One rep max calculator
Enter any hard set — weight and reps — and get your estimated 1RM plus a full loading table. Same maths as the PRE SZN app: Epley, Brzycki and Wathan blended where they are most accurate.
Estimated one-rep max
115.2 kg
| Reps | Weight (kg) | % of 1RM |
|---|---|---|
| 1 | 115.2 | 100% |
| 2 | 109.9 | 95% |
| 3 | 106.4 | 92% |
| 4 | 103.1 | 89% |
| 5 | 100.0 | 87% |
| 6 | 97.0 | 84% |
| 8 | 91.4 | 79% |
| 10 | 86.1 | 75% |
| 12 | 81.4 | 71% |
| 15 | 76.4 | 66% |
| 20 | 70.1 | 61% |
Stopped the set short of failure? The RPE calculator takes your reps in reserve into account. New to effort ratings? Read RIR and RPE explained.
Your 1RM, without the spreadsheet
PRE SZN estimates your 1RM from every hard set you log and keeps it current — then scores each set against your own four-week average, prescribes program percentages off your true numbers, and flags records the moment they happen.
Questions
How accurate is a one rep max calculator?
Very good between 2 and 10 reps, where this calculator averages three well-validated formulas (Epley, Brzycki, Wathan). Above 10 reps it switches to Wathan alone and precision falls — a 20-rep set says more about endurance than maximal strength. No formula replaces an actual max attempt; it replaces needing one.
Which formula does this calculator use?
One rep is taken at face value. From 2–10 reps it averages Epley (1 + 0.0333r), Brzycki (36 / (37 − r)) and Wathan; from 11–50 reps it uses Wathan alone, which stays smooth and monotonic at high reps. It is the exact model inside the PRE SZN app.
Why do programs prescribe percentages of 1RM?
Because intensity drives adaptation, and percentage of max is the cleanest way to prescribe it. 5x5 at 75% and 10x10 at 60% are different programs because of those numbers. The table above converts your estimate into working weights for any rep target.
Do I ever need to test a true 1RM?
Only if the number itself matters — competition, or curiosity. For training, an estimated 1RM that updates with every hard set is safer and just as useful. PRE SZN re-estimates yours continuously from the sets you log, so your percentages never go stale.