How accurate is a 1RM calculator?

Close up to about five reps, loose after ten. Every formula is a curve fitted to averages, and you are one person: your own ratio of strength to endurance decides whether the estimate lands above or below what you can really lift. Use it to pick working weights, not to put a number in your head you have never touched.

Why can I not lift what the calculator says?

A single maximum attempt is a skill of its own, and the estimate assumes you have it. Bracing, the walkout, a tight setup and the willingness to stay under a bar that is not moving yet are all things you only own if you practise them. Someone who trains in sets of ten has the muscle for the number and not the habit, and the first attempt usually shows it.

Should I test my real one-rep max?

Rarely, and never on a whim. A true attempt costs a session and takes a few days back, so it earns its place a handful of times a year — before a meet, at the end of a block, when a lift has stalled and you need to know where it really sits. The rest of the time an estimate from a hard set of three or five tells you everything you need in order to choose the next weight, and costs nothing.

One-rep max calculator

Put in a set you actually did — the weight and the reps — and get the single rep you could probably hit. Below it, what that maximum means for every other rep count.

What your maximum means for other rep counts

Reps	% of max	Weight
1	100%	—
2	97%	—
3	94%	—
4	92%	—
5	89%	—
6	86%	—
8	81%	—
10	75%	—
12	71%	—
15	67%	—

How it is calculated

Two formulas, switched at the point where they agree. Brzycki up to ten reps, Epley above that. At exactly ten reps both return 1.3333 × the weight, so the switch adds no jump.

Brzycki: 1RM = w × 36 / (37 − r)
Epley: 1RM = w × (1 + r / 30)

This is an estimate, not a measurement. It is at its best up to about five reps — a set of fifteen says more about your endurance than about your single. Nothing here replaces a spotter.

Questions this raises

How accurate is a 1RM calculator?: Close up to about five reps, loose after ten. Every formula is a curve fitted to averages, and you are one person: your own ratio of strength to endurance decides whether the estimate lands above or below what you can really lift. Use it to pick working weights, not to put a number in your head you have never touched.
Why can I not lift what the calculator says?: A single maximum attempt is a skill of its own, and the estimate assumes you have it. Bracing, the walkout, a tight setup and the willingness to stay under a bar that is not moving yet are all things you only own if you practise them. Someone who trains in sets of ten has the muscle for the number and not the habit, and the first attempt usually shows it.
Should I test my real one-rep max?: Rarely, and never on a whim. A true attempt costs a session and takes a few days back, so it earns its place a handful of times a year — before a meet, at the end of a block, when a lift has stalled and you need to know where it really sits. The rest of the time an estimate from a hard set of three or five tells you everything you need in order to choose the next weight, and costs nothing.

The same number the app shows

SyzUp estimates your max from every set you log, with this exact formula — so the figure here and the figure on your phone will not disagree.

Log the set instead of retyping it. The app keeps the history and draws the curve.

Get the app

Last updated: 2026-11-05