How accurate is an estimated 1RM?

It is quite good for sets of around 2 to 8 reps. The higher the reps, the less accurate the estimate becomes, so a heavy set of 3 to 5 gives the best read.

What are the percentages for?

Strength programs prescribe work as a percentage of your max, for example 5 reps at 80 percent. The percentages here give you those working weights without a true max attempt.

Verified formulaLast reviewed July 2026

One-Rep Max Calculator

Estimate the most you can lift for a single rep from a set you have already done.

Your details

Weight lifted (lbs)

Reps completed

Calculated live in your browser. Nothing is stored or sent anywhere.

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Fill in your details to see your result.

A mathematical estimate based on validated equations, for education only. It is not a diagnostic tool.

The science

Your one-rep max is the heaviest weight you could lift once with good form, and it is the reference point most strength programs are built around. Rather than risk a true max attempt, this calculator estimates it from a set you have already completed, and gives you the training percentages to plan your workouts.

The formula

Epley formula: weight x (1 + reps / 30)

Sources

1
Epley B. Poundage chart (origin of the Epley 1RM formula).
2
National Strength and Conditioning Association guidelines

Common questions

This tool is for education only and is not a diagnostic device. Consult a licensed clinician before making health decisions.