Rate of evaporation?

I am sure there are a million threads about this all ready so I will make it short and sweet..... I have a 15 gallon brewpot and am wondering if anyone else with the same can help me figure out roughly how much evaporation occurs with both 60 mins and 90 mins boil?

IMHO there are too many variables to get a definitive answer. How vigorously you boil, and the diameter of the kettle factor in. Perhaps if you post that information, someone can chime in with their resulsts. Even then, your mileage may vary.

It depends on the shape of the pot/kettle, burner, how high you run the gas/flame/heat to the wort and such.

Probably your best option would be to put a good amount of water into the pot, get it to boil on your brew rig/burner and see what your boil-off rate is at 15 minutes. That will give you a basic idea of your wort boil-off rate. It won't be exact due to the density differences between plain water and wort.

I agree with the above post, I would plan on around 8-10% and just watch your your boil. Halfway through you should be able to adjust your boil is to adjust for the correct amount of evaporation.

I just had a thought... would it be better to calculate modestly and in the event of being on the lower side after boil just adding cold water to get to your desired volume? Assuming that your OG is high enough before the addition....

Sounds crazy but I boiled 1 gallon of water last week for an hour and measured how much was left. Then i measured what was left and made the fraction my boil off rate?