Gravity Points from a Grain Bill

This is the equation a recipe is actually built on. Each malt carries a potential — its maximum extract, quoted as gravity points per pound per US gallon — and a grain bill delivers that potential scaled by how efficiently your brewhouse extracts it and divided by how much wort you are making. Pale malt runs about 37 PPG, wheat about 39, crystal malts in the mid thirties, and roasted grains lower still because much of their starch has been destroyed by the kiln.

Working in POINTS rather than in specific gravity is not a stylistic preference; the arithmetic only works in points. A gravity of 1.048 is 48 points, and points are proportional to the mass of dissolved extract, which is what makes them add and scale linearly. Specific gravity is not proportional to anything useful, because the 1.000 in front is the water and it does not participate. Anyone who has tried to average two specific gravities and got a nonsensical answer has run into this. Convert to points, do the arithmetic, convert back.

The unit-boundness is severe and worth stating plainly: PPG means points per POUND per US GALLON, three imperial units compressed into one figure, and there is no metric equivalent in common use. The nearest thing is the European convention of quoting extract as a percentage — HWE or fine-grind dry-basis extract — which is a genuinely dimensionless number and arguably the better one, but it is not what the malt bag in a North American homebrew shop says. This page converts your kilograms and litres into pounds and gallons before multiplying, so both conventions arrive at the same answer.

Three practical cautions. Which volume you use matters: gravity points into the fermenter, into the kettle before the boil, and out of the mash tun are three different numbers on the same brew day, and quoting an efficiency without saying which one was measured makes it uncomparable. Measure volume at a consistent temperature, because hot wort occupies about 4% more space than cool wort and that error goes straight into the gravity. And remember the direction the equation is usually run — solve it for weight, not for points, because the recipe starts with the gravity you want and asks how much malt buys it.