This guide looks at one rep max formula accuracy: how close the popular estimates really get to a tested max. 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.
One rep max formula accuracy: the same set, three estimates
Here is what the three formulas predict for a 100 kg set at different rep counts:
| Set | Epley | Brzycki | Lombardi | Spread |
|---|---|---|---|---|
| 100 kg × 3 | 110.0 kg | 105.9 kg | 111.6 kg | 5.7 kg |
| 100 kg × 5 | 116.7 kg | 112.5 kg | 117.5 kg | 5.0 kg |
| 100 kg × 10 | 133.3 kg | 133.3 kg | 125.9 kg | 7.4 kg |
At low reps the formulas sit within a few kilograms of each other, and Brzycki is the most conservative. At ten reps Epley and Brzycki agree exactly, while Lombardi is about 7 kg lower. The formulas are written as: Epley = weight × (1 + reps ÷ 30), Brzycki = weight × 36 ÷ (37 − reps), and Lombardi = weight × reps0.10.
How to get a more reliable estimate
- Use sets of 2–6 reps. Estimates from heavy, low-rep sets are consistently closer to a tested max than estimates from sets of 10 or more.
- Use a true all-out set. If you stopped with two reps left in reserve, add those reps before estimating, or the result will underestimate your max.
- Average across formulas. When the formulas disagree, the average is usually a sensible middle ground.
- Track the trend, not one number. A rising estimated max over several weeks tells you more than any single calculation.
- Remember individual differences. Lifters with more fast-twitch muscle often do fewer reps at a given percentage, so formulas may overestimate their max from high-rep sets.
Frequently asked questions
Which 1RM formula is the most accurate?
No single formula wins for everyone. Brzycki tends to be the most conservative at low reps, while Epley and Lombardi read slightly higher. For sets under about six reps the differences are small enough that any of them is useful.
Can I use a 1RM estimate to pick competition attempts?
Yes, as a starting point. Pair it with recent heavy singles and use our guide to picking openers to build a conservative plan.
Why does my estimate change between exercises?
Rep-to-max relationships differ by lift. Many lifters can do more reps at a given percentage on the squat and deadlift than on the bench press, so the same formula can be more accurate for one lift than another.