Guide
1RM formulas explained
Every one rep max calculator runs one of a handful of equations. Here's what each one does, where they agree, and why this site averages them.
In all of the formulas, w is the weight lifted and r is the number of reps completed. Each predicts the weight you could lift for a single rep.
Epley formula (1985)
1RM = w × (1 + r / 30)
The most widely used formula. Simple, and close to the average at 3–10 reps.
Brzycki formula (1993)
1RM = w × 36 / (37 − r)
Popular for bench press. Reads a little low at higher reps and breaks down completely near 37 reps.
Lombardi formula (1989)
1RM = w × r0.10
A power curve that rises slowly — tends to give higher numbers at very low reps and lower at high reps.
Lander formula (1985)
1RM = 100 × w / (101.3 − 2.67123 × r)
Similar in shape to Brzycki, slightly more generous.
O'Conner formula (1989)
1RM = w × (1 + 0.025 × r)
A gentle straight line — the most conservative of the group at moderate reps.
Mayhew formula (1992)
1RM = 100 × w / (52.2 + 41.9 × e−0.055 × r)
Fitted on bench press data; one of the more generous estimates.
Wathen formula (1994)
1RM = 100 × w / (48.8 + 53.8 × e−0.075 × r)
An exponential model that tracks Epley closely across most rep ranges.
How they compare
Estimated 1RM for a set done with 100 (kg or lb — the units don't matter) at different rep counts:
| Formula | 2 reps | 3 reps | 5 reps | 8 reps | 10 reps | 12 reps |
|---|---|---|---|---|---|---|
| Epley | 106.7 | 110.0 | 116.7 | 126.7 | 133.3 | 140.0 |
| Brzycki | 102.9 | 105.9 | 112.5 | 124.1 | 133.3 | 144.0 |
| Lombardi | 107.2 | 111.6 | 117.5 | 123.1 | 125.9 | 128.2 |
| Lander | 104.2 | 107.2 | 113.7 | 125.1 | 134.1 | 144.4 |
| O'Conner | 105.0 | 107.5 | 112.5 | 120.0 | 125.0 | 130.0 |
| Mayhew | 111.4 | 114.0 | 119.0 | 126.3 | 130.9 | 135.4 |
| Wathen | 105.1 | 109.0 | 116.6 | 127.7 | 134.7 | 141.5 |
| Average | 106.1 | 109.3 | 115.5 | 124.7 | 131.0 | 137.6 |
Which one should you use?
At 2–5 reps the spread between formulas is only a few percent, so any of them is fine. Past 8 reps the spread grows: at 12 reps the highest and lowest estimates differ by about 12%. That is why this site reports the average and the range rather than trusting one equation — and why the best input is always a heavy, low-rep set.
Ready to try it? Use the one rep max calculator.
Questions lifters ask
What is the most accurate 1RM formula?
Studies disagree, and the answer changes with the lift and the lifter. Epley and Brzycki are the most validated and are within a few percent of each other below 6 reps. Averaging several formulas reduces the error of any one.
Why do the formulas give different answers?
Each was fitted to a different group of lifters and exercises. At low reps the differences are small; past 10 reps endurance varies so much between people that no single curve fits everyone.
Which formula does this site use?
All seven. The headline number is their average, and the range shows the lowest and highest estimate. You can see each formula's result in the "By formula" table under the calculator.