Your one-rep max, or 1RM, is the heaviest weight you can lift for exactly one clean repetition of an exercise. It is the reference point for many training programs that prescribe loads as a percentage of your max.
Estimate instead of test
You do not need to test your 1RM to build muscle. The heavier a weight is relative to your max, the fewer reps you can do. Formulas use this relationship to work back from a set of several reps to a single max rep. The most common ones come from Epley (1985) and Brzycki (1993):
Epley: 1RM = weight × (1 + reps ÷ 30)
Brzycki: 1RM = weight × 36 ÷ (37 minus reps)
Example: You lift 100 kg for 5 reps. Epley gives about 116.7 kg, Brzycki 112.5 kg, and the average is about 114.6 kg. This is a worked example.
How accurate it is
With 3 to 8 clean reps close to failure, the estimate usually lands fairly close to your true max. From about 10 reps the formulas drift apart and get less accurate, because endurance and daily form play a bigger role. They work best for barbell compound lifts and are less reliable for isolation exercises and machines.
An estimated 1RM based on weight, reps and RPE is also called an e1RM. With it you can plan loads for other sets without ever attempting a true max. A real max attempt is mainly of interest to strength athletes and belongs in training only with a warm-up, clean technique and a spotter or safeties.