How One-Rep Max Calculator works

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

Data sources

1. 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.
2. Brzycki M. Strength testing — predicting a one-rep max from reps-to-fatigue. JOPERD. 1993;64(1):88-90.
3. 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.
4. 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.
5. Epley B. Poundage chart. Boyd Epley Workout. 1985. — Original Epley chart; no peer-reviewed primary source — a paperback training guide.
6. 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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Worked example

Computed by the same engine bundle served at /engines/one-rep-max-calculator.js. Re-runnable: the values below are the literal output of compute(engineInput).

FAQ

What formulas does this calculator use and how do they differ?

This calculator compares six validated formulas: Epley (1RM = weight x (1 + reps/30)), Brzycki (1RM = weight x 36 / (37 - reps)), and Lombardi (1RM = weight x reps^0.10), plus the Mayhew, Wathen, and Lander equations. The Epley formula, published by Boyd Epley at the University of Nebraska in 1985, is the most widely cited in strength training literature. Brzycki tends to produce slightly lower estimates at higher rep counts (8-12 reps), making it more conservative. Lombardi uses an exponential model rather than linear. The formulas agree within 2-3% for sets of 3-6 reps but diverge at 10+ reps. The calculator presents all six so you can see the range and use the most conservative estimate for safety.

How accurate are 1RM estimates compared to actual tested maximums?

Research by Reynolds, Gordon, and Robergs (2006) in the Journal of Strength and Conditioning Research found that estimation accuracy is highest for sets of 2-6 reps, where predictions typically fall within 3-5% of a tested 1RM. At 7-10 reps, accuracy drops to within 5-8%. Above 10 reps, estimates can overpredict by 10-15% because muscular endurance becomes a larger factor and the linear relationship between reps and maximal capacity breaks down. For the most reliable estimate, use a recent set of 3-5 reps performed at true maximal effort with good form.

Should I actually test my one-rep max in the gym?

For most training purposes, an estimated 1RM is safer and more practical than a true tested maximum. Actual 1RM attempts carry meaningful injury risk, particularly on compound lifts like squats and deadlifts, and produce significant central nervous system fatigue that can impair training for several days afterward. The NSCA recommends that only experienced lifters with spotters and proper equipment attempt true 1RM testing. For programming purposes, an estimated 1RM derived from a 3-5 rep set provides sufficient accuracy to set training loads.

Why does my estimated 1RM differ between exercises?

Different exercises have different rep-to-max relationships based on muscle groups involved and movement patterns. Isolation exercises and machine movements tend to allow more reps at a given percentage of 1RM than compound barbell movements. Deadlifts often produce lower rep-to-max ratios than squats because grip fatigue and spinal loading accumulate faster. The formulas were primarily validated on compound barbell movements, so they are most accurate for squat, bench press, deadlift, and overhead press.

How often should I update my estimated 1RM?

Update your estimated 1RM every 4-6 weeks, typically at the end of a training block before starting a new cycle. Intermediate lifters can expect 3-5% improvements per block on major lifts, while beginners may see 5-10% gains. If your estimated 1RM has not improved across two consecutive training blocks (8-12 weeks), your programming likely needs adjustment in volume, intensity, exercise selection, or recovery management.

What is the difference between a training max and a true 1RM?

A training max is typically set at 85-90% of your estimated true 1RM and is used as the reference point for calculating working weights in many popular programs like 5/3/1 by Jim Wendler. Using a training max instead of a true 1RM provides a built-in buffer that reduces injury risk and allows for productive training even on suboptimal days. If your estimated 1RM is 150 kg, your training max at 90% would be 135 kg, and your working sets would be calculated from that lower number.