By the way, points per pound per gallon (PPG) is calculated by taking the number to the right of the decimal point in a specific gravity reading and making it an integer (i.e., whole number). That value is multiplied by the batch volume, yielding total gravity points. Dividing total gravity points by the weight of the grist yields points per pound per gallon (PPG).

Example

Volume = 6.0 gallons

O.G. = 1.046

O.G. in gravity points = 46

Pounds of Grist = 9.0

Total Gravity Points = 46 * 6 = 276

Points Per Pound Per Gallon (PPG) = 276 / 9 = 30.37

PPG is a quick and easy method for determining extraction rate. In my humble opinion, it is a much more usable metric than extraction efficiency in a home brewery. Once one knows one's extraction rate for a given base malt formulating and scaling recipes becomes simple enough to do in one's head due to the fact that in the extract produced from base malt in a recipe dominates a batch's gravity points. For example, with a PPG of 30.37, we know that we can produce a beer with an O.G. of 1.03037 using a one pound of malt per gallon of post-boil wort; therefore, we know that we produce a 1.064 beer by using 64 / 30.37 = 2.1 pounds of malt per gallon of post boil wort.

PPG is a direct measurement. Extraction efficiency is an approximation due to the fact that the weighted calculations are made against a set of values that may or may not reflect the actual maximum yield of the malts used in a recipe. Extraction efficiency is a brew house metric, not a recipe or grain metric. Extraction efficiency should not vary from recipe to recipe. A brew house with a 75% efficiency rating should maintain that rating across all recipes. If it does not, then one knows for certain that the maximum yield weighting table values that one is using in the extraction efficiency algorithm are incorrect, making extraction efficiencies about as accurate as IBU approximations. Contrary to the false sense of security that brewing software gives a brewer, extraction efficiencies and IBU ratings require an analytical laboratory to analyze ingredients and the finished product in order to be accurate.