A one rep max is the heaviest weight you could lift once with correct technique. Estimating it from a lighter set lets you plan training percentages without the fatigue and injury risk of repeatedly testing a true maximum.
Why two formulas
They were fitted to different data and diverge as repetitions increase. Brzycki tends to give lower estimates at high rep counts and breaks down entirely at 37 reps, where its denominator reaches zero. Averaging the two is a reasonable practical compromise, which is what the headline figure above does.
The estimate degrades with reps
Below about five repetitions, both formulas are usually within a few percent. Above eight the relationship between endurance and maximal strength becomes highly individual, and an estimate from a set of fifteen tells you more about your muscular endurance than your maximum.
Lift type matters too. Squat and deadlift estimates from higher rep sets tend to run high because those lifts are limited by fatigue as much as by force production; bench press estimates tend to be closer.
Using it in training
Percentage-based programmes reference the 1RM: strength work usually sits at 80–95%, hypertrophy work at 65–80%. Because a true maximum fluctuates day to day with sleep, food and stress, many lifters now use rate of perceived exertion alongside percentages rather than trusting a single calculated number.
The set the estimate comes from should be taken close to failure with good form. A comfortable set of five with three reps left in reserve will give a figure well below what you could actually lift.