How accurate are one rep max estimates?

Within a few per cent in the two-to-five repetition range, and progressively worse above that. The honest way to read this page is to look at the spread between the five formulas: when they agree closely, the estimate is reliable; when they diverge, it is not.

Which formula is best?

Wathan and Epley track measured data well across the widest range. Brzycki is the most conservative and exact at a single repetition. For programming purposes, the average of all five is a more defensible single number than any one of them.

Why do the formulas disagree more at higher repetitions?

Because above roughly ten repetitions the limiting factor stops being maximal strength and becomes muscular endurance, which varies enormously between people and between exercises. The equations were fitted in the low-repetition range and are extrapolating beyond it.

Does the exercise matter?

Yes, considerably. These equations were derived largely from bench press and squat data. Exercises with a smaller range of motion or more technical demand — deadlifts, Olympic lifts — typically produce fewer repetitions at the same percentage, so estimates read high.

Should I test my actual one rep max?

Only if you need the number for competition, and only with a spotter and a proper warm-up. For programming purposes an estimate from a set of three to five is nearly as useful and carries far less risk.

Why is Epley not exact at one repetition?

Because its formula adds r/30 to a multiplier of 1, so at r = 1 it adds 3.3% rather than nothing. Brzycki, Lombardi and Wathan all return the load itself at a single repetition. It is a known quirk, not a bug in this calculator.

One rep max calculator

Enter a weight and the repetitions you managed with it. All five published equations are shown, because they agree at low repetitions and diverge sharply above ten.

Result

These are estimates from population-level equations, not a measurement of you and not medical advice. Individual results vary widely. Talk to a doctor or a registered dietitian before making significant changes to how you eat or train.

How this is calculated

Epleydefault

              1RM = w × (1 + r / 30)

Source: Epley B., "Poundage Chart", Body Enterprises, Lincoln NE, 1985.
Assumes: A linear relationship between repetitions and load, which holds reasonably to ten.
Accuracy: The most widely used, and the default here. Note it is not exact at a single repetition: it returns 103.3 kg for a 100 kg single, which is a property of the equation rather than an error.

Brzycki

              1RM = w × 36 / (37 − r)

Source: Brzycki M., Journal of Physical Education, Recreation and Dance 64(1), 1993, pp. 88–90.
Assumes: The same linear model, expressed as a ratio.
Accuracy: Exact at one repetition, and generally the most conservative in the two-to-ten range. Undefined at 37 repetitions, where its denominator reaches zero.

Lombardi

              1RM = w × r^0.10

Source: Lombardi VP., "Beginning Weight Training", Wm. C. Brown, Dubuque IA, 1989.
Assumes: A power relationship rather than a linear one.
Accuracy: Reads highest of the five at moderate repetitions. Its power form makes it better behaved at high repetition counts than the linear equations, though all five are unreliable there.

O'Conner

              1RM = w × (1 + r / 40)

Source: O'Conner B, Simmons J, O'Shea P., "Weight Training Today", West Publishing, 1989.
Assumes: The same linear model as Epley with a gentler slope.
Accuracy: The most conservative of the five at higher repetitions, since dividing by 40 rather than 30 credits each repetition less.

Wathan

              1RM = 100w / (48.8 + 53.8 × e^(−0.075r))

Source: Wathan D., "Load assignment", in Baechle TR (ed), Essentials of Strength Training and Conditioning, Human Kinetics, 1994, pp. 435–446.
Assumes: An exponential decay in the relationship between repetitions and load.
Accuracy: The most sophisticated model of the five, and it tracks measured data well across a wider repetition range than the linear equations. Its exponential form means it never becomes undefined.

Which formula should you use

This page defaults to Epley. The most widely used, and the default here. Note it is not exact at a single repetition: it returns 103.3 kg for a 100 kg single, which is a property of the equation rather than an error.

Brzycki — Exact at one repetition, and generally the most conservative in the two-to-ten range. Undefined at 37 repetitions, where its denominator reaches zero.
Lombardi — Reads highest of the five at moderate repetitions. Its power form makes it better behaved at high repetition counts than the linear equations, though all five are unreliable there.
O'Conner — The most conservative of the five at higher repetitions, since dividing by 40 rather than 30 credits each repetition less.
Wathan — The most sophisticated model of the five, and it tracks measured data well across a wider repetition range than the linear equations. Its exponential form means it never becomes undefined.

Estimated one rep max by load and repetitions (Epley)

Read across from the weight you lifted and down from the repetitions you managed. Above ten repetitions the estimate is dominated by endurance rather than maximal strength.

Estimated one rep max by load and repetitions (Epley)
Weight lifted	1 reps	2 reps	3 reps	5 reps	8 reps	10 reps	12 reps
60 kg	62 kg	64 kg	66 kg	70 kg	76 kg	80 kg	84 kg
80 kg	82.7 kg	85.3 kg	88 kg	93.3 kg	101.3 kg	106.7 kg	112 kg
100 kg	103.3 kg	106.7 kg	110 kg	116.7 kg	126.7 kg	133.3 kg	140 kg
120 kg	124 kg	128 kg	132 kg	140 kg	152 kg	160 kg	168 kg
140 kg	144.7 kg	149.3 kg	154 kg	163.3 kg	177.3 kg	186.7 kg	196 kg

Questions

How accurate are one rep max estimates?: Within a few per cent in the two-to-five repetition range, and progressively worse above that. The honest way to read this page is to look at the spread between the five formulas: when they agree closely, the estimate is reliable; when they diverge, it is not.
Which formula is best?: Wathan and Epley track measured data well across the widest range. Brzycki is the most conservative and exact at a single repetition. For programming purposes, the average of all five is a more defensible single number than any one of them.
Why do the formulas disagree more at higher repetitions?: Because above roughly ten repetitions the limiting factor stops being maximal strength and becomes muscular endurance, which varies enormously between people and between exercises. The equations were fitted in the low-repetition range and are extrapolating beyond it.
Does the exercise matter?: Yes, considerably. These equations were derived largely from bench press and squat data. Exercises with a smaller range of motion or more technical demand — deadlifts, Olympic lifts — typically produce fewer repetitions at the same percentage, so estimates read high.
Should I test my actual one rep max?: Only if you need the number for competition, and only with a spotter and a proper warm-up. For programming purposes an estimate from a set of three to five is nearly as useful and carries far less risk.
Why is Epley not exact at one repetition?: Because its formula adds r/30 to a multiplier of 1, so at r = 1 it adds 3.3% rather than nothing. Brzycki, Lombardi and Wathan all return the load itself at a single repetition. It is a known quirk, not a bug in this calculator.

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Last reviewed August 20, 2026. Category: Training.