Boil-off rate is how much wort volume evaporates per hour during an active boil — a figure that varies genuinely by equipment (kettle diameter, burner intensity, ambient humidity and altitude all play a role) rather than being one fixed universal number every setup shares.
Why guessing this number wrong throws off the whole batch
A brewer who doesn't know their own actual boil-off rate and just guesses a standard "1 gallon per hour" figure risks starting with the wrong pre-boil volume, which cascades into a final post-boil gravity and volume that both miss target even though the recipe itself, the grain bill, and the mash were all executed correctly — this is a genuinely common, entirely avoidable source of batches landing off-spec.
Measuring your own rate once pays off on every future batch
Marking a kettle at known volume intervals and timing how much boils off over a standard 60-minute boil on a specific piece of equipment gives a real, system-specific number worth reusing in every future recipe calculation — a boil-off rate measured once on your own setup is far more useful than any generic published average, since burner output and kettle geometry both vary enough between setups to make a borrowed number unreliable.