Points times gallons is the only quantity worth tracking
A gravity reading on its own tells you nothing about how well the mash worked, because a small volume of strong wort and a large volume of weak wort can come from the same sugar. The quantity that survives the brew day intact is gravity points times gallons. A gravity point is the third decimal place of specific gravity multiplied out: 1.052 is 52 points. Five and a half gallons at 52 points is 286 points, and that number does not change when you boil, chill, dilute or transfer. Only adding or removing sugar changes it.
Efficiency is that number divided by what the grain could have given. Malt potential is quoted as points per pound per gallon — one pound of a malt rated 36 ppg, fully converted and completely rinsed into one gallon, would give a gravity of 1.036. Eleven pounds at 36 gives 396 points as an absolute ceiling. Landing 286 of them is 72.2%, and that is a perfectly ordinary result on a home system.
The potential figure is the input people get wrong, because it is copied from a forum rather than read off a spec sheet. Base malts sit in a narrow band, but a bill with a meaningful share of crystal and roasted malt has a lower weighted average, and using a base-malt figure for the whole bill quietly flatters the efficiency. Weight each malt potential by its pounds and average them. If you do not have the sheets, the number is a guess and the efficiency inherits the guess.
Two efficiencies, and the gallon between them
The gap that confuses people is between mash efficiency and brewhouse efficiency, and it is not a mystery: it is the wort you leave in the kettle. Work the default case through. Seven gallons of 1.044 pre-boil is 308 points, which is 77.8% of the 396 the grain offered. After the boil the wort is 1.052; five and a half gallons reach the fermenter and four tenths of a gallon stay behind with the trub. Those 5.9 gallons hold 307 points, near enough the 308 that went in — the boil moved none of them.
But the efficiency you are charged for is only the 286 that fermented. The four tenths of a gallon left in the kettle carried 21 points out of the calculation, and that is worth 5.3 percentage points of brewhouse efficiency on its own. It is not a conversion problem, it is not a sparge problem, and no amount of crushing finer will touch it. It is a gallon you paid grain for and poured out.
That distinction is the practical value of running both numbers. A low mash efficiency and a small gap points at the crush, the sparge or the mash. A decent mash efficiency and a large gap points at the kettle, the whirlpool and the hops, and the fix is a different one entirely.
| What moved | Effect on mash efficiency | Effect on brewhouse efficiency |
|---|---|---|
| Finer crush | Rises | Rises with it |
| Longer or better sparge | Rises | Rises with it |
| Bigger grain bill, same system | Usually falls | Falls with it |
| More trub left in the kettle | Unchanged | Falls |
| Heavier hop charge | Unchanged | Falls — cones hold a lot of wort |
| Smaller batch, same losses | Unchanged | Falls sharply |
The check that catches a bad reading
Because points times gallons is conserved through the boil, entering both a pre-boil and a post-boil measurement gives you a free consistency check on four numbers at once. If the pre-boil count and the post-boil count disagree by more than a few percent, one of them is wrong, and the calculator says so rather than averaging the error away.
In practice the offender is nearly always a volume, and nearly always the pre-boil one. Wort at a rolling boil occupies about four percent more space than the same wort cold, so a dipstick calibrated at room temperature over-reads while the kettle is going. Read seven gallons hot and you have about 6.73 gallons of cold wort, which is a 4% error landing directly in a number you are dividing by. The volume side of that correction lives on the brewing water calculator.
What to do with the number once you have it
Nothing, if you are only brewing one batch. Efficiency is a planning tool: its whole use is that once you know your own effective points per pound, you can size a grain bill for a target gravity without guessing. At 72.2% of a 36 ppg average, each pound is worth 26 effective points. A 1.060 batch at 5.5 gallons needs 330 points, so it needs 12.69 pounds of grain. The recipe, written as if the mash were perfect, would have said 9.17 pounds, and 9.17 pounds at your efficiency lands the batch near 1.043 — seventeen gravity points short of where you aimed.
The trap is treating your figure as a constant. It is stable for batches that resemble each other and it is not stable across a jump in grain bill size, because a bigger bed absorbs more water and rinses less completely in the same tun. If your normal batch is eleven pounds and you brew a twenty-five pound bill, expect the efficiency to drop, and plan the grain with a lower figure rather than being surprised at the hydrometer.
Questions people ask
What counts as a good brewhouse efficiency?
The question has less content than it looks like, because efficiency is a ratio between your equipment and a potential figure you chose, and both halves move. Home all-grain systems commonly land somewhere in the sixties and seventies, and the number people quote is often flattered by using a base-malt potential across a bill that contains crystal and roast. What matters is that your figure is repeatable, because a repeatable 65% builds accurate recipes and an unstable 78% does not. Chasing the number upward has real costs too: a very fine crush and a long sparge both fix efficiency and both create their own problems.
Why did my efficiency drop when I brewed a bigger beer?
Because the same tun and the same amount of sparge water now have to rinse a deeper grain bed. A larger bill absorbs more water in absolute terms, and the wort left behind in that grain is a larger share of the total sugar, so less of it reaches the kettle. The effect is real and well known, and it is why high-gravity recipes are usually written with a lower efficiency assumption than the same brewer uses for ordinary strength. Enter the efficiency you measured on a batch of similar size rather than your usual figure when planning a big one.
Should I use mash efficiency or brewhouse efficiency to size a grain bill?
Brewhouse efficiency, because it is the one that includes the wort you never ferment. A grain bill sized on mash efficiency assumes everything that came out of the tun ends up in the fermenter, and it will not, so the batch lands short by whatever the kettle keeps. Mash efficiency is a diagnostic figure, useful for telling a conversion or sparge problem apart from a kettle loss. Size recipes with the brewhouse number and use the mash number to work out which end of the process to fix.
My efficiency came out over 100%. What did I do wrong?
One of four inputs. The most common is a malt potential set too low for what is actually in the bill, which shrinks the denominator. Next is a volume measured hot and entered as cold, which inflates the points count by about four percent. Third is a gravity sample drawn before the wort was mixed through, particularly after a top-up or a late sugar addition, which reads whatever layer the thief happened to be in. Fourth is simply entering the pre-boil volume where the fermenter volume was asked for. Check the potential figure first; it is the one that comes from memory rather than from a measurement.
Does squeezing the bag or pressing the grain improve efficiency?
It recovers wort that is otherwise absorbed in the grain, so yes, it moves both efficiency figures up, and brew-in-a-bag brewers who squeeze hard commonly see absorption nearer 0.08 gallons per pound than 0.12. The important part is consistency rather than the gain itself: if you squeeze sometimes and not others, your efficiency moves between batches for a reason that never appears in your notes, and every recipe you size from it will be a little wrong in an unpredictable direction. Pick one habit and measure the efficiency it gives you.