How one rep max calculator works
Your one-rep max (1RM) is the heaviest weight you can lift for a single repetition with good form. Testing a true 1RM is risky and tiring, so it is usually estimated from a lighter weight you lift for several reps. Enter the weight and the reps you performed, pick a formula, and the tool estimates your 1RM.
Four formulas are supported. Epley (1RM = weight x (1 + reps/30)) is the most common and works well for 1-10 reps. Brzycki (1RM = weight x 36 / (37 - reps)) is similar and preferred for low reps. Lombardi (1RM = weight x reps^0.10) and Lander are alternatives. The tool shows the result of the chosen formula plus all four estimates so you can compare.
The tool also builds a percentage-of-1RM training table using the Brzycki inverse relationship (percent = (37 - reps) / 36), so you can see the load that corresponds to each rep target (1, 2, 3, 4, 5, 6, 8, 10, 12 and 15 reps). For example, a load you can lift for 8 reps is roughly 80% of your 1RM. Estimates grow unreliable above about 10 reps and are for education only, not medical or coaching advice.