Formula to determine how much DME to add when pre-boil gravity is missed.

So I brewed a partial mash a few weeks ago and ran into a problem when I took the pre-boil gravity. I chilled the sample down to 70F and got 1.042. BeerSmith gave me a preboil gravity of 1.064. My target OG was 1.073. I wasn't sure how to figure out how much DME to add to bring up my gravity so I just added 2 lbs. I ended up obtaining an OG of 1.070. My preboil volume was 6 gallons and my post-boil was 5 gallons.

So my question is what formula do I use to determine how much DME to add so that I reach my target OG?

Thanks for any help. I plan on going all grain for my next batch and wanted to figure this out in case my efficiency is lower than expected.

Extract needed in lbs = (Total gravity(target) - Total gravity(from mash))/45 for DME, or 38 for Liquid extract

Total gravity is the volume of wort X gravity units of wort.

1.042 specific gravity = 42 gravity units

so from your example it would be

Total gravity(target) = 6gal (pre boil volume) x 64 (target pre boil gravity units) = 384
Total gravity (from mash) = 6 (pre boil volume) x 42 (actual pre boil gravity units) = 252


SO your equations with number is

(384 -252)/45 = 2.93 lbs of DME

Awesome, thanks for the help. I"ll make sure to note this down or create a quick web app to compute it for me.

If you get something like this, you can use the slider to plug in the OG boost and batch size you desire and it will tell you how many lbs of DME/LME/whatever to add: http://www.northernbrewer.com/default/beer-recipe-calculator.html
I like mine!

Thanks! This just saved me on my first AG brew today

The way I do it is to tweak my efficiency in Beersmith to match the actual pre-boil gravity, then add DME to my recipe until the right post-boil gravity is achieved, since pre-boil gravity is kind of a pain to see.

If I am doing a partial mash and know what gravity my wort should be before adding extract and topping off, can I run these same calculations? I figure this way I wouldn't have to worry about trying to make sure that the water is mixed well with my wort and possibly throwing off the hydro reading.