One rep max calculator: estimate your 1RM from submax reps

Four published 1RM formulas (Epley, Brzycki, Lombardi, O'Conner) plus a cross-formula average and a confidence band that widens with reps. The math is anchored on LeSuer 1997 validation — reliability is HIGH at ≤5 reps and degrades from there.

Estimated bench press 1RM

114.8kg

Confidence band: 112.5–117.1 kg (±2%, reliability HIGH).

Epley

116.7 kg

Brzycki

112.5 kg

Lombardi

117.5 kg

O'Conner

112.5 kg

Reliability HIGH: ≤5 reps. Validated prediction error from LeSuer 1997 sits within ~2% of measured 1RM in this range.

Adjust

The lift label tunes the headline copy. The math is identical across lifts — 1RM formulas are weight × f(reps) regardless of exercise.

kg

Total weight on the bar (bar + plates). For dumbbell pressing, use the dumbbell calculator instead — the math is different.

reps

Reps performed at that weight before form broke down or you hit failure. Reliability drops sharply past 5 reps; past 10 the estimate is a programming hint, not a real 1RM prediction.

The four published formulas (Epley, Brzycki, Lombardi, O'Conner) return slightly different numbers from the same input. The cross-formula average is the most defensible single number when you don't have a strong reason to prefer one. See per-formula breakdown above.

How to use this number

  • Programming reference, not a meet attempt.Use the estimate to set 70–85% working weights. Don't walk up to the bar and try the headline number cold.
  • Prefer ≤5-rep input. The validation literature holds within ~2% in that range. A 5-rep set is the highest-signal single set you can run for 1RM estimation.
  • Read the four-way spread.If Epley and Brzycki disagree by >5%, your input rep count is in a noisy band. Re-run at fewer reps.
  • Re-test every 4–8 weeks. Strength changes; stale 1RM estimates produce stale percentages. Pair with the percentage calculator.

What this calculator does NOT do

  • Predict a real meet attempt. Calculator output is a programming reference. Real 1RM attempts need a structured warm-up, a spotter, and a peak-week protocol.
  • Account for technique drift. A grindy 5-rep set with bar-path drift on rep 5 will over-estimate. The math assumes form held throughout.
  • Handle dumbbell pressing. Dumbbells need different math — stability demand, ROM differential, increment granularity. Use the dumbbell 1RM calculator instead.
  • Replace coaching judgment. Programming a peak or selecting attempts in a meet is a coach decision. The number is one input, not the whole answer.

Ask a StrengthMath question

Quick answers about StrengthMath's calculators and how the numbers work. Free, no signup. Not professional advice — for regulated decisions, talk to a licensed professional.

Hi, I'm the StrengthMath assistant. I answer questions about strength-training math — 1RM estimation, percentage-of-1RM programming, plate loading, dumbbell-vs-barbell comparison, strength-standards reading — and how the calculators on this site work. I'm not a strength coach or sports-medicine professional and can't program for your specific physiology, training history, or competition goals. For programming or pain/injury, work with a qualified strength coach (NSCA CSCS, USAW, equivalent) or a sports-medicine physician.

The four formulas, side by side

Each formula is an algebraic curve fit to historical submax-rep data. They're mathematical, not empirical, and they all return slightly different numbers from the same input. The differences matter most at high reps, where prediction error grows for everyone.

Reliability bands — why the confidence widens

LeSuer 1997 validated multiple 1RM prediction equations against true 1RM testing across bench, squat, and deadlift. The headline finding: prediction error within ~2% at ≤5 reps, ~5% at 6–10, growing sharply past 10. The reliability bands surfaced above are StrengthMath methodology grounded in those directional findings — labeled as such in the engine output.

A worked example — 100 kg × 5 bench

Run 100 kg × 5 reps in the calculator above with the lift set to bench. The math:

Cross-formula average ≈ 114.8 kg. Confidence band at HIGH reliability (±2%) ≈ 112.5–117.1 kg. That's a tight 5 kg window, which is what you want a 1RM estimate to look like.

Now bump reps to 12 with the same 100 kg. The four formulas spread from ~125 to ~135 kg — a 10 kg disagreement that the ±10% NOISY band honestly admits. Higher rep input, wider error bar.

Frequently asked

Which 1RM formula is most accurate?

It depends on the rep range, and the honest answer is that no single formula wins across all reps and all athletes. LeSuer 1997 validated multiple equations against true 1RM testing across bench, squat, and deadlift and found prediction error within ~2% at ≤5 reps and growing sharply past 10. Epley tends to overshoot at high reps; Brzycki holds up well at low reps but has an asymptote at 37 reps that breaks the math; Lombardi runs flatter; O'Conner is the most conservative. Average across all four is the most defensible single number when you don't have a strong reason to prefer one — the per-formula breakdown above shows the spread so you can see how much agreement you're getting.

How many reps should I use?

≤5 if you want a tight estimate. The reliability band on this calculator is HIGH at ≤5 reps (~±2% per LeSuer 1997), MEDIUM at 6–10 (~±5%), NOISY at 11–15 (~±10%), and VERY NOISY past 15. A 5-rep set with a real working weight is the most informative single set you can run for 1RM estimation. A 12+ rep set is a programming hint, not a 1RM prediction.

Is the cross-formula average better than picking one?

For most people, yes. Each formula was fit to a different historical dataset and tends to over- or under-shoot in different rep ranges. Averaging across all four cancels some of that systematic bias. The four-way spread itself is also a signal — if all four formulas agree closely, your input is in a range where the math is well-behaved; if they disagree by 5–10%, you're in a noisier rep range and the headline number deserves wider error bars.

Should I actually max out to verify?

Only with a coach, a spotter, and a structured warm-up. A real 1RM test is high-CNS, high-injury-risk, and ruins the rest of your session. The calculator above is for programming purposes — knowing roughly where 80% sits so you can prescribe a working weight. For an actual one-rep-max attempt (powerlifting meet, programmed peak), follow a tested warm-up protocol and have someone watching the bar.

Why does the calculator cap reps at 20?

Because past ~15 reps the four formulas diverge sharply and the underlying validation literature thins out. Brzycki specifically has an asymptote at 37 reps where the math becomes undefined; staying well clear of that boundary keeps the estimate sane. If you're hitting 20+ reps with a given weight, that weight is closer to 50% 1RM than to anything you can productively extrapolate from. Re-test with a heavier weight in the 3–8 rep range.

Does the lift type change the math?

No. The 1RM formulas are weight × f(reps) — the lift label is descriptive only. A 5-rep estimate from a bench press uses the same Epley/Brzycki/Lombardi/O'Conner math as a 5-rep estimate from a squat. The reason for separate landing pages (incline-bench, dumbbell-bench, etc.) is that the surrounding context differs: incline bench has its own strength relationship to flat bench, dumbbell pressing needs different math because of stability and ROM differentials, and so on. The calculator engine is shared.

Related

By Jimmy L Wu. The four 1RM prediction formulas (Epley, Brzycki, Lombardi, O'Conner) are reproduced from their original publications. Reliability bands are StrengthMath methodology grounded in LeSuer 1997 (“Accuracy of prediction equations for strength performance”) — directional findings across the validation literature, labeled as such. Engine logic in lib/strength/oneRepMax.ts. Not medical advice — for max attempts, work with a qualified strength coach. Methodology covers the full sourcing posture.