StrengthCalc

Methodology

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StrengthCalc estimates a one rep max by running six published formulas on a set you lifted, then reports every result, their plain average (the “Average Estimate”) and the range between them. Everything runs in your browser; nothing you type is sent to us.

The six formulas

These are the only formulas used. Each is cited in the sources below.

The six 1RM estimation formulas used by StrengthCalc
FormulaEquationOriginHow it behaves
Epley1RM = w × (1 + r ÷ 30)Boyd Epley’s 1985 poundage chart — a coaching tool rather than a peer-reviewed study.Linear: adds 3.33% per rep. Sits in the middle of the six from 2 to 10 reps.
Brzycki1RM = w × 36 ÷ (37 − r)Matt Brzycki, Journal of Physical Education, Recreation & Dance (1993).Lowest of the six at 2–5 reps (tied with O’Conner at 5), identical to Epley at 10 reps, and highest at 11–12 reps as its curve steepens. Undefined at 37 reps.
Lombardi1RM = w × r^0.10V. P. Lombardi’s textbook Beginning Weight Training (1989).Power curve that flattens as reps rise: mid-pack at low reps, lowest of the six at 11–12 reps.
Mayhew1RM = 100w ÷ (52.2 + 41.9e^(−0.055r))Mayhew and colleagues (1992), fitted to bench-press data from college men and women.Exponential. Highest of the six at 1–6 reps: it is not anchored to a single (it gives about 109% of the weight for 1 rep), so it reads high on very heavy sets.
O'Conner1RM = w × (1 + 0.025 × r)O’Conner, Simmons and O’Shea’s textbook Weight Training Today (1989).Linear: adds 2.5% per rep. Lowest of the six at 6–10 reps.
Wathan1RM = 100w ÷ (48.8 + 53.8e^(−0.075r))Dan Wathen’s chapter in the NSCA textbook Essentials of Strength Training and Conditioning (1994); often spelled “Wathan”.Exponential. Within about 1% of Epley at 3–8 reps and highest of the six at 7–10 reps.

w = weight lifted, r = reps completed, e ≈ 2.718.

Lander’s formula is deliberately excluded until its equation is verified against its original published source. We would rather show six checked formulas than seven unchecked ones.

The Average Estimate

The Average Estimate is the arithmetic mean of the six formula results. It is a summary of where the formulas cluster — not a scientifically validated consensus, and we never describe it as more accurate than any single formula. It is the default headline number because it is less sensitive to any one formula’s quirks; you can switch the headline to any individual formula.

Alongside it we show the lowest and highest formula and the spread: the gap between them as a percentage of the lowest. A larger spread means the formulas disagree more about your set.

Input rules

Rounding rules

How the numbers are checked

All calculation code is a small set of pure JavaScript functions covered by an automated test suite. The reference case is 100 kg × 5 reps:

Reference test case: 100 × 5 reps
FormulaExpected result
Epley116.67
Brzycki112.50
Lombardi117.46
Mayhew119.01
O'Conner112.50
Wathan116.58
Average Estimate115.79

The tests also cover invalid input, the 1-rep rule, unit conversion, rounding, the plate solver (including cases where “largest first” gives the wrong answer) and the warm-up generator. Statements on this site that compare formulas — such as “Mayhew is highest at 1–6 reps” — are checked against the formula outputs by the same tests.

What the numbers are not

Every result is an estimate. It is not a tested max, not a prediction of what you will lift on a given day, and not medical or coaching advice. Technique, fatigue, experience and the lift itself all move your real number. See the disclaimer.

Editorial policy and corrections

StrengthCalc is an independent site. Content is written and reviewed by its editor, Muhammad Khuzaima, and we do not claim professional credentials on the site. Factual claims are tied to the published sources listed on each page, and we separate what research found from our own reasoning. If you find an error, please contact us — corrections are made promptly and the review date above is updated.

Sources

  1. Epley, B. (1985). Poundage Chart. Boyd Epley Workout. Lincoln, NE: Body Enterprises. (A coaching chart, not a peer-reviewed study.)
  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. Link
  3. Lombardi, V. P. (1989). Beginning Weight Training: The Safe and Effective Way. Dubuque, IA: Wm. C. Brown.
  4. Mayhew, J. L., Ball, T. E., Arnold, M. D., & Bowen, J. C. (1992). Relative muscular endurance performance as a predictor of bench press strength in college men and women. Journal of Applied Sport Science Research, 6(4), 200–206.
  5. O’Conner, B., Simmons, J., & O’Shea, P. (1989). Weight Training Today. St. Paul, MN: West Publishing.
  6. Wathen, D. (1994). Load assignment. In T. R. Baechle (Ed.), Essentials of Strength Training and Conditioning (pp. 435–446). Champaign, IL: Human Kinetics.
  7. LeSuer, D. A., McCormick, J. H., Mayhew, J. L., Wasserstein, R. L., & Arnold, M. D. (1997). The accuracy of prediction equations for estimating 1-RM performance in the bench press, squat, and deadlift. Journal of Strength and Conditioning Research, 11(4), 211–213.
  8. Reynolds, J. M., Gordon, T. J., & Robergs, R. A. (2006). Prediction of one repetition maximum strength from multiple repetition maximum testing and anthropometry. Journal of Strength and Conditioning Research, 20(3), 584–592. Link