One-Rep Max (1RM) Calculator

Background.

This 1rm calculator estimates your one-rep max — the maximum weight you could lift for a single repetition with perfect form — without forcing you to grind out an actual maximal attempt under the bar. The one-rep max (1RM) is the single most important benchmark in resistance training because virtually every periodised strength program prescribes loads as a percentage of it: Westside Barbell's dynamic-effort method calls for 50–60% 1RM, Wendler's 5/3/1 cycles through 65/75/85% and 70/80/90%, classical Russian-style strength blocks live in the 80–95% range, and hypertrophy work typically sits between 65 and 80%. Knowing your 1RM is the foundation that turns guesswork into programming.

Yet attempting a true 1RM is risky, time-consuming, and disruptive: a maximal single requires a long warm-up ramp, an experienced spotter, central nervous system recovery of 4–7 days afterwards, and exposes you to injury rates that climb sharply above 90% intensity, especially on the bench press, squat, and deadlift. The solution, validated in the strength-and-conditioning literature since Boyd Epley's original 1985 University of Nebraska paper, is to perform a sub-maximal set to technical failure in the 3–10 rep range and feed the weight and rep count into a rep-max formula that maps the rep-strength continuum back to a single-rep estimate.

This calculator runs seven peer-reviewed formulas in parallel — Epley (1985), Brzycki (1993), Lander (1985), Lombardi (1989), Mayhew (1992), O'Conner (1989), and Wathan (1994) — so you can see at a glance how they agree and disagree. All seven formulas converge within 1–2% of each other at a single rep (where they are tautologically anchored), spread to a 3–8% range across 3–8 reps, and diverge sharply above 10 reps where the relationship between strength endurance and maximal strength becomes non-linear and individual.

We also return a back-calculated working load at any target rep count from 1 to 20 — pick 5 for Wendler-style strength blocks, 8 for classic hypertrophy sets, or 3 for top-end singles work — so you can program your next session directly from the result. The methodology is anchored in the NSCA's Essentials of Strength Training and Conditioning (4th edition, 2016), which remains the certifying body's official position on load prescription.

What is one-rep max calculator?

Your one-rep max (1RM) is the heaviest load you can move through a complete range of motion for exactly one repetition with technically sound form. It is the conventional unit of strength in barbell sports — powerlifting, weightlifting, strongman — and the reference intensity that virtually all modern periodisation programs are written against. Estimating it from a sub-maximal set to failure works because there is a roughly logarithmic relationship between load and maximum repetitions: a lifter who benches 100 kg for 1 rep typically benches around 95 kg for 2, 92 kg for 3, 87 kg for 5, 80 kg for 8, and 75 kg for 10. That relationship is consistent enough across lifters and across compound lifts (squat, bench, deadlift, overhead press, row) that linear and exponential regressions fit the population data with R² values typically above 0.95 in the 1–10 rep range. Within that window, Epley and Brzycki — the two most widely used formulas — sit within about 2 kg of each other for any realistic input. Epley's linear form (1RM = w × (1 + r/30)) is slightly more generous and tracks better for hypertrophy-range sets of 8–10 reps; Brzycki's hyperbolic form (1RM = w × 36 / (37 − r)) is slightly more conservative and was specifically validated against low-rep bench press performance, making it the better choice for 1–5 rep inputs. Above 10–12 reps, the rep-max continuum breaks down because muscular endurance, lactate clearance, and mental tolerance start dominating the limit instead of maximal force production. LeSuer et al. (1997, JSCR 11(4):211–213) compared seven formulas against measured 1RMs in 67 college-aged lifters and found mean absolute errors of 1.0–2.5% for the squat, bench, and deadlift when reps were ≤ 10 — accuracy that is more than adequate for programming working sets, though not for setting world records.

How to use this calculator.

1. 01Pick a compound lift you can perform with good technique to failure — bench press, back squat, deadlift, overhead press, or a barbell row. Single-joint isolation lifts (curls, lateral raises) are poorly predicted by 1RM formulas and should not be tested.
2. 02Warm up thoroughly: 5 minutes general cardio, then 2–3 progressively heavier warm-up sets at 40%, 60%, and 80% of your estimated working weight, with 2–3 minutes of rest between sets.
3. 03Choose a weight you believe you can complete for 3–10 reps with maximal effort. Below 3 reps is essentially a max attempt; above 10 reps the formulas lose accuracy. Most coaches recommend 5 reps as the sweet spot.
4. 04Perform a single set to technical failure — the point at which you cannot complete another rep without form breakdown, bar deviation, or assistance from a spotter. Count only complete reps with full range of motion.
5. 05Enter the weight lifted, the unit (kg or lb), the exact number of reps completed, and your preferred primary formula. Epley is the safest default for general use; Brzycki for 1–5 rep sets; Wathan for NSCA-aligned programming.
6. 06Enter a target rep count to back-calculate the load you should use for your next working set. Programming examples: target 5 reps for Wendler 5/3/1, target 8 for hypertrophy blocks, target 3 for top-end strength singles.
7. 07Re-test every 4–8 weeks under matched conditions (same time of day, same warm-up, same sleep/nutrition state) to track adaptation. Note any rep that ended with form breakdown — the formulas assume clean execution.

The formula.

The calculator dispatches seven peer-reviewed rep-max formulas from the single `oneRepMax.calculate` formula identifier. Each accepts the weight (w) and reps performed (r) and returns an estimated 1RM. The selected formula becomes the primary output; the other six are returned alongside for comparison.

Epley (1985) — University of Nebraska: 1RM = w × (1 + r / 30) Linear form, the most widely used field formula. Slightly over-predicts at high rep counts.

Brzycki (1993) — Journal of Physical Education, Recreation & Dance 64(1):88–90: 1RM = w × 36 / (37 − r) Hyperbolic form, undefined at r = 37 (hence the input cap at 36). Most accurate of the major formulas at 1–5 reps; validated against bench press performance.

Lander (1985) — NSCA Journal 6(6):60–61: 1RM = (100 × w) / (101.3 − 2.67123 × r) Linear-denominator regression. Sits between Epley and Brzycki across the 1–10 rep range.

Lombardi (1989) — Beginning Weight Training, Wm. C. Brown Publishers: 1RM = w × r^0.10 Power-law form. Conservative at high reps; tracks well at 5–8 reps.

Mayhew et al. (1992) — Journal of Sports Medicine and Physical Fitness 32(1):37–42: 1RM = (100 × w) / (52.2 + 41.9 × e^(−0.055 × r)) Exponential decay form, validated on bench press in 434 male collegiate athletes. Slightly conservative.

O'Conner et al. (1989) — Weight Training Today, West Publishing: 1RM = w × (1 + r / 40) A more conservative linear form than Epley — useful as a lower bound when prescribing a first attempt at a new program.

Wathan (1994) — NSCA Essentials of Strength Training and Conditioning, Human Kinetics: 1RM = (100 × w) / (48.8 + 53.8 × e^(−0.075 × r)) Exponential decay form; the NSCA's reference equation and the most accurate of the seven for multi-joint compound lifts according to LeSuer et al. (1997).

All seven formulas converge to within 1% at r = 1 (tautological — they all return w when r = 1, except for Brzycki which returns w × 36/36 = w). At r = 5, the spread across formulas is typically 4–6%. At r = 10, the spread widens to 10–15%. At r = 15+, the formulas diverge by 20% or more and should not be used for programming.

Target weight back-calculation: targetWeight = 1RM / (1 + targetReps / 30) The Epley inverse is applied regardless of which primary formula was selected, because Epley is the most widely used relationship for projecting weights at sub-maximal rep counts in the strength-and-conditioning literature.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Epley, B. (1985). Poundage Chart. Boyd Epley Workout. Lincoln, NE: Body Enterprises. The original linear regression 1RM = w × (1 + r/30) developed at the University of Nebraska strength program.
2. [2]Brzycki, M. (1993). Strength testing — predicting a one-rep max from reps-to-fatigue. Journal of Physical Education, Recreation & Dance, 64(1), 88–90. The hyperbolic 36/(37 − r) regression validated on bench press performance.
3. [3]Lombardi, V. P. (1989). Beginning Weight Training: The Safe and Effective Way. Dubuque, IA: Wm. C. Brown Publishers. ISBN 0697106772. Source of the power-law form 1RM = w × r^0.10.
4. [4]Baechle, T. R., & Earle, R. W. (Eds.). (2016). Essentials of Strength Training and Conditioning, 4th edition. National Strength and Conditioning Association. Human Kinetics. ISBN 978-1492501626. Chapter 17 — Resistance Training, including the Wathan equation and NSCA load-prescription matrix.
5. [5]LeSuer, D. A., McCormick, J. H., Mayhew, J. L., Wasserstein, R. L., & Arnold, M. D. (1997). The accuracy of prediction equations for estimating 1RM performance in the bench press, squat, and deadlift. Journal of Strength and Conditioning Research, 11(4), 211–213. Comparison of seven 1RM formulas against measured maxes in 67 college lifters.
6. [6]Mayhew, J. L., Ball, T. E., Arnold, M. D., & Bowen, J. C. (1992). Muscular endurance repetitions to predict bench press strength in men of different training levels. Journal of Sports Medicine and Physical Fitness, 32(1), 37–42.
7. [7]Lander, J. (1985). Maximums based on reps. National Strength & Conditioning Association Journal, 6(6), 60–61.