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http://www.braukaiser.com/wiki/index.php?title=Understanding_Efficiency

http://www.homebrewtalk.com/wiki/index.php/efficiency

So how is mash efficiency calculated? Well, each grain has a theoretical yield that is specified by the malt house. Actually there are more than one yield specified and these yields differ by the coarseness of the grind and moisture content. But the one most often used by homebrewers is the fine crush dry yield. This yield is specified in gravity points per lb per gallon of wort. If you have purchased malt in 50 or 55# bags you will find this yield on a label usually attached to the bag. If you don't have the specific theoretical yield for the grains in your brewhouse you can just use the typical values that are widely published for each type of brewing grain. Your brewhouse efficiency is then calculated as a percent of the theoretical yield. Most brewing software can do this calculation for you. Following is an example of how this calculation is done.

The following grains were mashed :

* 4 lbs. Pilsner malt, theoretical yield 37 points/lb-gal

* 2.5 lbs. Munich malt, theoretical yield 33 points/lb-gal

* 14 oz (0.86 lb) Cara-Vienne, theoretical yield 34 points/lb-gal

* 2 oz (0.13 lb) Cara-Munich, theoretical yield 34 points/lb-gal

The final volume was 3.5 gallons, at a specific gravity was 1.058. So what was our efficiency?

Well, if we had perfect (100%) extraction, our total gravity points would have been:

( 4 x 37 ) + ( 2.5 x 33 ) + ( 0.86 x 34 ) + ( 0.13 x 34 ) = 264 points

Dividing by the volume of wort gives us 264 / 3.5 = 75 points; this means that -- in a perfect world -- we would have expected to get a specific gravity of 1.075. But our actual gravity was 1.058, so our efficiency is 58 / 75 = 0.77, or 77%.