Type “5 reps at 100kg” into any one-rep max calculator and you’ll get a confident-looking number back. What most calculators don’t tell you is that they’re often averaging (or silently picking) between formulas that can disagree by 5–10kg on the same set.
The three most common formulas
Epley: 1RM = weight × (1 + reps ÷ 30) — linear, simple, tends to run slightly high at higher rep counts.
Brzycki: 1RM = weight × 36 ÷ (37 − reps) — built from a percentage-of-max table, tends to run slightly low at higher rep counts, and becomes undefined at 37+ reps.
Lombardi: 1RM = weight × reps^0.10 — a power-law model that sits between the other two for most common rep ranges.
Why they disagree
Each formula was fit to a different dataset, at a different time, often on a specific lift (usually bench press) and population. None of them account for the fact that fatigue accumulates differently depending on the lift (a 5RM squat and a 5RM bench don’t relate to their respective 1RMs the same way), your training history, or how close to technical failure the set actually was.
Where the estimate breaks down
All rep-max formulas get noticeably less reliable above about 10 reps — the relationship between reps and %1RM stops being as predictable once you’re deep into a set, largely because muscular endurance and pacing start to matter as much as raw strength. If you want an accurate 1RM estimate, a set of 1–5 reps taken close to failure will give you a far more trustworthy number than back-calculating from a set of 15.
Our approach
Rather than pick one formula and hope, our One Rep Max Calculator averages all three and shows you each individual estimate, so you can see the spread rather than a single falsely-precise number. If the three formulas agree closely, trust the estimate. If they spread out significantly, that’s a signal your rep count is pushing past where any of them are reliable.