how can I determine post-boil gravity from a pre-boil gravity reading?

If I drain my mash-tun and take a gravity reading of the runnings at 1.057, is there a way I can formulate or estimate the what gravity after the boil will be (AKA - the OG of the wort in fermentor)? Thanks guys.

Without any losses it's easy: just multiply the 'points' (aka gravity units) by the ratio of starting:ending volumes. So if you had 6 gal. of 1.057 wort and you were boiling it down to 5 gal. you would multiply 57 * 6/5= 68.4 so your gravity @ 5 gal. would be 1.068. But you might have some losses such as trub and hops absorbing some wort.

Make sure if you measure the wort volume when hot you account for the expansion. If it's at lauter temp (~170 F) multiply the volume measured by .97 and if it's near boiling multiply the measured volume by .96. Cool your hydro samples before reading...and not in a container that allows a lot of surface area or the evaporation can skew your reading. All this I got from BrauKaiser's site...good stuff there.

why not just draw a sample post boil?

IrregularPulse said:

why not just draw a sample post boil?

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I'm guessing he forgot and it is now fermenting.

actually, I did measure and got 1.066 post boil (and 1.057 from the lautering tun). The reason I'm asking is that I'd like to be able to determine whether or not I need to add any DME to hit my desired gravity. I just converted a cooler to a Mash tun and this was my first AG run (outside of some 1/2 batches using Deathbrewer's awesome technique). I will of course eventually get my efficiency up and dialed-in, and then it won't matter, but I missed my OG by 10+ points on this IPA I did yesterday, and I think it may be a bit over-bittered now because of it.....I would have dialed back the bittering hops or added some DME to make up the lost gravity points if I'd been able form an educated guess about my gravity after the boil.....

thanks for your responses.....I do know to either cool the wort or adjust for the high laugtering temp.

ok that makes sense. My recommendation is to get some brewing software such as beeersmith. You enter your recipe and it tells you what your preboil OG should be right on your brew sheet. You can download the free trial before dropping the $20 on it if you'd like.