One-Rep Max Formulas Compared: Epley, Brzycki and Friends

Each formula converts a weight w lifted for r reps into a predicted max. Epley: w × (1 + r ÷ 30). Brzycki: w × 36 ÷ (37 − r). Lombardi: w × r^0.10. O’Conner: w × (1 + 0.025 r). Mayhew and Wathan use exponential curves fitted to lifting data. For 100 kg lifted 5 times: Epley 116.7 kg, Brzycki 112.5 kg, Lombardi 117.5 kg, giving an average of 115.8 kg in the one rep max calculator.

Plot each formula’s estimate for 100 kg across different rep counts:

For sets of 3 to 10 reps the six formulas stay within roughly 7–10 kg of each other (8.1 kg at 3 reps, 6.5 kg at 5 and 9.7 kg at 10), but at 15 reps they span 32.5 kg. Brzycki shoots up as reps approach 37 because its denominator shrinks, while Lombardi and O’Conner climb slowly. This is why the standard advice is to use sets of fewer than about 10 reps.

Reaching 15 or 20 reps depends more on muscular endurance, pacing, fatigue and technique than on maximal strength, and people differ widely in how their endurance relates to their strength. A set taken to genuine failure is also more accurate than a set stopped with “a few left in the tank”. For best results, use a heavy set of 3–8 reps, taken close to failure with good form.

The calculator’s table lists weights for each percentage with approximate reps. These are starting points: adjust to how the bar feels and follow a programme.

• An estimate is not a promise that you can lift that weight once; technique, fatigue, sleep and exercise differences all matter.
• Never attempt a true max without a spotter or safety bars, a progressive warm-up and a rested body.
• Different lifts behave differently: the formulas are most often validated on bench press and squat.
• Re-estimate every few weeks as you progress.

This article is educational and not coaching or medical advice; a qualified coach and the guidance of bodies such as the American College of Sports Medicine are good starting points.