Here is a form letter about how to calculate extraction efficiency when
mashing.

The formula that most people seem to use is:

Extraction=[ ( OG-1.000 ) * 1000 * Vol ] / pounds of grain used

For example, if you made 5 gal of 1.050 wort with 10 pounds of grain:

Ex = [ ( 1.050 - 1.000 ) * 1000 * 5 ] / 10 = 25 points gal / pound

(This would not be very good.  32+ is quite good, 29-31 is OK, etc.)

This method is oversimplified, however, because it assumes that all
grain has the same expected extract.  You can find tables of expected
extract for various grains, for example in Miller's book.  For example,
2-Row has an expected extract of 35 points gals / pound; for crystal the
number is 24.

How do you use these numbers?  For each type of grain you add the
product of the amount of that type of grain and its expected extract.
This gives you the total extract you expect.  For example, if you used
10 pounds of 2 row and a pound of crystal, you would expect
10*35+1*24=374 points.  In 5 gallons this would give you an OG of
374/5=75 --> 1.075 if you got 100% yield (you won't).

If you in fact got 1.055, you would know that your efficiency is about
55/75=73%.  (Mine are usually about 85-90%.)  This tends to be pretty
constant and lets you plan subsequent batches accordingly.  For
example, if you wanted to make 5 gallons of 1.060 wort, you would need
300 points of extract.  If you normally get 85% efficiency, you would
need to plan on 300/.85=353 points of theoretical extract.  If you had
a recipe that you wanted to copy, you could use the same amount of
specialty malts specified in the recipe and increase or decrease the
amount of pale malt until the theoretical extract was 353.

This strategy can also be used for partial mash beers.  Malt extract,
and other sugar sources, have expected extracts.  Liquid malt's
extract is 35-38 and dry's is 45.  The yield of these is always 100%
so these need to be figured separately.  Using the above example, if
you wanted to use liquid malt extract in place of half of the pale
malt, you could figure that your theoretical extract as 5*35+1*24=199.
If you typically get 90% yield, you should get ~180 points of extract
from the grains.  Since the desired total was 300, you need 120 more.
This is 120/36=3.3 pounds of typical liquid extract.

Table of typical possible extracts (and colors just for fun)

2-row            35   1.5-3
6-row            33   1-2
Vienna           33   3-7
Munich           33   5-20
Cara-Pils        30   1-10
Crystal (caramel)24   10-120
Chocolate        24   350-400
Black Patent     24   450-550
Roasted Barley   24   500-550
Special B        24   200-2??
Malted Wheat     38   1-2
Torrefied Barley 30   <1
Torrefied Corn/  40   <1
           Rice
Sucrose          45   <1
Glucose          40   <1
Honey           ~35   <1
Malt Extract,Liq~36   ?
  "    "     Dry~45   ?

Jeremy Bergsman
jeremybb@leland.stanford.edu