What is a one-rep max?
Your one-rep max (1RM) is the heaviest weight you can lift once, with good form, on a given exercise. It is the most common way to express intensity in strength training: “3 × 5 at 80 %” means the same thing whatever someone’s level.
Testing it directly takes a long warm-up, a spotter and solid technique. Estimating it from a lighter set taken close to failure avoids all that and gives a value close enough to program from.
How the calculation works
Three well-known formulas are applied: Epley (weight × (1 + reps / 30)), Brzycki (weight × 36 / (37 − reps)) and Lombardi (weight × reps^0.10). None of them fits everyone; averaging them smooths out their differences. The result is rounded to 0.5 kg or 1 lb.
Programming with the percentage table
Each row gives the load for a percentage of 1RM and the number of reps usually achievable at that intensity. In practice you rarely train to failure: pick a load in the zone you want, then leave one to three reps in reserve.
The zones are guidelines: strength is mostly built above 85 %, hypertrophy happens across a wide range, from 65 to 80 % and beyond, and muscular endurance below that. Recalculate the 1RM every four to six weeks, or as soon as sets feel clearly easier.
Limits of an estimate
The formulas are most reliable between 2 and 10 reps, and on compound lifts such as the squat, bench press or deadlift. They are less reliable on isolation exercises, for beginners whose technique still varies, or when the set was not taken close to failure. An estimated 1RM is not a record you should go and chase.
Safety
Do the reference set after a gradual warm-up, with clean form and, on lifts that call for it, a spotter or safety bars. Stop as soon as your technique breaks down. If you have pain, a recent injury or a health condition, check with a health professional before training at high intensity.