What is a one rep max?
Your one rep max (1RM) is the heaviest weight you can lift for a single repetition with good form. It is the standard measure of maximal strength, and the anchor for percentage-based programs: when a plan prescribes “5 reps at 75%”, that percentage refers to your 1RM.
You do not need to actually test it. A hard set of 2–10 reps predicts your max closely enough for programming, without the fatigue and injury risk of grinding out a true single.
How the estimate works
Each formula models the relationship between the reps you can do at a weight and your maximum. This calculator runs seven published formulas — Epley, Brzycki, Lombardi, Lander, Mayhew, O'Conner, and Wathen — and headlines the average, showing the min–max spread so you can see the uncertainty honestly.
The two most common formulas: Epley estimates 1RM = weight × (1 + reps ÷ 30), and Brzycki estimates 1RM = weight × 36 ÷ (37 − reps). They agree exactly at 10 reps and drift slightly apart elsewhere.
If you log RPE or RIR, use the reps-in-reserve field: a set of 5 with 2 reps left in the tank is treated as 7 effective reps, which stops submaximal sets from underestimating your max.
Reps to percentage of 1RM
The relationship also works as a quick reference. Roughly, each rep you can do at a weight maps to a percentage of your max (Brzycki formula, rounded):
| Reps | % of 1RM | Reps | % of 1RM |
|---|---|---|---|
| 1 | 100% | 7 | 83% |
| 2 | 97% | 8 | 81% |
| 3 | 94% | 9 | 78% |
| 4 | 92% | 10 | 75% |
| 5 | 89% | 11 | 72% |
| 6 | 86% | 12 | 69% |
So a weight you can lift for 5 clean reps is about 89% of your max. For the full breakdown from 100% down to 30% with training zones, use the percentage chart.
Using your 1RM in training
Most working weights are percentages of your 1RM — heavy strength work at 80–95%, hypertrophy work at 60–80%. Once you have your estimate, the percentage chart turns it into every training load, and the warm-up calculator builds the ramp to get there.