One-Rep Max Calculator

Which formula to trust

All four are empirical fits to data, not derivations. They diverge at the extremes:

• Epley — the most common, widely cited. Slightly aggressive at high reps.
• Brzycki — slightly more conservative for sets of 5+. Used in many strength-training textbooks.
• Lombardi — exponential rather than linear; behaves better for very high or low rep ranges.
• O'Conner — linear, the simplest. Pretty close to Epley.

For sets of 1–5 reps, all four agree to within 2–3%. Above 10 reps, divergence grows and the estimate gets noisier — what you can do for 12 reps says less about your true 1RM than what you can do for 3.

Important caveats

• Don't actually test 1RMs frequently. They're stressful and injury-prone. Train with 70–85% loads.
• Form failure ≠ true rep failure. A grindy ugly last rep tells you something different than a smooth one.
• 1RM varies day to day by 5–10%. Sleep, nutrition, stress, time of day.