Hong Life CalculatorsUnderstanding the one-rep max (1RM) calculator — concept and principle
Your one-repetition maximum (1RM) is the heaviest weight you can lift once with good form. Testing it directly is tiring and carries some risk, so this calculator estimates it from a lighter set taken close to failure. Enter the weight and the number of reps you completed, and it shows seven published prediction equations and their average.
How it is calculated
With w = weight lifted and r = reps:
- Epley: w × (1 + r ÷ 30) (for one rep the calculator returns w)
- Brzycki: w × 36 ÷ (37 − r)
- Lombardi: w × r^0.1
- Mayhew: 100w ÷ (52.2 + 41.9 e^(−0.055r))
- O'Conner: w × (1 + 0.025r)
- Wathan: 100w ÷ (48.8 + 53.8 e^(−0.075r))
- Lander: 100w ÷ (101.3 − 2.67123r)
- Average 1RM: the mean of the seven results
The unit of the result is the unit you entered, kg or lb.
The principle
Each extra rep you can do means the load is a smaller fraction of your maximum. Researchers tested lifters at several loads, recorded reps to failure, and fitted curves to the data. Epley and O'Conner are straight lines (each rep adds a fixed percentage), Brzycki and Lander are nearly linear over low reps, and Mayhew and Wathan use exponential curves that flatten at higher reps. All of them agree closely at 2–6 reps and diverge as reps increase, because endurance, muscle fibre type and technique start to matter more than maximal strength. Averaging smooths out the bias of any single equation.
Worked example
- Inputs: 100 kg for 5 reps.
- Epley: 100 × (1 + 5/30) = 116.67. Brzycki: 100 × 36 ÷ 32 = 112.5.
- Lombardi: 100 × 5^0.1 = 117.46. Mayhew: 10,000 ÷ 84.03 = 119.01.
- O'Conner: 100 × 1.125 = 112.5. Wathan: 10,000 ÷ 85.78 = 116.58. Lander: 10,000 ÷ 87.94 = 113.71.
- Sum = 808.43; ÷ 7 = 115.49 kg average 1RM.
| Weight lifted (kg or lb) | Average 1RM |
|---|---|
| 25 | 28.87 |
| 50 | 57.75 |
| 75 | 86.62 |
| 100 | 115.49 |
| 125 | 144.36 |
| 150 | 173.24 |
| 200 | 230.98 |
How to read the result
Treat the average as a range of a few kilograms, here about 112–119 kg. The estimate is most reliable for sets of 10 reps or fewer, performed to genuine failure with consistent technique. Use it to choose working weights, for example around 80–85% of 1RM for sets of five, and recalculate every few weeks as you get stronger.
Common mistakes
- Using a set that stopped well short of failure, which underestimates 1RM.
- Entering high-rep sets (15 or more), where all equations become unreliable.
Frequently asked questions
Which 1RM formula is the most accurate?
None is best for everyone. Studies comparing them, such as LeSuer et al. (1997), found accuracy varies by lift, so the average is a sensible default.
Can I use reps in reserve instead of going to failure?
Yes, add your reps in reserve to the reps completed before calculating, although this adds some uncertainty.
Sources: Brzycki M. Strength testing: predicting a one-rep max from reps-to-fatigue. JOPERD 1993;64(1):88–90. · LeSuer DA et al. The accuracy of prediction equations for estimating 1-RM performance in the bench press, squat, and deadlift. J Strength Cond Res 1997;11(4):211–213.