Recently I brewed an ale yesterday and after 6.25g I was able to stop the runoff because I could just add water to get to my target SG (and target volume). However, my runoff from the MT was still reading 18 BRIX... That, to me, meant that there was still a lot of good wort available. I discovered that Beer Tools Pro had my efficiency at 75% (I'm usually well north of 80-85%). This lead me to creating a recipe that was a little grain heavy.

Does anyone know a good way to figure out how to determine the actual efficiency, given the fact that I probably could have made 15+ gallons with that grain bill? I would like to reduce the grain bill (save $) and I even loaded up BTP, changed the efficiency and then dropped the base malt until the gravity came in line, but this wouldn't work for complicated recipes.

You need four things to calculate your mash efficiency: Grain bill weight, weighted average grain potential, collected wort volume, and collected wort SG. You can use either pre-boil volume and pre-boil SG, or post-boil volume and post-boil SG (aka OG), but you cannot mix pre and post boil values. Using post-boil values can be more accurate because you don't have to worry about SG errors due to wort stratification if you sparged.

Mash efficiency is defined as Extract Collected in BK / Max Potential Extract. (Many use "sugar" as a synonym for "extract", but while extract is roughly 90% sugar, it also contains significant amounts of protein, plus other minor components.) Max Potential Extract is equal to Grain Bill Weight * Weighted Average Grain Potential. And, Extract Collected in BK is equal to Volume in BK * 1000 * (SG - 1). The "1000 * (SG - 1)" may look complicated, but it is just the second and third digits after the decimal point in the SG value (1.037 --> 37.) Mash efficiency then becomes:

Mash Efficiency = Vol in BK [gal] * 1000 * (SG - 1) / Grain Bill Weight [lbs] * Weighted Ave Grain Potential

If you want Brewhouse Efficiency it is just:

Brewhouse Efficiency = Mash Efficiency * Vol in Fermenter / Post-Boil Volume

Weighted Average Grain Potential is calculated as follows: multiply the lbs of each grain by the points per pound for that grain, and total the results. Then divide that total by the total weight of all the grain. Pts/lb is just 1000 * (SG potential of the grain - 1). For example if you have 8 lb of grain with a potential of 1.038, and 4 lb with a potential of 1.035, the weighted average grain potential is:

(8 * 38 + 4 * 35) / (8 + 4) = 37

If you want the most accurate results, then you should correct for the moisture content of the grain, which is usually about 4% +/- 1%. Correct the weight by multiplying the total grain weight by (1 - moisture content). So, for the 12 lbs of grain above the corrected dry weight would be 12 * (1 - 0.04) = 11.52 lb. Then use this value in the denominator of the mash efficiency formula. You do not need to use the corrected weights in the Weighted Average Grain Potential formula, since the correction gets applied equally to the numerator and denominator, so cancels out.

Brew on