Preboil ----> OG calculation

is there a chart, calculator, or formula to figure out what your starting gravity is after mashing to figure out to what volume you will need to end with? Do you guys take a gravity reading after mashing?

for example:i mash my grains and have 7 gallons of wort at 1.021 and my recipe calls for an OG of 1.054... Someway/ thing to tell me I need to boil till I get to 5.5 gallons to reach my OG?

also woukd be helpful using a recipe that assumes a certain % efficiancy and I don't know what mine will be

thanks,
sean

(im guessing once I reach a consistent efficiancy I will know but just starting out I have no idea)

Good news: There is a simple calculation that you can perform.

(Current volume) * (Current gravity points) = (Desired Volume) * (Expected gravity points)

Bad news: Per the calculation, you need to boil down to ~2.7 gallons.

(7 gal * 21 points)/(54 points) = 2.72 gal

That being said, it looks like your mash got screwed up somewhere. A post SG of 1.021 for 7 gallons is really low. What was the recipe/process you followed? Did you crush the grains yourself?

the really good news is my numbers were all hypothetical... Plans are set in stone to do my first AG batch Saturday afternoon!

if my efficancy was this bad or had other issues... If I did boil down to 2 3/4 gallons I should be ok though right?

oh yeah and thanks for the formula... Although now that I think about it... It makes total scense!

sean