formula is:
Total volume of sparge water = Target pre-boil volume – volume of first runnings
In our example, the target pre-boil volume may be 28.0 quarts (7 gallons) and the volume of first runnings is 6.3 quarts (1.58 gallons). Therefore the total volume of sparge water is 28.0 – 6.3 = 21.7 quarts (5.43 gallons). For the purposes of providing an adequate volume of sparge water, add to the calculated value the volume of any dead space in the hot liquor tank or sparge water vessel.
Putting it all together
The total water needed consists of the sum of the volumes of the strike water, any additional mash water infusions, the sparge water (including any sparge water vessel dead space), any water added to the kettle prior to the boil, and any water added to the kettle or fermenter after the boil. Expressed as a formula, it is:
Total volume of water needed =
Volume of strike water +
volume of any additional mash water infusions +
volume of sparge water (including any sparge water vessel dead space) +
any water added to the kettle pre-boil +
any water added to the kettle or fermenter post-boil
In the example, there were no additional mash water infusions, sparge water dead space, or water added to the kettle or fermenter either prior to or after the boil. This results in the calculation of the total water volume for the brew session as the simple addition of the strike water and the sparge water, or:
12.5 + 6.2 = 18.7 quarts (4.68 gallons)
While the hypothetical calculations above have been expressed to a precision of 0.1 quart, in real-world home-brewing situations you may find it necessary only to measure the volume of the actual water additions to the nearest whole unit. For the 5–10 gallon batches of many home-brewers, this represents an accuracy of about 5% or less and is likely to produce meaningful and useful results.