Water Volume Calculator

Where the water goes

Every brew starts with more water than ends up in the fermenter, because water is lost at every stage:

1. Grain absorption — roughly 1.04 L per kg of grain. The grain soaks it up and keeps it.
2. Boiloff — typically 3.5–4.5 L per hour on a home system. Wider kettles and stronger burners lose more.
3. Trub and kettle loss — the sludge of hops and protein left in the kettle after transfer.
4. Fermenter loss — the yeast cake and trub at the bottom of the fermenter.

This calculator works backwards from your target batch volume, adding each loss to arrive at the total water you need to start with.

Mash water and sparge water

In a sparge mash:

Mash water = grain weight × water-to-grain ratio. At the default 2.6 L/kg, a 5 kg grain bill takes 13 liters of mash water.

Sparge water = pre-boil volume − first runnings. First runnings are the mash water minus what the grain absorbed and any mash tun dead space.

The split matters because mash water has to be heated to strike temperature (use the strike water calculator for that), while sparge water is typically heated to 76–78 °C.

In a full-volume mash there is no split. Mash water = pre-boil volume + grain absorption + dead space — for that same 5 kg bill and a 19 L batch, 26.9 liters, a ratio of 5.4 L/kg rather than 2.6. All of it is heated to strike temperature, and because the mash is so much thinner it loses less heat to the grain: hitting a 66 °C rest takes 72.8 °C strike water when sparging and 69.3 °C at full volume. Switch the mash type above and watch the ratio move; the total water does not change, only where it goes.

Adjusting for your system

The defaults assume a typical home brewery: 2.6 L/kg water-to-grain ratio, 3.8 L/hr boiloff, 0.5 L trub loss and 0.5 L fermenter loss. If your system differs — and it will — measure your actual boiloff rate and grain absorption once, then use those numbers going forward. The brewhouse efficiency calculator will tell you whether your recipe gravities match reality.