Work backwards from the fermenter, never forwards from the tap
Every brew day loses water at four separate points, and the only volume you actually care about is the last one. So the calculation runs in reverse: decide what you want in the fermenter, then add back each loss in the order it occurs, ending at the tap.
Start at the fermenter with your target — say 5.5 gallons. Add the wort you will abandon in the kettle under the cold break and hop debris, typically half a gallon on a homebrew system with a dip tube, more with a lot of whole hops. That gives the cooled volume the kettle must hold at the end of the boil. Convert it to a hot volume, because wort measured at boiling occupies roughly 4% more space than the same wort at room temperature and your kettle markings were almost certainly made with cold water. Add the boil-off. Add the water the grain will soak up and keep. Add whatever sits below your mash tun's outlet and never runs off. That total is what you heat.
Splitting it is the easy part. Mash water is grain weight times your chosen thickness — 11 lb at 1.25 qt/lb is 13.75 qt, or 3.44 gallons. Sparge water is simply the remainder. There is no separate sparge formula, and that is the insight that makes the whole thing tractable: the sparge is a residual, not a target.
Notice which of these losses scale with what. Boil-off scales with time and kettle geometry but not with batch size, so a one-gallon test batch loses nearly as much as a five-gallon batch on the same burner. Grain absorption scales with grain weight, so a strong beer loses far more water proportionally than a session beer. Trub and deadspace are fixed volumes that do not scale at all, which is why small batches are so much less efficient.
The four losses, and what a realistic number looks like
Grain absorption is the largest loss in most all-grain batches. The standard figure is 0.125 gal per pound of grain, which is 1.04 L/kg — half a quart per pound. It is not a constant of nature: a bag squeezed hard drains to nearer 0.08 gal/lb, while a deep bed left to drip under gravity holds more. Measure yours by weighing the spent grain wet and dry if you want to be exact, or accept 0.125 and adjust after three brews.
Boil-off is the loss people most often get wrong, because it is usually quoted as a percentage. A percentage is meaningless: evaporation depends on the surface area of the boil and the vigour of it, not on how much wort is underneath. Measure it once as gallons per hour on your own kettle and it will hold across batch sizes. Homebrew kettles typically run 0.75 to 1.5 gal/hr uncovered; wide flat kettles on strong burners run higher.
Cooling shrinkage is about 4% between 212 °F and room temperature. It is not a loss so much as a bookkeeping trap: it only bites you if you measure hot volume against a cold-calibrated dipstick, which almost everybody does. Include it, or measure everything hot and set it to zero — but pick one and stay with it, because mixing the two conventions is the single most common source of a batch that comes up half a gallon short.
Trub and deadspace are just geometry. Deadspace is whatever sits below your false bottom or manifold in the mash tun; measure it by filling an empty tun until wort starts to run off. Kettle loss is the cone of break material and hops around the pickup, typically 0.25 to 0.75 gallons and much more if you use a lot of whole hops without a hop spider.
Worked example: 5.5 gallons of an 11 lb pale ale
Target 5.5 gallons into the fermenter, 11 lb of grain mashed at 1.25 qt/lb, 60-minute boil, kettle boils off 1.2 gal/hr, 0.5 gallon of kettle loss, 0.25 gallon of tun deadspace, 4% shrinkage.
- Cooled wort needed in the kettle. 5.5 + 0.5 = 6.00 gal.
- Same wort measured hot. 6.00 × 1.04 = 6.24 gal.
- Boil-off. 1.2 gal/hr × 1.0 hr = 1.20 gal, so pre-boil volume = 6.24 + 1.20 = 7.44 gal.
- Grain absorption. 11 × 0.125 = 1.375 gal.
- Total water. 7.44 + 1.375 + 0.25 = 9.065 gal, which is 34.3 L.
- Mash water. 11 × 1.25 ÷ 4 = 3.44 gal (13.75 qt, 13.0 L).
- Sparge water. 9.065 − 3.4375 = 5.63 gal.
Check the mash will fit. The mash water is 3.44 gallons and the grain itself displaces about 0.08 gal per pound — malt solids run near 1.55 g/cm³, so a pound occupies roughly 0.29 L — adding 0.88 gallons. Total mash volume is 4.32 gallons, comfortable in a 10-gallon cooler and a squeeze in a 5-gallon one. Now heat the mash water to the figure from the strike water temperature calculator and you are ready to dough in.
Checking the plan against your equipment
Three sanity checks before you light the burner.
Will the mash fit in the tun? Mash volume is the mash water plus roughly 0.08 gallons per pound of grain for the grain itself. A 15 lb grain bill at 1.5 qt/lb needs 5.6 gallons of water plus 1.2 gallons of grain — 6.8 gallons of mash, which will not go in a 5-gallon cooler. If it does not fit, mash thinner is the wrong answer (that makes it bigger); mash thicker and sparge more.
Will the pre-boil volume fit in the kettle? You need headroom above the pre-boil volume or you will boil over in the first five minutes. A 7.44-gallon pre-boil wants a 10-gallon kettle.
Is the sparge volume sensible? A very large sparge relative to the mash — say three times the mash volume — rinses the grain bed thin and risks pulling tannins from over-diluted, high-pH final runnings. Stop collecting when the runnings fall below about 1.008, whatever the calculator says. A very small sparge, or none, costs efficiency, which you can quantify afterwards with the brewhouse efficiency calculator.
If your gravity comes out consistently high, you collected less volume than you planned — usually because boil-off is greater than you assumed. Consistently low gravity with the right volume points at the mash rather than the water plan. Consistently correct gravity but short volume in the fermenter means your trub loss is bigger than you entered. Each of those symptoms points at a different input on this page, which is exactly why it is worth writing the numbers down.
Mash volume and grain absorption by grain bill
| Grain | Mash at 1.25 qt/lb | Mash at 1.5 qt/lb | Mash at 1.25 (L) | Water absorbed |
|---|---|---|---|---|
| 8 lb / 3.6 kg | 3.14 gal | 3.64 gal | 11.9 L | 1.00 gal |
| 10 lb / 4.5 kg | 3.93 gal | 4.55 gal | 14.9 L | 1.25 gal |
| 12 lb / 5.4 kg | 4.71 gal | 5.46 gal | 17.8 L | 1.50 gal |
| 14 lb / 6.4 kg | 5.50 gal | 6.37 gal | 20.8 L | 1.75 gal |
| 16 lb / 7.3 kg | 6.28 gal | 7.28 gal | 23.8 L | 2.00 gal |
| 18 lb / 8.2 kg | 7.07 gal | 8.19 gal | 26.7 L | 2.25 gal |
The mash volume columns are water plus grain displacement, which is what has to physically fit in the tun. The absorption column is what never comes back out, and is already counted in the total water figure the calculator returns.
Measure your boil-off rate once and stop guessing
Put a known volume of water in your kettle — six gallons, measured cold with a jug, and mark the level on a stick. Boil it as hard as you boil wort, uncovered as you normally would, for exactly one hour. Let it cool to room temperature and measure again. The difference is your boil-off rate in gallons per hour, and because it depends on kettle diameter and burner output rather than on batch size, it stays valid for every batch you brew on that kit. Do it once and you eliminate the most variable input on this page.
Where a water plan goes wrong
- Quoting boil-off as a percentage. Evaporation is driven by surface area and vigour, not by the volume underneath. A percentage is only valid at the batch size it was measured at.
- Mixing hot and cold volume measurements. Wort shrinks about 4% on cooling. Choose a convention and apply it to every measurement on the brew day.
- Forgetting the mash tun deadspace. It is invisible because you never see the wort that stays under the false bottom, but it comes straight off your pre-boil volume.
- Ignoring the volume a step infusion adds. Boiling water added for a mash-out is part of your total water, and comes out of the sparge; size it with the strike water temperature calculator before you split the totals.
- Sparging past 1.008. The last, thin runnings carry tannins and raise the wort pH. Stop and accept the short volume, then top up with plain water if you must.
- Planning a mash that will not fit. Add roughly 0.08 gal per pound for the grain's own volume before you assume the tun is big enough.
Batch sparge, fly sparge and no sparge
The totals on this page are the same whichever sparge method you use; only the way you deliver the sparge water changes.
Batch sparging drains the mash completely, adds the sparge water in one or two doses, stirs, rests and drains again. It is fast, needs no special equipment, and is very hard to do badly. Efficiency lands a few points below fly sparging, and splitting the sparge into two equal doses recovers a little of that.
Fly sparging trickles the sparge water on top of the bed at the same rate the wort runs off, keeping an inch or two of liquid above the grain for forty-five minutes or so. It rinses more thoroughly and gives the best efficiency, at the cost of time, a sparge arm and enough attention to keep the flow balanced. Run it too fast and the water channels straight through, giving you neither the efficiency nor the time back.
No sparge, including full-volume brew-in-a-bag, mashes with all the water at once. It is the simplest possible brew day and produces excellent beer, but the grain keeps its full share of a much larger sugar-bearing volume, so efficiency drops eight to twelve points. Compensate by adding grain, which is exactly what the ratio between your planned and actual efficiency tells you to do.
Whichever you use, the water plan feeds the rest of the brew day. Pre-boil volume and gravity set the average boil gravity that decides your hop utilization in the Tinseth IBU calculator, and the volume that lands in the fermenter sets the pitch you need from the yeast pitching rate calculator. Getting the volumes right is what makes every other calculation on brew day mean something.