How the one rep max calculator works
Testing a true one-rep max is tiring and carries some risk, so lifters often estimate it from a heavier set of several reps. Two of the most used formulas are Epley (1985) and Brzycki (1993).
Brzycki: 1RM = Weight × 36 ÷ (37 − Reps)
The two agree closely at around 10 reps and differ slightly at lower reps, where Epley is a little higher. Both are most accurate between 2 and 10 reps done to or near failure with good form. The percentages are taken from the Epley estimate and work with pounds or kilograms, since the formulas do not depend on units.
Worked example
You bench press 225 lb for 5 reps. Epley gives 225 × (1 + 5/30) = 262.5 lb and Brzycki gives 225 × 36 ÷ 32 = 253.1 lb. For a strength block, sets of 5 at 80% would be about 210 lb and heavy triples at 90% about 236 lb.
Frequently asked questions
How accurate are 1RM calculators?
For sets of 10 or fewer reps they are usually within about 5% of a tested max for trained lifters. Accuracy drops for high-rep sets.
Which formula is best?
No single formula wins for everyone. Epley and Brzycki are the most common; showing both gives a sensible range.
Should I test my real 1RM?
Not necessarily. Many programs use estimated maxes. If you test, use a spotter and proper warm-up.
What percentage of 1RM builds muscle?
Hypertrophy occurs across a wide range, roughly 30% to 85% of 1RM, as long as sets are taken close to failure.
Updated October 2026. Results are estimates for education, not financial or medical advice. All math runs in your browser; nothing you type is sent anywhere.