Why Your Estimated One-Rep Max Is Never Exact — and That's the Point

Testing a true 1RM is its own risk

The most direct way to find your one-rep max is to actually attempt heavier and heavier singles until you fail — which is exactly the scenario most likely to cause an injury, since a true maximal attempt pushes form and joints to their absolute limit with no safety margin left if something goes slightly wrong.

Two formulas, one underlying idea

Both the Epley formula (weight × (1 + reps/30)) and the Brzycki formula (weight × 36/(37 − reps)) work from the same basic observation: there's a fairly predictable relationship between how many reps you can do at a given weight and what your absolute single-rep max is, derived by fitting a curve to real lifting data from trained lifters.

Why they agree at low reps and diverge at high ones

Both formulas were built and validated primarily on sets of 10 reps or fewer, which is why they track closely together in that range. Push them to estimate from a 20-rep set and they diverge sharply — because at that point you're no longer measuring primarily strength, you're measuring muscular endurance, which follows a different relationship to 1RM entirely.

An estimate to plan around, not a certificate

A 1RM estimate is most useful as an input to percentage-based programming — "work at 80% of 1RM this week" — where being off by a few pounds doesn't change the training outcome much. Treat it as a planning tool, not a number to defend in an argument about how much you can actually lift.

Calculate yours

Our One-Rep Max Calculator runs both formulas from a single weight-and-reps input, plus a percentage breakdown table for programming different training zones.