One Rep Max Calculator
The most you could lift once, worked out from a set you have already done — plus what to load for any other rep count.
Runs on your device — nothing you type is sent anywhereThe set has to have been close to failure. Between three and eight reps is where these estimates are at their most reliable.
An estimate from a set you took close to failure. A set you stopped three reps early will read low, and no formula can tell the difference.
How it works
Two numbers in, and nothing you type leaves your computer.
Put in the set you did
The weight on the bar and how many reps you got. It has to be a set you took close to failure, or the answer comes out low.
Read the number, and the spread beside it
The estimate is the middle of seven published formulas. The range next to it is how far apart those seven actually are.
Use the table to load your next session
What to put on the bar for any other rep count, worked out from the same estimate.
When people work this out
Following a percentage-based programme
5/3/1, Starting Strength, Sheiko and most of the rest prescribe loads as a percentage of your max. You need the max before the programme means anything.
Not wanting to test a true single
A genuine one-rep attempt is the most demanding and highest-risk thing you can do in a gym, and it costs you the session. An estimate from a set of five costs nothing.
Coming back after a break
Testing a max on your first week back is a poor idea. A moderate set tells you roughly where you are without finding out the hard way.
Comparing lifts across a training block
Sets at different weights and rep counts are hard to compare directly. Converted to an estimated max, they are on the same scale.
The number every other calculator hides
There is no formula for a one rep max. There are at least seven, they were published by different people over about forty years, and they do not agree with each other.
That is not a flaw in any of them. Each was fitted to a particular group of lifters doing particular exercises, and people simply differ — two athletes with the same five-rep max can have genuinely different singles, because muscular endurance and peak strength are not the same quality.
How far apart they are
Take one set and run it through all seven. The disagreement between the highest and the lowest estimate depends almost entirely on the rep count:
| Reps in the set | How far apart the seven formulas are |
|---|---|
| 3 to 8 | About 6% — they broadly agree |
| 10 | About 8% |
| 12 | About 13% |
| 15 | About 25% |
| 20 | Around 57% — from 135 kg to 212 kg on the same set |
So a single figure from a set of twenty is not an estimate. It is one author's opinion, printed without the six that disagreed.
This page shows all seven and the range they cover. Below about ten reps that range is narrow enough to plan with. Above it, the number on this page gets a warning, because the honest answer is that nobody knows.
Why three to eight reps is the sweet spot
At one or two reps the set is nearly a max already, so there is little to estimate — but the formulas are fitted to multi-rep sets and one or two of them overshoot slightly at the bottom.
Beyond ten, what limits you stops being strength and starts being endurance, and endurance varies far more between people than peak strength does. That is exactly where the formulas separate, because each author's lifters had different endurance.
Between three and eight, both problems are small. That is where the seven land within about six percent of each other.
The set has to be honest
Every one of these formulas assumes you stopped because you could not do another rep. A set of eight that you racked with three left in the tank is arithmetically a set of eight, and the estimate will be low by roughly the weight of those three reps.
This is the largest single source of error in practice, and no calculator can detect it. If the number looks low, ask how close that set really was.
What the percentages underneath are for
Most training programmes are written as percentages of a max: five sets of three at 85%, that sort of thing. The table converts the estimate into actual weights so you do not have to.
Those percentages are worked out from the same seven formulas rather than copied from a chart — the median of the seven, run backwards. They come out at roughly 95% for a double, 92% for a triple, 86% for five and 75% for ten, which is what the standard chart in every coaching manual says. The agreement is worth noting: it is the same arithmetic arriving from the other direction.
What this is not
It is an estimate, from one set, of one lift, on one day. It does not know your technique, your sleep, whether you are on your third exercise, or that a max on a leg press and a max on a squat behave differently.
Treat it as a starting weight to plan around, not a target to attempt. If you are going to test a real single, that is a session with a spotter and a warm-up, not a number from a web page.
Nothing you type is sent anywhere
The whole calculation is a few lines of arithmetic running in your browser. No account, no logging, and nothing to transmit — which is true of everything on this site, and is simply the obvious way to build a calculator.
Common questions
How do I calculate my one rep max?
Take a set you did close to failure — the weight and the number of reps — and run it through a formula. This page runs seven of them and shows the middle figure along with the range they cover, because they disagree. Between three and eight reps they land within about six percent of each other, which is close enough to plan training around.
How accurate is a one rep max calculator?
Accurate enough to programme with from a set of three to eight reps, and progressively less so above that. The seven formulas here differ by about 6% at five reps, 13% at twelve and around 57% at twenty. The larger error in practice is not the formula at all: it is a set that stopped short of failure, which reads low and which nothing can detect.
Which 1RM formula is the best?
None of them is best for everybody, which is why this shows all seven. Epley and Brzycki are the most widely used and are nearly identical up to about ten reps — they cross over at exactly ten and then diverge. Rather than picking one, the figure here is the middle of the seven, which is less exposed to any single one being wrong for you.
How many reps should I use for the estimate?
Between three and eight. Below that the set is nearly a max already and a couple of the formulas overshoot. Above ten, what limits you is muscular endurance rather than peak strength, and endurance varies far more between people — which is exactly where the formulas separate.
Is it safe to test a true one rep max?
It is the most demanding and highest-risk thing most people do in a gym, and it uses up the session. That is the reason estimating from a moderate set exists. If you do test one, it belongs in a planned session with a full warm-up and someone spotting — not decided on the basis of a number from a web page.
Does this work for any exercise?
The arithmetic does not care which lift it is, but the relationship it models does. These formulas were fitted mostly to bench press and squat. Lifts with a short range of movement or a lot of technique in them — deadlifts, olympic lifts — tend to hold fewer reps at a given percentage, so an estimate from a high-rep set will read high.
Does it work in pounds as well as kilos?
Yes, and the choice changes nothing but the label. Every one of these formulas is a ratio — it multiplies the weight you lifted by a number that depends only on the reps — so it gives the same answer in whichever unit you put in.
Is anything I type sent anywhere?
No. It is a few lines of arithmetic running in your browser, with no account, no logging and nothing transmitted. You can disconnect from the internet after the page loads and it carries on working.
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