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This calculator estimates the total amount of water needed for an all-grain batch. The main variables are the target volume plus water lost to boil-off and water absorption. The calculator does not include possible losses from shrinkage when boiled wort is cooled or equipment losses (e.g., false bottoms, transfers).
Batch and no-sparge calculations assume that the grain bed will be run dry, with no free wort being left in the mash tun. Beechum and Conn (2020) give the estimate of .12 gallons per pound of grain of water absorbed (1 liter per kilogram). In contrast, continuous (fly) sparging requires more water because the grains are floating in a medium-thick mash until a run-off gravity of 1.008 is reached. The sparge is then stopped, resulting in some non-absorbed water being left behind in the mash tun. In my system, this is about .24 gallons per pound (2 liters per kilogram). The calculations use a mash thickness of 2.5 liters per kilogram to be conservative.
The boil off evaporation percentage is calculated as (1 - (final boil volume / pre-boil volume)) * 100. My system boils off about 1.0 gallon/hour with a medium intensity boil. An example batch would be starting at 6.5 gallons and ending at 5.5 gallons. The proportion boiled off is 1 - (5.5 / 6.5) = 1 - .85 = .15 or 15% expressed as a percentage. Small homebrewing systems may boil off more, like 1.5 to 2.5 gallons per hour (23% to 42% per hour). The default value of 20% seems realistic for most homebrewing systems.
The hop losses were empirically determined from boiling 10 g of CTZ whole cone hops in 500 ml of water for 1 minute. This absorbs 100 ml. The relationship is that 1 g of hops absorbs 10 ml, which is 10X the weight in metric units. This can be understood as reversing the dehydration process used in fresh hops. When hops are dried to an 8 to 10% water content, the hops will absorb water equal to about 10 times the dry weight during the boil. When dry hopping is done, the dry hop weight could be included in the total hop weight.
Beechum, D. & Conn, D. (2020, January-February). Sparge ho! The many ways of rinsing grain (or not). Brew Your Own, 79-81.
Carr, N. (2020). How to calculate water loss during brewing
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