Homebrewers can expect to see 1-1.5 gallons of wort boil off per hour. Since most recipes recommend an hour-long boil, you should expect just over a gallon to boil off.
Factors such as kettle size, burner type, temperature, and humidity will all affect the boil-off rate.
For instance, it will take longer to bring 30 gallons of wort to a boil than five gallons. If brewing outside, homebrewers have to take the temperature and wind into account, as well.
Many recipes are written for about a 5 gallon batch of beer. In an hour, just over a gallon of wort will boil off.
Expect nearly 2 to 2.5 gallons of wort to boil off in 90 minutes. For a 5 gallon batch, it is recommended that you start with close to 8 gallons of pre-boil wort.
In order to determine how much liquid you will need to start with in order to accommodate for the boil-off rate, you can set up an experiment ahead of time using water.
With enough headspace to prevent any accidents, the boil should be higher than a simmer but not an out-of-control boil. A consistent, rolling boil is preferred. This is to ensure sterilization of the beer as well as clarity and development of flavors.
Wort boils at over 212℉. The exact temperature at which it will reach a boil is determined by the wort’s gravity as well as your elevation.