One-Rep Max Estimator
Five formulas, a full training percentage table, and an RPE converter for prescribing working sets.
- 01Enter a weight and the reps you completed with it, taken close to failure.
- 02Sets of two to six reps give the most reliable estimate.
- 03Use the RPE section to convert your max into a working load for a target set.
What the terms mean
- 1RM
- One-rep max: the heaviest weight you could lift for a single repetition.
- RPE
- Rate of perceived exertion, 1–10. RPE 8 means roughly two reps left in the tank.
- RIR
- Reps in reserve — how many more you could have done. RPE 10 minus RIR.
- Rep max
- The most weight you can lift for a given number of reps, e.g. a 5RM.
From 100 kg × 5 reps. Range across formulas: 113–118 kg.
Method & sources
All five equations are curve fits to population data, so they disagree — most at high rep counts. Wathan is shown as the default because it tends to track well across the widest rep range, but the spread above is the honest uncertainty. Estimates are most reliable from sets of 2–6 reps.
Epley B 1985 · Brzycki M 1993 · Lombardi VP 1989 · Wathan D 1994 · O'Conner B et al. 1989 · RPE chart after Helms ER et al.
What these numbers
actually tell you.
Read the range, not one number
All five formulas are curve fits to different population samples, so they disagree by a few percent. That spread is the genuine uncertainty in predicting a maximal effort from a submaximal one. Wathan is shown as default because it tends to track well across the widest rep range.
Where estimates stop being trustworthy
Accuracy falls away above roughly ten reps and becomes poor beyond twelve, because high-rep sets are limited by fatigue and technique rather than by maximal force production. The tool warns you when you cross that line. Novice lifters also tend to read high, because they cannot yet express a true maximum.
Prescribing by RPE is usually better than by percentage
A fixed percentage ignores daily readiness — 80% after a bad night is a very different set from 80% when fresh. Prescribing "five reps with two in reserve" lets the load self-adjust. Research validating the scale found trained lifters judge proximity to failure reasonably well within a few reps of it.
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