Stupid Mash Effeciency Question

If your pre-boil gravity reading matches what Beersmith tells you your pre-boil gravity should be based on 71% efficiency, then yes, you have 71% efficiency.

 

you can just use your pre boil but you need to adjust your volume.

 

the "efficiency" listed on the main recipe tab in Beersmith 2.0 is the "Brewhouse Efficiency" (which I believe you are referring to as system efficiency) and the Mash Efficiency is actually listed on the "Mash" tab. The Mash efficiency will be based on both your preboil volume and your preboil gravity. The brew house efficiency is based on your volume in the fermenter(s) (ie your batch size) and your post boil gravity (ie your Original Gravity).

 

I just calculate efficiency manually. If you already know how to do this, I apologize, but maybe someone else can learn from this.

To do this, you need to know the potential extract points of the grain you're using. Sometimes home brew stores will give the average potential standard gravity on the grain's label; or you can find a sheet on line that gives typical yields for each grain. So let's say you're making a pale ale with the following grains (with potential standard gravity indicated):

7 lbs American 2-Row (1.036)
2 lbs Munich Malt (1.037)
1 lb Crystal 40 (1.034)

To find your efficiency, you first need to figure out the total number of potential gravity points from your recipe by adding up the potential gravity points of each grain. To do so, multiply (number of pounds of grain) x (potential standard gravity - 1 * 1000). That (potential standard gravity - 1 * 1000) sounds more complicated than it is. If it's 1.037, then you use 37. If it's 1.034, use 34, etc. So here we go:

American 2-Row: (7 lbs) x (36) = 252
Munich: (2 lbs) x (37) = 74
Crystal 40: (1 lb) x (34) = 34
Total number of potential gravity points = 360

It doesn't matter how much volume that you have for the total extract potential. 360 points is the total number of points you're shooting for. However, the volume of your actual wort is important to calculate your efficiency. So let's say you collected 6.5 gallons of pre-boil wort, and you take a gravity reading and get 1.039. Take your volume (6.5 gallons) and multiply that by (your reading - 1 * 1000). Again, that sounds complicated, but just use 39 if it's 1.039, or 50 if it's 1.050, etc.

So (6.5 gallons) x (39) = 253.5 points

To find your efficiency, take this number (253.5) and divide by the total potential gravity points (360).
So 253.5/360 = .704
Multiply by 100 to get your percent efficiency: 70.4%

Let's say you boiled down that same wort to 5.5 gallons with an O.G. of 1.046. You still end up with 253 points (5.5 gallons * 46) with an efficiency of about 70%. Hope that makes sense.

It seems very cumbersome at first and is sort of complicated the first couple times you do it, but soon it becomes a very quick and easy calculation.