Calculation for Boil off?

[arell12]

I was wondering if there is a calculator or formula to determine your exact boil off if you have Pre boil gravity and post boil gravity?

I do BIAB and I try to have 7G Pre boil. I just want to double check some measurements.

 

[olotti]

Brewers friend boil off calculator.

 

[pricelessbrewing]

I was wondering if there is a calculator or formula to determine your exact boil off if you have Pre boil gravity and post boil gravity?

I do BIAB and I try to have 7G Pre boil. I just want to double check some measurements.

From pre boil and post boil gravity all you can figure out is the ratio of pre boil to post boil. If you have one, either one will work, you can determine the other one.

Example: (Assuming same temperature pre boil and post boil)
Pre boil 7 gallons, sg is 1.052
Post boil sg is 1.066

Post boil would then be (7*52)/(66)=5.5151515....
However this has a good margin of error on it, as the volume is likely 7G +- 0.05 and the gravity readings are likely +- 0.001 ish. Which could easily change the estimate maybe 0.2~

(I'm lazy and don't feel like doing error analysis on someones homebrew measurements)

What's your kettle diameter? This is usually a good way to estimate boil off as well, assuming it's add a good boil and not a weak boil or super violent boil.