Bench press training image accompanying the comparison of 1RM prediction equations. Photo source: Pexels.
Formula
Equation
Epley
weight × (1 + reps ÷ 30)
Brzycki
weight × 36 ÷ (37 − reps)
Lander
100 × weight ÷ (101.3 − 2.67123 × reps)
Lombardi
weight × reps0.10
O’Conner
weight × (1 + 0.025 × reps)
Why they disagree
Each equation represents a different mathematical model of the relationship between load and repetitions. Differences become more noticeable as the repetition count rises.
Why we do not crown one formula as universally best
Published research has found differences in prediction performance by exercise and population. Rather than making a universal accuracy claim, the calculator shows the five estimates and the arithmetic mean.
