Lets say you were aiming for 1.047 but only hit 1.032. In a 5 gallon batch, how much malt extract would you need to add to the fermenter to bring it up to the desired gravity?
If we steal the numbers from this question, we can assume that liquid malt extract has a value of about 37 points per pound per gallon, and dry has about 44. This is not exact.
total gravity points needed = Desired total gravity - current total gravity total gravity points needed = 47 * 5 (desired reading * desired volume) - 37 * 5 (current reading * current volume) total gravity points needed = 235 - 185 total gravity points needed = 50
When we talk about total gravity, we don't need to worry about the effect in volume. The amount of sugar in 50 gallons of water is the same as if you boil it down to 1 gallon. It's total gravity, or total sugar content, essentially.
So, you need to add 50 points to the batch.
Dry = 50 / 44 = 1.14 lbs Liquid = 50 / 37 = 1.35 lbs
I think my math is right. But it's early :) The numbers will honestly depend on the manufacturer of your malt, but you can plug in those numbers instead.
Let me know if that all makes sense.
In my experience:
1 lb of Dry extract will add 44 points per gallon (potential = 1.044), or 8.8 points in 5 gallons.
47-32 = 15 points needed to get where you want to be.
15/8.8 ~ 1.7 lb of DME.
Make a mini-wort - boil and cool the DME in as little water as possible and then add it to the fermenter slowly to minimalize the splash factor.