What is a one rep max?
Your one rep max (1RM) is the heaviest weight you can lift for a single rep with good form. It's the reference number behind nearly every strength program — when a plan says "squat 5×5 at 80%," that's 80% of your 1RM. Testing a true max is taxing and carries injury risk, so most lifters estimate it from a hard set instead: lift a weight you can handle for 2–10 clean reps, enter it above, and you get a reliable max without grinding a maximal single.
The four formulas
- Epley: 1RM = w × (1 + r ÷ 30). The most widely used; slightly generous at high reps.
- Brzycki: 1RM = w × 36 ÷ (37 − r). Very accurate in the 2–10 rep range.
- Lombardi: 1RM = w × r^0.10. Conservative at moderate reps.
- O'Conner: 1RM = w × (1 + r ÷ 40). The most conservative of the four.
The formulas agree closely at low reps and drift apart as reps climb, which is why the calculator shows all four and headlines the average.
Using your 1RM in training
Match the percentage to the adaptation you want: heavy singles to triples (90%+) build maximal strength and skill under load, 70–85% for 5–10 reps drives most muscle growth, and lighter sets develop work capacity. Recalculate every 4–8 weeks — as your estimated max moves up, your working weights should follow.
Once you know your working weight, the barbell plate calculator tells you exactly which plates to load on the bar — no gym math between sets.