Two entirely separate mechanisms shrink your water volume between strike and finished beer, and conflating them (or forgetting one) is the most common reason a brew day ends with less beer than planned.
Grain absorption
Grain retains roughly 1.04 L of water per kg — John Palmer's published 0.5 quart per pound converted to metric, the standard figure reproduced across brewing software — water that never makes it into the kettle as collected wort, no matter how thoroughly you sparge. For common grain-bill weights: 4.5 kg (10 lb) absorbs roughly 4.7 L; 9.1 kg (20 lb) absorbs roughly 9.5 L. (Brew-in-a-bag with a hard bag squeeze is commonly reported lower, closer to 0.5-0.75 L/kg, because squeezing physically recovers wort a standard lauter leaves behind — a different technique, not a different physical constant.) See how much water do my grains absorb for the fuller explanation and how this figure varies with crush and grain type.
Boil-off rate
Boil-off is water lost to evaporation during the boil, and it's a rate, not a fixed volume — it depends on your boil's vigour, your kettle's surface area, and your local altitude and humidity, so it varies more system-to-system than grain absorption does. A commonly cited planning range for a typical homebrew kettle is roughly 3.5-4.5 L per hour (roughly 1-1.2 gal/hour) at a good rolling boil, though a wide, shallow kettle with strong airflow can boil off meaningfully faster, and a covered or gently-simmering boil noticeably slower.
Why both matter for the same number
Both mechanisms shrink volume between your strike water and your finished, packaged beer, and both need separate line items in a water-volume plan — grain absorption during the mash and lauter, boil-off during the boil, plus whatever's lost to kettle dead space, chiller loss, and trub/hops left behind at transfer. Skipping either in your planning produces a batch that lands short of its intended final volume even when strike water and boil temperature both went exactly to plan; skipping boil-off specifically is the more common miss, since it's easy to plan pre-boil volume without separately accounting for how much a full 60-90 minute boil actually removes.
A rough example, put together
For a 19 L (5 gal) target final volume with a 60-minute boil at a mid-range 4 L/hour boil-off rate: that's 4 L boiled off, putting the target pre-boil volume at 23 L, not 19 L — before adding whatever your specific system loses to trub and dead space (commonly another 1-2 L on top). For a 4.5 kg (~10 lb) grain bill on the same batch, add the 4.7 L grain absorption figure above and the total mash-and-sparge water requirement comes to roughly 27.7 L — well over 8 L more than the 19 L final-volume figure a brewer planning only the finished batch size might reach for. Every system differs enough in dead space and boil vigour that measuring your own actual boil-off rate over a batch or two — noting pre- and post-boil volume at a known boil time — is worth doing once, rather than relying on a generic range indefinitely.
Turning this into an actual strike-water plan
Once you know your grain absorption and boil-off numbers, the strike water calculator still only handles the temperature and volume of the strike water itself — see how much water for a 10-pound grain bill for how that figure combines with absorption into a fuller brew-day water plan.