Mash efficiency is calculated from the maximum Ppg of the grain bill you are using, the actual points you get from your pre-boil gravity, and the pre-boil volume. You can research maximum extractable sugar, or Ppg, online for the grains you are using. You then calculate the Maximum Ppg As-Is for each grain on the bill with the following equation:
Max-Ppg As-Is= (Max Ppg*lbs of grain)/pre boil volume
Then you sum up all of the Max-Ppg As-Is for each grain in your grain bill and that is the maximum points you can get from your grain bill and pre-boil volume.
You then measure the pre-boil gravity to calculate your efficiency as below:
If your summed Max-Ppg As-Is = 49 points or a gravity of 1.049 and you measured a pre-boil gravity of 1.040, the mash efficiency is calculated as:
Mash Efficiency= 40/49 = .816 or 81.6%
Beersmith uses brewhouse effiiciency and estimates a mash efficiency. For this to be correct, you must go to your equipment profile and enter your system losses correctly or the estimated mash efficiency will not be right. The brewhouse efficiency is a combination of mash efficiency and system losses to calculate an overall total efficiency. They have some helpful aids on the Beersmith web page to describe this in more detail.