Total water needed for an all-grain batch = batch size + boil-off + trub/deadspace loss + grain absorption. Grain absorption runs about 0.125 gallons per pound of grain (0.5 liters per kilogram). For a 5-gallon batch with a 10-pound grain bill, a 60-minute boil, and typical equipment losses, that works out to roughly 7.75 gallons (29.3 liters) total.
What actually eats into your water total
Four things separate your final batch size from the water you start with:
- Grain absorption — grain soaks up and holds onto water that never drains into the kettle, roughly 0.125 gal/lb (0.5 L/kg)
- Boil-off — water that evaporates during the boil, typically 0.75–1.5 gal/hr (2.8–5.7 L/hr) depending on kettle diameter and boil vigor
- Trub and deadspace loss — wort left behind in the kettle and chiller that never makes it to the fermenter, usually 0.25–0.75 gal (1–2.8 L)
- Batch size itself — the actual volume you want in the fermenter
Mash ratio (how thick or thin you mix the mash) doesn't change your total water — it only changes how that total splits between mash water and sparge water. A thicker mash just means more sparge water later to reach the same pre-boil volume.
Worked example: 5-gallon batch, 10 lb of grain
Assume a 60-minute boil, boil-off of 1 gal/hr, and 0.5 gal of trub/deadspace loss:
- Batch size: 5 gal
- Boil-off (60 min @ 1 gal/hr): 1 gal
- Trub/deadspace loss: 0.5 gal
- Grain absorption (10 lb × 0.125 gal/lb): 1.25 gal
Total water = 5 + 1 + 0.5 + 1.25 = 7.75 gal (29.3 L)
Splitting total water into mash and sparge
Pick a mash ratio — 1.25 qt/lb (2.6 L/kg) is a common default that balances easy stirring against mash tun capacity. For 10 lb of grain, that's 12.5 qt, or 3.125 gal (11.8 L) of mash (strike) water.
Sparge water is just what's left: 7.75 − 3.125 = 4.625 gal (17.5 L) of sparge water to reach your 6.5-gallon pre-boil target.
Boil-off rate varies a lot by setup — a wide, shallow kettle on a strong burner boils off faster than a narrow pot on a weak stovetop element. Measure your own kettle's boil-off rate once (mark the pre- and post-boil volume on a dowel) and use that number instead of a generic estimate for accurate results going forward.
Skip the manual math
Pick a BJCP style in the recipe calculator and get a full grain bill, hop schedule, and brew-day plan scaled to your batch size.
For the full mash and water chemistry chapter — including strike temperature and sparge technique — see Home Brewing Science.
Boil-off and trub loss figures are typical ranges, not universal constants — equipment varies. Measure your own setup after a batch or two and adjust these numbers for consistently accurate results.