One-Rep Max Calculator
How it works
The Epley formula (weight × (1 + reps/30)) and the Brzycki formula (weight × 36 / (37 - reps)) both take one working set — a weight and how many reps you got with it — and project what you could lift for a single rep. They agree closely at low rep counts and diverge further apart the more reps the set involved. Above 30 reps neither formula is treated as meaningful here, since extrapolating that far from a true one-rep effort produces a number precise in appearance only.
Brzycki's denominator, 37 - reps, shrinks toward zero as reps approaches 37 and goes negative beyond it — mathematically that would print a huge or negative "estimate", so it is withheld (shown as n/a) once reps reaches 36 rather than shown at all.
Frequently asked questions
Why do Epley and Brzycki give different numbers?
Both are empirical curve fits to the same relationship — how much load you can move for a given number of reps — built from different data and different assumptions about how that curve bends. Neither is "the" correct one-rep max; they are two independent estimates, and how far apart they land is itself a rough signal of how much uncertainty is in the estimate.
Why is the estimate less accurate at higher reps?
Every rep-max formula is extrapolating from a set you actually did to a single rep you did not. The further that set is from one rep — a set of 3 versus a set of 15 — the further the extrapolation reaches, and the more room there is for it to drift from what you could really lift once. Both formulas are built for lower rep ranges and get shakier well before their mathematical limits.
What is the percentage table for?
Strength programs are usually written as a percentage of your one-rep max — "5 sets of 5 at 80%" — rather than as an absolute weight, so the same program scales to any lifter. The table converts your estimated max into the actual weight each common training percentage works out to, using the Epley estimate as its basis.