If I know my pre-boil gravity and know my boil evap losses, is there a way to calculate / guesstimate the post-boil OG?? I may be trying an all-grain batch for the first time and really have no idea what the efficiency may be. I would like to be able to adjust my hop additions if I feel I need to because I did not hit my target OG. If I get close to the target OG I'm not going to obsess over it, but if I'm off by say 10 points (1.070 vs 1.080), I may want to change a few things.

Divide your preboil volume by your expected post boil volume. (The answer will be > 1.) Multiply that by (1 - preboil gravity). Add 1. So let's say your preboil volume is 6.5 gallons and your pre-boil gravity is 1.050. And you're expecting to boil off 1.5 gallons, leaving 5 gallons post boil. 6.5/5.0 = 1.3 1.3 x (1 - 1.050) = 1.3 x 0.050 = 0.065 0.065 + 1 = 1.065 post boil gravity

Or multiply your preboil volume by your preboil gravity to get total gravity points, then divide by postboil volume. If you've got 6.5g of 1.050 wort preboil, you've got 325 gravity points preboil (6.5 x 50). Divide by postboil volume to get 1.065 (325 / 5 = 65).

It's really the same math, though I'd suggest being more careful in the description. (6.5 X 1.050 ≠ 6.5 x 50) OP: Whatever you do, don't forget to subtract your gravity reading from 1.0 before multiplying it by anything.

Oh right, I see what you did in yours now. I don't even think in terms of 1.xxx anymore. We're not hitting 2.xxx, so I'm used to just dropping everything in front of the decimal point.

I like to keep it simple. You only get so much sugar in your volume. If you have 100 sugar points in 6 gal, you'll still only have 100 sugar points in 5 gal after a boil. I suck at math and need something a bit more tangible.