Dilution to a Target Gravity

Sooner or later a batch comes in stronger than the recipe asked for, usually because the efficiency was better than the one the recipe assumed. The fix is water, and the arithmetic is conservation of extract: adding water adds volume but no sugar, so the product of volume and gravity points stays constant. Nineteen litres at 60 points diluted to 50 points becomes $19 \times 60/50 = 22.8$ litres.

The formula only works in POINTS, and running it in specific gravity is the mistake this page exists to prevent. Diluting 1.060 by a factor of 1.2 does not give 1.050; it gives something closer to 1.050 only by accident of the numbers, and at other gravities the error is glaring. The reason is that the 1.000 is water and does not dilute — only the 60 points above it do. Strip the 1.000 off, do the arithmetic on what is left, and put it back afterward. The same reasoning is why gravity points, not specific gravities, are what you add when combining two worts.

Run the other direction — target gravity higher than the starting gravity — the equation returns the volume you would have to boil DOWN to. That is a real operation and brewers use it, but the equation is no longer telling the whole truth, because boiling concentrates the bitterness, drives Maillard browning that darkens the wort, and takes time the brew day may not have. Dilution has the mirror problem: water is the one ingredient that dilutes everything at once. Cutting a wort by 20% cuts its IBUs by 20% too, and a beer diluted to gravity without recalculating the hops will taste thin and under-bittered even though the numbers on the sheet look right.

Two mechanical notes. Use water of known quality and known temperature — cold tap water carries dissolved oxygen and chlorine, both of which belong nowhere near hot wort, and boiled-then-cooled water or reverse-osmosis water is the safe choice. And dilute before pitching rather than after: adding water to fermenting or finished beer introduces oxygen at the worst possible moment, and oxidised beer is a far worse outcome than a batch that came out two points strong.