How to use it
- Type the weight you lifted for one set, and choose kilograms or pounds.
- Enter the Reps completed in that set, stopping at the last rep with good form.
- Read the One-rep max at the top, then the estimate of each formula in the table.
- Use the Weights for each percentage of the max table to plan your working sets.
How the estimate is calculated
A one-rep max (1RM) is the heaviest weight you can lift once. A set of several reps at a lighter weight predicts it, using a formula fitted to lifting data. This calculator uses six formulas with w as the weight and r as the reps.
Epley: 1RM = w × (1 + r / 30)
Brzycki: 1RM = w × 36 / (37 − r)
Lombardi: 1RM = w × r^0.10
Mayhew: 1RM = 100 w / (52.2 + 41.9 e^(−0.055 r))
O'Conner: 1RM = w × (1 + 0.025 r)
Wathan: 1RM = 100 w / (48.8 + 53.8 e^(−0.075 r))- The average of the six results is shown first, because no single formula is best for every lifter.
- The percentage table takes the average as 100 %. The reps beside each percentage come from the Epley formula turned around: reps = 30 × (1 / percentage − 1).
What the estimate does not cover
The formulas were fitted mostly to trained lifters. A beginner's real one-rep max may be lower than the estimate, because the nervous system needs practice to handle a near-maximal load. The Wikipedia article on the one-rep max reports that an estimate can differ from a real test by 10 % or more.
The estimate is for the exercise you tested. A squat, bench press and deadlift each need their own set. Stop a set before form breaks down. A real one-rep test with a spotter and a warm-up is more exact, but it is also riskier.
Worked example: 100 kg for 5 reps
Epley gives 100 × (1 + 5 / 30) = 116.7 kg, and Brzycki gives 100 × 36 / 32 = 112.5 kg. The six results run from 112.5 kg (Brzycki and O'Conner) to 119.0 kg (Mayhew). Their average is 115.8 kg.
On the percentage table, 80 % of 115.8 kg is 92.6 kg, and the Epley inverse gives 30 × (1 / 0.8 − 1) = 7.5, about 8 reps.
| Formula | One-rep max (kg) |
|---|---|
| Epley (1985) | 116.7 |
| Brzycki | 112.5 |
| Lombardi | 117.5 |
| Mayhew et al. | 119.0 |
| O'Conner et al. | 112.5 |
| Wathan | 116.6 |
| Average | 115.8 |
Frequently asked questions
Which formula is the most accurate?
No single formula is right for every lifter and every rep range. The Wikipedia article on the one-rep max notes that the formulas diverge after about 10 reps, which is why this calculator shows all six and their average.
Why do the formulas give different answers?
Each one was fitted to different data and uses a different shape of curve. Epley and O'Conner rise in a straight line with reps, while Mayhew and Wathan use an exponential curve. The gap is largest at high reps.
Can I use it for squats or deadlifts?
Yes, the formulas take any lift. A result is only specific to the lift you tested, so use a set of that same exercise for each estimate.
Is it safe to test a true one-rep max?
The estimate is the safer way to plan. A real one-rep test needs a warm-up, a spotter or safety stands and good technique, and it should be done under a coach's eye if you are new to the lift.
What is the best rep range for an estimate?
A set of about 3 to 5 reps gives the most stable estimate, because the formulas agree most closely there. Above 10 reps the estimates spread widely.
Sources
Figures and text last checked on Tuesday, November 10, 2026.