I use the following method (quoted from S. cerevisiae in an older post) for determining extraction rates. It's simply determining how many points (example: 1.050 is 50 points) you get out of one pound of grain mashed in one gallon of water. This works out to points per pound per gallon or (point x gal / lb) or PPG. I don't worry about what types of grains go into the recipe because generally most recipes are heavily weighted toward base malt anyway (you could go and get the weighted diastatic power of each individual grain in the recipe but it's a lot of extra work for not much increase in accuracy - the wiggle room on your batch to batch process is probably more than the increase in accuracy you'll get by doing it this way). Calculate the PPG for several batches and you'll notice it doesn't change much as long as your process is pretty much the same (if it does change drastically you might want to look at your notes to see if something went astray or maybe you got a bad crush from the LHBS). Then you can look at a recipe as a percentage and just say you want 6 gallons of 1.060 wort at the end of boil. Using your averaged PPG it is then trivial to determine the total grist weight you'll need to hit that target (6 gal. x 60 points / PPG = total lbs needed). Total grist weight can then be multiplied by the percentages of each grain in the recipe and you're done. See below for more detail.

flbrewer, if it helps, you can bypass efficiency percentages and work directly in points per pound per gallon. I prefer to use extraction rates in points per pound per gallon because it is a directly applicable value. Extraction efficiency is an indirect value that requires an additional weighted computation to be performed.

total_gravity_points = 6.25 * 60 (1.060 in gravity points) = 375

points_per_pound_per_gallon = total_gravity_points / grist_weight

points_per_pound_per_gallon = 375 / 16 = 23.44

What this value means in layman's terms is that the original recipe was formulated to achieve an extraction rate of 23.44 gravity points per pound of grain, that is, the extract from one pound grain dissolved in a one gallon solution has a specific gravity of 1.02344 That's a value that can be easily applied in one brew house. Working the other way yields:

original_gravity = points_per_pound_per_gallon * grist_weight / batch_volume / 1000 + 1.0

original_gravity = 23.44 * 16 / 6.25 / 1000 + 1.0 = 1.060

The way to determine one's brew house extraction rate in points per pound per gallon is to simply track the original gravity and total final boil volume minus the break and hops for each batch. For each batch in the sample, calculate a batch extraction rate using the following formula:

batch_extraction_rate = original_gravity_in_points * batch_total_volume / grist_weight

I calculate this value when I take my original gravity reading. After a few batches have been brewed, it's time to calculate an average batch extraction by summing the batch extraction rates and dividing by the number of batch extraction rates.

Batch Extraction Rates

Batch #1 - 28 points per pound per gallon

Batch #2 - 27 points per pound per gallon

Batch #3 - 29 points per pound per gallon

Batch #4 - 30 points per pound per gallon

Batch #5 - 29 points per pound per gallon

average_batch_extraction_rate = 28 + 27 + 29 + 30 + 29 / 5 = 28.6

Now, we can convert the recipe to our brew house by calculating a grist scaling factor. Our extraction rate is higher than the original extraction rate; therefore, this value will be less than one.

grist_scaling_factor = 23.44 / 28.6 = 0.82

All we need to do from this point forward is to multiply every quantity in the grist by 0.88 to scale the grist to our brew house. We do not need to adjust the hop bill because the bitterness unit to gravity unit ratio will remain the same.

While what I just wrote seems like a lot of work, what you will find over time is that these calculations become automatic, and you will no longer have to compute an average batch extraction rate because your extraction rates will converge within a close enough range that you can ball park it without actually performing the calculation. When that happens, you will be able to look at a recipe for 6.25 gallons 1.060 wort, and think I need to use 60 / 28.6 = 2.1 pounds of malt per gallon to hit that gravity. At that point, you will prefer to look at recipes as percentages, which is the way that professional brewers state recipes. For example, we are making a bigger version of SNPA, which is often quoted as having a grist that is 95% 2-Row and 5% C60.

total_grist_weight = 2.1 * 6.25 = 13.125 pounds

pounds_of_2Row = 13.125 * 0.95 = ~12.5

pounds_of_C60 = 13.125 * 0.05 = ~0.65

As you have more than likely already ascertained, the values add up to more than 13.125 pounds. I rounded to make measuring a little easier.