How accurate is an estimated one rep max?

Close within about five reps and increasingly loose after that. The four formulas here disagree by two or three percent at five reps and by much more at twelve, which is why the tool shows all four rather than one number.

Which formula should I use?

Epley if you want one. It is the most widely used and the percentages most programs are written against assume something close to it. The average of all four is a safer number to train off.

Why does the answer need a hard set?

Because the estimate assumes the set was taken close to failure. A set of five with three reps still in the tank will return a number well under your real max, and training off it will feel too easy.

Should I test a true one rep max instead?

Rarely, and not as a beginner. A true single needs a spotter, a warm-up ramp and a recovery day, and it tells you roughly what a hard set of three already told you.

Calculator

One rep max calculator

Estimate the most you could lift once, from a weight you lifted for a few reps. Four formulas, the spread between them, and the training percentages.

Estimate your max

Weight lifted

Clean reps

Estimated one rep max

114.8 kg

The four formulas span 112.5 to 117.5 kg. Train off the average.

Epley: 116.7
Brzycki: 112.5
Lombardi: 117.5
O'Conner: 112.5

Training weights

Reps	Percent	Weight
1	100%	114.8
2	95%	109
3	93%	106.7
4	90%	103.3
5	87%	99.9
6	85%	97.6
8	80%	91.8
10	75%	86.1
12	70%	80.3
15	65%	74.6

Percentages are the ranges programs are written against. They are a starting point for picking a weight, not a promise about how many reps you will get on the day.

What the four formulas are

Epley: The most widely used. Runs high above about ten reps.
Brzycki: Close to Epley at low reps, more conservative as reps climb.
Lombardi: A power curve. The most conservative of the four at high reps.
O'Conner: Epley flattened. Sits lowest across most of the rep range.

How it works

A one rep max is the heaviest weight you could lift once. Most people never test it, because testing costs a training session and needs a spotter, so the number is estimated from a set you already did. Lift a weight for five clean reps and the formulas below turn that into a max.

Enter the weight and the reps, and the tool shows what each of the four published formulas returns, the average, and the spread between the highest and lowest. It then prints the training percentages, so the number becomes weights you can actually put on a bar.

One hundred kilos for five reps gives 116.7 from Epley, 112.5 from Brzycki, 117.5 from Lombardi and 112.5 from O’Conner. The average is about 115, and five percent is the honest width of the answer.

The reason for showing four numbers is that a single number invites false confidence. Each formula is a curve fitted to a different set of lifters, and they diverge as the reps climb because high-rep performance depends on conditioning as much as on strength. Under about six reps they agree closely enough to train from. Past ten, treat the result as a rough direction.

Two things break the estimate. A set that stopped well short of failure returns a number too low, and a set on a machine or a heavily assisted lift returns one too high. Use a free-weight set that genuinely ran out of reps.

Questions

How accurate is an estimated one rep max?: Close within about five reps and increasingly loose after that. The four formulas here disagree by two or three percent at five reps and by much more at twelve, which is why the tool shows all four rather than one number.
Which formula should I use?: Epley if you want one. It is the most widely used and the percentages most programs are written against assume something close to it. The average of all four is a safer number to train off.
Why does the answer need a hard set?: Because the estimate assumes the set was taken close to failure. A set of five with three reps still in the tank will return a number well under your real max, and training off it will feel too easy.
Should I test a true one rep max instead?: Rarely, and not as a beginner. A true single needs a spotter, a warm-up ramp and a recovery day, and it tells you roughly what a hard set of three already told you.