A one rep max is the heaviest weight you can lift once with good form. It is the reference number for almost every strength programme: percentages of your max decide whether a set builds strength, muscle, or endurance. The problem is that testing a true max is risky and tiring, especially without a spotter. This estimator solves that by working backwards from a normal set: tell it the weight you lifted and how many repetitions you managed, and it estimates your max for you.
The tool uses two well-known formulas and averages them. The Epley formula multiplies your weight by one plus repetitions divided by thirty, so one hundred kilograms for five repetitions gives one hundred times 1.167, which is about 116.7 kilograms. The Brzycki formula divides your weight by 1.0278 minus 0.0278 times repetitions, which for the same set gives about 112.9 kilograms. The average, about 114.8 kilograms, smooths out the quirks of each formula and is usually closer to reality than either one alone.
Here is how to use the result. The tool also prints a training table: ninety percent of 114.8 is about 103.3 kilograms, eighty percent is about 91.8, seventy percent is about 80.4, and sixty percent is about 68.9. Heavy strength work usually lives around eighty-five to ninety-five percent, muscle building around sixty-five to eighty-five percent, and endurance work below sixty-five percent. These bands are general coaching guidance, not a prescription for you, but they turn one estimated number into a whole programme of working weights.
Accuracy depends heavily on how many repetitions you base the estimate on. Estimates from two to ten repetitions are the most reliable, because the formulas were built around that range and the effort is close to maximal. Estimates from a single repetition need no formula at all, since that repetition is your max. Estimates from more than ten repetitions tend to overestimate, because endurance and technique start to matter more than pure strength, and the formulas assume a straight-line relationship that breaks down at high repetitions.
Different lifts also give different accuracy. The formulas work best on big barbell lifts like the squat, bench press and deadlift, where the weight is the main limit. They work less well on single-joint exercises, machines with unusual resistance curves, or lifts where balance or grip fails before the muscles do. If your estimated max for a machine exercise looks far above anything you have ever lifted, trust your experience over the formula, and use a conservative number for programming.
Never use an estimate as permission to attempt a weight you are not ready for. An estimated max is a planning number, not a guarantee that you can lift it safely today. Fatigue, poor sleep, a new technique, or a small injury can all move your real capacity well below the estimate on a given day. Always warm up in steps, use a spotter for heavy barbell work, use safety bars or a rack where possible, and stop a set the moment your form breaks down rather than grinding out an ugly repetition.
This tool is an educational estimator only, never medical advice. It does not assess your health, your readiness, or whether maximal lifting suits you, and it cannot replace a qualified coach. Seek professional advice before attempting maximal lifts, training with a health condition, or returning after injury, and stop immediately if you feel pain. Used with that care, a one rep max estimate is a practical shortcut that keeps your training percentages honest without the risk of a true max test. Re-run the estimate every few months from a fresh submaximal set, and your training weights will track your real progress instead of freezing around an old, stale number.