One-rep max calculator
Seven published formulas at once, with the range they actually give — because they disagree by more than you would guess, and every other calculator picks one and does not tell you which. Nothing you type here leaves your browser — it is worked out on your own device, so there is nothing to send and nothing to log.
Seven formulas, and none of them agrees with another
There is no single one-rep max formula. There are at least seven in the published literature — Epley, Brzycki, Lander, Lombardi, Mayhew, O’Conner and Wathen — all in use, all in the textbooks, and all giving different answers.
They agree best at seven repetitions, and even there they differ by 5.6%. There is no rep count at which these formulas agree. At twelve repetitions the spread is 12.6%, at fifteen it is 24.9%, and at twenty it is 57.1%.
Every other calculator picks one and shows you a single number. If you have used two of them and wondered why they disagreed, this is why.
At one repetition, five of them are simply wrong
If you lifted a weight once, that weight is your one-rep max. It is what the words mean, and there is nothing left to estimate.
Brzycki and Lombardi return exactly that. The other five do not. Lander adds 1.4%, Wathen 1.3%, O’Conner 2.5%, Epley 3.3% and Mayhew 8.9% — which is to say Mayhew reports a lift you have just completed as being 8.9% below your maximum for it.
That is not a criticism of Mayhew, who fitted a curve to 434 college students and never claimed it should be evaluated at one. It is a criticism of every calculator that will happily accept a 1 in the repetitions box and hand the answer back without comment.
Every author gave a repetition limit. No calculator enforces one
This is the part almost nobody mentions. The authors did not publish these as universal laws; they published them with ranges.
From the comparison table in Reynolds, Gordon and Robergs: Brzycki and Epley up to 10 repetitions, Lombardi up to 11, Mayhew up to 15. Lander and O’Conner stated no limit at all.
So the range where the formulas disagree most is mostly the range their own authors said not to use them in. At twenty repetitions the spread is 57% and only three of the seven are still inside their stated limits. This page shows you which, on every row, every time.
And two of them return negative numbers
Brzycki and Lander both divide by something that shrinks as the repetitions rise. Brzycki’s denominator reaches zero at 36.97 repetitions and Lander’s at 37.92. Past that they return a one-rep max below zero.
Nobody does a 38-rep set with a meaningful load, so this is not advice about training. It is a demonstration that these are curves fitted over a range and not truths, and that a calculator which accepts any number you type has not thought about where its own arithmetic stops working. Those two figures were solved for, not looked up.
So which one should you use?
The honest answer is that the question is slightly wrong. A one-rep max estimated from a multi-rep set is an interval, not a number, and the interval is what this page gives you.
If you want a single figure anyway: test at five or fewer repetitions, where the spread is smallest and every formula is inside its stated range, and use whichever your training programme already assumes — consistency between sessions matters more than which curve you picked, because you are tracking a change rather than measuring a constant.
Where these came from
Six of the seven are transcribed from the comparison table in Reynolds, Gordon and Robergs (2006), which prints them side by side with each author’s stated repetition guideline. Wathen is from the same literature.
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. · Epley, B. (1985). “Poundage Chart.” Boyd Epley Workout. Lincoln, NE: Body Enterprises.
Two things we checked rather than copied. Lombardi’s exponent is 0.10; it is widely reproduced as 0.13, which is wrong by about 8% at ten repetitions and looks entirely plausible. And Reynolds et al. print Epley’s constant as 0.033, which is their rounding of Epley’s own 1 ÷ 30 — building from the table rather than from Epley costs 0.33 kg at ten repetitions on a 100 kg lift. This page uses Epley’s figure.
The rearrangements from the published percentage forms into the kilogram forms used here are proved symbolically, for all weights and all repetitions, rather than checked at a few sample values.