Run the brew day backwards
Volume calculations go wrong when people work forwards, because every loss compounds and there is no point at which you can check. Start from the end instead: the volume you want in the fermenter is fixed, and everything before it is that number plus a loss.
- Cold wort in the fermenter — the number you actually care about.
- Plus kettle trub and whirlpool loss, which stays behind the pickup with the break material.
- Plus hop absorption, which is small with pellets and significant with whole cones.
- That is the cold wort in the kettle. Multiply by about 1.04 to get what it measured while boiling.
- Plus what boiled off, which is your rate times the boil length.
- That is the pre-boil volume, measured hot.
- Plus grain absorption and mash tun deadspace.
- That is the total water to measure out.
The worked default here — 5.5 gallons wanted, 1.25 gal/hr for 60 minutes, 11 lb of grain — comes to 8.43 gallons of water in for 5.5 gallons of beer out. Thirty-five percent of what you measure into the tun never reaches the fermenter, and that is a normal, well-run brew day rather than a problem.
Hot to cold shrinkage, the loss nobody accounts for
Water at boiling temperature occupies about 4% more volume than the same water at room temperature. If you dip a marked stick into a boiling kettle and read 6 gallons, you have about 5.75 gallons of cold wort. People who measure hot and plan cold consistently come up a quarter gallon short and never work out why.
There are two ways to handle it and only one of them is safe. Either measure everything hot and apply the correction once, as this calculator does, or wait until the wort is chilled and measure then, which is accurate but far too late to fix anything. Marking your kettle with a stick calibrated at room temperature and then reading it while boiling is the common mistake — the marks are right, the wort is expanded, and the difference lands in your final volume.
Boil-off rate is yours, not a published number
Boil-off depends on kettle diameter far more than on burner output, because evaporation happens at the surface. A wide, shallow kettle boiling gently can lose more per hour than a tall narrow one at a rolling boil. Published figures range from about 0.8 to 1.5 gallons per hour and none of them are about your equipment.
Measuring it takes one afternoon. Put a known volume of water in the kettle, mark the level with a dipstick, boil uncovered at the vigour you actually use for an hour, let it settle, and measure again while it is still hot. The difference is your rate. Do it once, write it on the kettle, and stop guessing. Note that the same kettle boils off differently outdoors on a windy day than in a garage, and differently with the lid partly on, so if your conditions vary, so does the number.
| Loss | Typical range | How to pin it down |
|---|---|---|
| Boil-off | 0.8 to 1.5 gal/hr | Boil water for an hour and measure the drop |
| Grain absorption | 0.10 to 0.13 gal/lb | Compare water in against runnings out on a real mash |
| Mash tun deadspace | 0.1 to 0.5 gal | Fill with water, drain fully, measure what is left |
| Kettle trub and whirlpool | 0.25 to 0.75 gal | Measure what stays in the kettle after transfer |
| Hop absorption | 0.01 to 0.02 gal/oz pellets, far more for cones | Weigh the spent hops or accept the range |
Volume error is gravity error
The gravity points in a batch are set by the mash. The boil does not create sugar, it removes water, which concentrates what is already there. So gravity points times gallons is a constant from the end of the mash onward, and any volume error becomes a gravity error of the same proportion in the opposite direction.
That is a useful property, because it makes the problem fixable while you can still see it. Take a pre-boil gravity reading, compare it against the figure this calculator gives, and you learn which of two very different things went wrong. Low gravity at the right volume means the mash under-converted, and boiling longer will only make a small strong beer. The right gravity at too much volume means the mash was fine and a longer boil lands you exactly where you wanted. Diluting an over-strong wort with water works too, and is the easier direction.
Downstream
The mash side of the same calculation — strike temperature, strike volume and the sparge — is on the strike water calculator, which takes the pre-boil figure from here as its input. Once the batch is fermented, the ABV and attenuation calculator turns the two gravity readings into an alcohol figure, and the priming sugar calculator needs the volume that actually reaches the bottling bucket rather than the recipe number. For water volume outside brewing, the pipe volume calculator and the rainwater harvesting calculator handle different questions with the same arithmetic.
Questions people ask
I keep ending up short in the fermenter. Which number is wrong?
Almost always boil-off or trub loss, and the way to tell them apart is to measure the volume at the end of the boil before transfer. If the post-boil volume is right and the fermenter is short, the loss is in the transfer — trub, whirlpool and hops. If the post-boil volume is already short, it is boil-off, and your rate is higher than you entered. A third possibility is that you are measuring the kettle hot and planning cold, in which case you are 4% short on every batch and nothing you change about the process will fix it.
Does boil-off rate change with the size of the batch?
The rate in gallons per hour stays roughly constant for a given kettle and boil vigour, because evaporation is driven by surface area and that does not change with fill depth. What changes is what that rate means proportionally. Losing 1.25 gallons from a 7 gallon pre-boil is 18%; losing the same 1.25 gallons from a 3 gallon partial boil is 42%. If you brew both full and partial batches in the same kettle, use the same rate in gallons per hour rather than a percentage, and expect the small batches to be far more sensitive to boil length.
Should I squeeze the grain bag to get more wort back?
You can, and it reduces the absorption figure meaningfully — brew-in-a-bag brewers who squeeze hard often see absorption nearer 0.08 gal/lb than 0.12. The old warning that squeezing extracts tannins has not held up well in practice at normal mash pH, but the point that matters here is simply that if you squeeze, measure your absorption with squeezing included, and if you sometimes squeeze and sometimes do not, your volumes will vary batch to batch by the difference.
How much does whole hop absorption really cost me?
A great deal more than pellets, which is the practical argument for pellets in a small batch. Pellets soak up something like 0.01 to 0.02 gallons per ounce. Whole cones retain far more because of the physical bulk of the flower, and figures several times higher are commonly reported. For a heavily hopped batch with whole cones this stops being a rounding error and becomes half a gallon or more. If you use cones, either measure what a hop stand costs you or plan the batch a half gallon larger.
Does this work for extract brewing?
Yes — set the grain bill to zero and the mash tun deadspace to zero, and the calculation reduces to boil-off, shrinkage, trub and hops. Extract brewing usually adds a step this calculator does not model, which is topping up the fermenter with cold water after a partial boil. If you do that, set the target volume to whatever you want the concentrated wort to be before top-up, and add the top-up separately. The gravity relation still holds: points times gallons is constant, so a partial boil topped to full volume dilutes exactly in proportion.