Efficiency is the ratio of sugar obtained from the grain to the maximum potential sugar in the grain.
Efficiency = Actual Sugar / Potential Sugar * 100%
There are several important efficiency metrics in brewing:
- Conversion Efficiency - amount of sugar actually created in the mash divided by the potential sugar in the grain
- Lauter Efficiency - the amount of sugar that makes it into the boil kettle divided by the actual amount of sugar created in the mash
- Mash Efficiency - the amount of sugar that makes it into the boil kettle divided by the potential sugar in the grain. Also known as the kettle efficiency.
- Transfer Efficiency - the amount of sugar that makes it into the fermenter divided by the amount of sugar in the boil kettle.
- Brewhouse Efficiency - the amount of sugar that makes it into the fermenter divided by the potential sugar in the grain.
Some of the above efficiencies are the product of other efficiencies
Mash Efficiency = Conversion Efficiency * Lauter Efficiency
Brewhouse Efficiency = Mash Efficiency * Transfer Efficiency
Brewhouse Efficiency = Conversion Efficiency * Lauter Efficiency * Transfer Efficiency
Unfortunately, the above definitions are not universal throughout brewing literature or internet forums. I believe the above definitions are consistent with those used by BeerSmith and BrewersFriend. Whenever reading about efficiency, it is essential to understand the definition of the terms being used.
To rigorously calculate efficiencies, you need to do a mass balance on all of the sugar in the system and track it from step to step. There is however, a simpler, approximate method that gives reasonable results. I will describe the simpler method here.
To calculate efficiency, you must know the total potential sugar available from all of your grains. Malts typically have what is known as a specific gravity potential per pound. The SG potential is the SG that you would obtain in one gallon of wort from one pound of grain if all of the available starch was converted to sugar (i.e. 100% mash efficiency.) To do calculations, the potential SG is converted to points per pound per gallon, abbreviated as ppg. The ppg for a grain is 1000 * (Potential SG - 1). So, a grain with a potential SG of 1.036 has a sugar potential of 36 ppg. The total points for a grain bill is the sum over all grains of each grain weight times that grain's potential. So, 10 lbs of grain with an SG potential of 1.036 would have 360 total points of potential.
The number of sugar points contained in a volume of wort is the volume in gal times (SG of the wort - 1) * 1000. Thus 6 gal of wort with an SG of 1.050 would have 300 sugar points. If we got that 6 gal pre-boil from our 10 lbs of grain above, then our mash efficiency would be 100% * 300 / 360 = 83.3%. Since no sugar is evaporated during the boil, you will have the same number of points in the wort post boil. In the example, we will have 300 points post boil, and if the post-boil volume is 5 gal, then the SG of the boiled wort would be: 1 + (300 / 5) / 1000 = 1.060. If the pre and post-boil points calculated from the volumes and SG's don't match, then there is a measurement error somewhere (or you spilled some wort.) All volumes have to be corrected to room temperature, and SG's must be corrected to the calibrated temperature of the hydrometer.
Brew on