One rep max calculator

Four formulas, shown side by side

A one rep max is estimated from a set you have already completed. Several formulas exist and they disagree — by a few percent at five repetitions and much more beyond ten. This shows four published ones together rather than picking one and presenting it as the answer:

• Epley — weight x (1 + reps / 30). The most widely used.
• Brzycki — weight x 36 / (37 - reps). Reads lower as repetitions climb.
• Lombardi — weight x reps to the power 0.10.
• O'Conner — weight x (1 + reps / 40). The most conservative.

They are not averaged

Averaging four estimates produces a number that looks more precise than any of them and is not. The spread between them is the most useful thing on the screen: a wide spread means the estimate is soft. Training percentages are calculated from Epley, and the tool says so, rather than from a blend nobody could reproduce.

Why it stops at fifteen repetitions

These formulas were fitted to sets of roughly ten repetitions or fewer. Past that they are extrapolating, and by twenty they disagree by twenty percent or more. Rather than print four confident numbers that cannot all be right, the tool refuses and suggests testing again with a heavier weight and fewer reps.

Example

100 kg for 5 repetitions gives roughly 117 kg by Epley, 112.5 by Brzycki, 117.5 by Lombardi and 112.5 by O'Conner — a five-kilogram spread, which is the honest picture.

Do not chase the number

An estimate is not permission to attempt that lift. A true maximum attempt needs a spotter, proper warm-up, safe equipment and experience of the movement. The estimate is for planning training percentages, not for deciding what to load on the bar today.

Related tools

See the heart rate zone calculator and the BMI calculator.