Estimating a one rep max
Your one rep max is the heaviest weight you can lift once with good form. Actually testing it is time-consuming, needs a spotter, carries real injury risk, and leaves you too fatigued to train properly afterwards. Most lifters, sensibly, would rather estimate it.
Estimation works because there is a fairly consistent relationship between the weight you can lift for a given number of reps and the weight you could lift once. Lift something for five reps and a formula can approximate your single.
The formulas, and why they disagree
We use the two best established: Epley and Brzycki. They are built on different curves and they do not agree, which is the genuinely interesting part.
At low rep counts they land within a couple of kilos of each other. As reps climb the gap widens, with Brzycki tending to return the more conservative number at higher rep counts. Neither is simply correct. Rather than hide that behind a single figure, we show both, because the spread between them is itself information about how much confidence the estimate deserves.
How accurate is any of it
Reliable within roughly five reps. Beyond that, accuracy degrades quickly, because sets of twelve or fifteen are limited as much by muscular endurance and by your tolerance for discomfort as by maximal strength. A twenty rep set tells you very little about a single.
Accuracy also varies by lift. Estimates hold up well for the bench press and reasonably for the squat. Deadlift estimates tend to overshoot, since grip and lower back fatigue accumulate faster than on other lifts.
Who publishes this site
RepMax is built and maintained by CJ Software Ltd, a UK software company that develops a small family of free, single-purpose calculators. We are a software business, not a coaching service, and we say so plainly because it matters when you decide how much weight to give a number on a screen.
The site is funded by advertising. Advertisers have no influence over the calculations, the guidance, or what we choose to write about.
Train sensibly
An estimated max is a planning tool for setting training percentages, not a number to attempt because a website suggested it. Programme from the conservative end, particularly on deadlifts, and progress gradually.
Nothing here is coaching or medical advice. If you are new to lifting, coming back from injury, or unsure about technique, work with a qualified coach. Good form under a moderate load beats a confident estimate under a heavy one.