One Rep Max Calculator

A one rep max is the number every strength programme is written in percentages of, and testing it properly is both risky and inconvenient — so almost everyone estimates it from a set they already did. This shows the three equations that estimate it, side by side, along with how far apart they are. That last number is the one worth looking at: it is the honest width of the answer, and no calculator that shows a single figure can tell you it.

How it is calculated

Epley: w × (1 + r ÷ 30) · Brzycki: w ÷ (1.0278 − 0.0278 r) · Mayhew: 100w ÷ (52.2 + 41.9 e^−0.055r)

Two of these have less behind them than their popularity suggests. The Epley equation was extrapolated from a poundage chart in a University of Nebraska training manual, with no documented empirical derivation. Brzycki published his in a practitioner article, also without a reported sample or method. Only the Mayhew equation was fitted to data — 435 college students on the bench press.

Source: A Weight-Dependent 1RM Prediction Equation Optimized on 303,494 Near-Failure Sets — the Epley, Brzycki and Mayhew equations as published, and what is known about where they came from

Questions people ask

Which of the three should I use?

Whichever your programme was written against, and then stay with it. The value of a one rep max estimate is that it is consistent from week to week, not that it is correct — swapping equations mid-programme changes every percentage you lift.

Why do they disagree most on a single repetition?

Because only Brzycki is built to return the weight itself at one repetition. At 100 kg for one rep it says 100, Epley says 103.3 and Mayhew says 108.9. They agree most closely around ten repetitions and part company at both ends, which is the opposite of what people usually assume.

Why does Brzycki stop giving an answer at high repetitions?

Because its divisor reaches zero. Past 36 repetitions the equation sends the estimate to infinity and then negative, so this page stops showing it rather than printing a number that means nothing. Epley grows without bound but stays positive.

How many repetitions should the set be?

Ten or fewer. Validation studies agree that accuracy degrades above roughly 10 repetitions, and one found the best predictions came from a five-rep set. Above that the page says so rather than quietly handing back a worse number.

Do these work for every lift?

No, and that is a documented weakness rather than a caveat. The equations were derived on the bench press and applied everywhere since; validation work has found they underestimate the deadlift in particular, and the assumption that reps relate to percentage the same way whether you are curling ten kilos or squatting a hundred and fifty has been questioned for decades.

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