Quote:
Originally Posted by IL1kebeer
Multiply your preboil og by your preboil volume. that will give you a your total gravity points. divide that by your target og then you will get the voluime that you need to shrink your wort down to after boil and cooling losses.
Example:
Preboil gravity = 1.055
Preboil volume = 6.5 Gallons
Target OG = 1.065
55 (preboil gravity) * 6.5 (preboil volume) = 357.5 total gravity points
357.5 (total gravity points) / 65 (target OG) = 5.5 gallons
6.5 (preboil volume)  5.5 (volume after boil and cooling) = 1 gallon
Keep in mind that this doesn't take into account any losses from trub or plate chilllers etc.
And also, like Revvy said, there are many different programs out there that will do these calculations for you.

Thanks a lot for the equation. I have already made a calculator from it using an excel file.
there is one thing I dont understand......
I started with a 1.032 preboil gravity and 5.81 gallons of wort. I wanted to reach 1.053....after the boil I was left with 0.8 gallon of trub and 1.5 gallons of wort boiled off. According to the calculator..I should have boiled off 2.3 gallons of wort to get to 1.053, but I got there with 1.5 gallons.
Is it because my system is not flawless or is this because the turb left from the grains doesnt really affect the final gravity. I guess it hardly has water, but mainly the dirt left over form the grains.