Grain holds onto water during the mash and lauter, and that water becomes part of your total water usage even though it never ends up in the kettle — the standard published figure, from John Palmer's "How to Brew" and reproduced across brewing software, puts absorption at 0.5 quart per pound of grain, roughly 1.04 L per kg, and it's worth building into your water planning rather than discovering the shortfall on brew day.
The arithmetic at a few common grain weights
| Grain weight | Absorbed water |
|---|---|
| 4.5 kg (~10 lb) | ~4.7 L |
| 5.4 kg (~12 lb) | ~5.6 L |
| 9.1 kg (~20 lb) | ~9.5 L |
This is straightforward multiplication against the published 1.04 L/kg rate, not a separate calculator — the value is worth knowing precisely because it's easy to forget when planning total water volume, and the error compounds as grain bills get bigger.
Why it isn't perfectly exact for every grain
1.04 L/kg is a widely-cited average for a standard mash and lauter, not a physical constant — the true absorption rate varies somewhat with crush consistency (a finer crush exposes more surface area and can hold slightly more water), grain type (a grain bill heavy in flaked adjuncts or wheat can absorb more than an all-base-malt bill), and how long the grain sits in contact with water before lautering. Brew-in-a-bag with a hard bag squeeze is commonly reported meaningfully lower, closer to 0.5-0.75 L/kg, because physically squeezing the bag recovers wort a standard lauter simply leaves behind — a different technique, not evidence the standard figure is wrong. Treat 1.04 L/kg as a reliable planning figure for a normal all-grain mash, and if your actual pre-boil volume consistently comes in a bit short or long relative to what this rate predicts, adjust your own personal absorption rate for future batches based on your specific system rather than assuming the published average is wrong.
Where this fits into total water planning
Absorption sits between your strike water and your total pre-boil volume: strike water goes into the mash, some of it stays behind in the grain (the absorption figure), and the rest — plus any sparge water — becomes your collected wort. See how much water for a 10-pound grain bill for how this combines with the strike water calculator's output into a full brew-day water plan, rather than treating strike volume alone as your total water requirement.
The other side of the same coin
Boil-off rate is the other major water-volume factor on brew day, working in the opposite direction — water lost to evaporation during the boil, rather than retained by grain during the mash. Both figures need to be accounted for separately when planning a target pre-boil volume that actually lands you at your intended final batch size; neither one substitutes for the other, and skipping either produces a batch that comes up short of its intended volume even when every other step went exactly to plan.