One Rep Max (1RM) is the maximum amount of weight you can lift for a single repetition of a given exercise with correct technique. It is the foundational metric of strength training — used to prescribe training loads, track progress, and compare strength relative to bodyweight. Most people never actually test their true 1RM (it requires maximum effort and a spotter), so submaximal estimation formulas were developed to predict it from lighter sets.

This calculator uses four validated formulas: the Epley formula (1985) — the most widely used in powerlifting and the default displayed — along with Brzycki, Lombardi, and O'Conner. Each formula was derived from different population studies and tends to agree well for sets of 1–6 reps. Accuracy decreases progressively above 10 reps — for sets of 15+ reps, the estimated 1RM can deviate significantly from reality.

The training percentage table — which maps percentages of 1RM to representative rep ranges — is based on NSCA (National Strength and Conditioning Association) and Prilepin's Chart guidelines. Strength phases typically use 85–95%, hypertrophy 67–85%, and muscular endurance below 65%. These percentages are the backbone of periodised programming used in all professional strength sports.