Methodology: One-Rep Max Calculator

Scope

Estimates one-rep max (1RM) from a sub-maximal set using six published rep-to-1RM formulas, then averages the results and reports the per-formula spread.

Tested on back squat, bench press, deadlift, overhead press, and common accessory lifts. Accuracy degrades as reps get further from the 3–5 range the formulas were originally fit on.

This is an estimator, not a training prescription. Do not attempt a true 1RM based on a calculator output without a coach and safety equipment.

Formula

Each formula takes lifted weight w and reps r and returns an estimated 1RM. The tool averages six formulas and highlights the per-formula spread.

epley      = w * (1 + r / 30)
brzycki    = w * 36 / (37 - r)
lombardi   = w * (r ** 0.10)
mayhew     = w * 100 / (52.2 + 41.9 * exp(-0.055 * r))
oconner    = w * (1 + 0.025 * r)
wathen     = w * 100 / (48.8 + 53.8 * exp(-0.075 * r))

Coefficients

Parameter	Value	Note
Epley constant	`30`
Brzycki constants	`36, 37`
Lombardi exponent	`0.10`
Mayhew constants	`52.2, 41.9, 0.055`
O'Conner constant	`0.025`
Wathen constants	`48.8, 53.8, 0.075`

Data sources

LeSuer DA, McCormick JH, Mayhew JL, et al. The accuracy of prediction equations for estimating 1-RM performance in the bench press, squat, and deadlift. J Strength Cond Res. 1997;11(4):211-213. — Empirical comparison of Epley, Brzycki, Lander, Lombardi, Mayhew, and O'Conner formulas vs measured 1RM.
Brzycki M. Strength testing — predicting a one-rep max from reps-to-fatigue. JOPERD. 1993;64(1):88-90.
Mayhew JL, Ball TE, Arnold MD, Bowen JC. Relative muscular endurance performance as a predictor of bench press strength in college men and women. J Appl Sport Sci Res. 1992;6(4):200-206.
National Strength and Conditioning Association. Essentials of Strength Training and Conditioning. 4th ed. Human Kinetics, 2016. — Source for the Wathen load-assignment equation and NSCA's current 1RM testing guidance.
Epley B. Poundage chart. Boyd Epley Workout. 1985. — Original Epley chart; no peer-reviewed primary source — a paperback training guide.
Lombardi VP. Beginning weight training: the safe and effective way. Wm. C. Brown, 1989. — Source for the Lombardi exponent; textbook, not a journal paper.

Assumptions

The input set was taken to near-failure (within 0–1 reps in reserve).
Lifter was warmed up and the rep execution quality did not degrade across the set.
Reps are whole numbers; the formulas were not designed for fractional reps.

Approximation range

Empirical validation work (Mayhew 1992, LeSuer 1997, Wood 2002) found all six formulas cluster within ~5% of each other in the 3–5 rep range.

Accuracy deteriorates noticeably past 10 reps: Epley and Brzycki tend to overestimate, Lombardi and Wathen tend to underestimate.

Per-lift accuracy typically ranks bench > squat > deadlift, with deadlift showing the most noise because of neural and grip-specific factors.

Limitations

Never used to set an attempt weight for a true 1RM without a coach in the room and proper safety equipment.
Formulas were fit on male collegiate populations; older, younger, and female lifters may show systematic bias.
The tool cannot see rep quality, so a grindy set of 5 and a clean set of 5 return identical estimates.
Bar speed / VBT estimates are not incorporated.

Reproducibility

Given weight = 100 kg, reps = 5: Epley = 100 * (1 + 5/30) = 116.67. Brzycki = 100 * 36 / 32 = 112.50. Average of all six formulas lands near 114 kg.

Change log

2026-04-24: methodology page first published.

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