One-Rep Max Calculator

How this is calculated

Two established formulas are used, both published and both widely applied in strength training. Epley: 1RM = weight x (1 + reps / 30). Brzycki: 1RM = weight x 36 / (37 - reps). They are shown side by side deliberately. Every 1RM formula is a curve fitted to how people generally behave, not a measurement of you, and these two curves cross. Setting them equal and solving gives r squared minus 7r minus 30 = 0, whose root is exactly 10 - so BELOW ten reps Epley reads higher, ABOVE ten reps Brzycki reads higher, and at exactly ten reps they agree to the decimal. That crossover matters for how you read the spread. At ten reps the two figures are identical, and that is a property of the arithmetic rather than a sign of a good estimate - the curves happen to intersect there. A wide spread does tell you the formulas are extrapolating differently, but a narrow one near ten reps tells you nothing at all. The most reliable estimates come from low reps, where the spread is moderate, not from ten reps where it is zero. At one rep Brzycki returns exactly the weight you lifted, which is correct by definition, while Epley adds about 3.3 percent. At that end Brzycki has the more sensible shape. Accuracy is best from about 1 to 5 reps, where published comparisons put the formulas within a few percent of a tested maximum. It is still reasonable through 10. Beyond that it degrades quickly, because a high-rep set measures muscular endurance and pain tolerance as much as it measures strength, and those vary far more between people than maximal strength does. The reps box stops at 15 for that reason - and Brzycki's denominator (37 minus reps) reaches zero at 37 reps, which is a good illustration that these are fitted curves rather than laws. The percentage figure is the estimate multiplied out - a starting point for programming, not a prescription. What percentage suits a given session depends on the lift, your experience and the goal of the block. ⚠️ ESTIMATING A ONE-REP MAX IS SAFE; TESTING ONE IS NOT NECESSARILY. That is rather the point of the calculation - it gives you the figure from a set you already completed, so you do not have to attempt a true maximal lift to find it. An actual 1RM attempt is where lifting injuries cluster, and it needs a competent spotter, a genuine warm-up, and technique that holds under load. If you are new to the lift, coming back from injury, or working around a joint that already complains, take the estimate and leave the attempt alone - and get an injury looked at by a clinician or a physiotherapist rather than training through it.

Common questions

Which formula should I believe?

Neither, exactly - believe the range. Published comparisons find Epley and similar formulas closest at very low reps and Brzycki and Mayhew stronger through the 6 to 10 range, with errors of a few percent either way. That is why both are shown, and the midpoint is a reasonable single number. One caution on reading the gap: the two curves CROSS at exactly ten reps, so the spread collapses to zero there for reasons that have nothing to do with confidence. Trust a low-rep estimate with a moderate spread over a ten-rep estimate with none.

Why does Brzycki return exactly my weight at 1 rep and Epley doesn't?

Because of how each curve is shaped. At one rep Brzycki's 36 / (37 - 1) is exactly 1, so it returns your weight - which is correct by definition, since a single rep IS your max for that day. Epley's (1 + 1/30) adds about 3.3 percent. Neither is wrong as such; they are curves fitted to real lifters, and Brzycki's happens to be anchored correctly at that end.

How many reps should I use?

As few as you can manage honestly. One to five is where these formulas are most reliable, and the estimate degrades from there. A set of 12 tells you a great deal about your endurance and rather less about your maximum strength, because at that rep count pain tolerance and conditioning start to dominate - and those differ between people far more than maximal strength does.

Should I go and attempt this weight?

Not on the strength of a calculator. This is an estimate from a curve fitted to other people, it assumes the reps were clean and close to failure, and it says nothing about whether your joints, technique or that particular day can support a maximal attempt. Genuine 1RM testing wants a proper warm-up progression, experience with the lift, and ideally a spotter or safety bars.

Why does the reps box stop at 15?

Because past that the estimate stops meaning much. Beyond roughly 10 reps the numbers drift as endurance takes over, and Brzycki in particular falls apart as reps approach 37 - its denominator reaches zero there and the formula returns nonsense. That is a useful reminder of what these are: curves fitted to observed data over a limited range, not laws that hold everywhere.

Does it work the same for every lift?

Not equally well. These formulas were largely derived from bench press and squat data, and they translate reasonably to most compound barbell lifts. They fit less well to deadlifts, where many lifters manage fewer reps at a given percentage, and poorly to isolation work and machines. Treat the estimate as most trustworthy on the big compound lifts.

Should I test my actual one-rep max?

Usually not, and avoiding it is the reason this calculator exists - it gives you the number from a set you have already done safely. A true maximal attempt is where lifting injuries concentrate: form degrades exactly when the load is highest, and a failed rep with a loaded bar is a different kind of problem from a failed rep at eighty percent. If you do test one, it needs a full warm-up, a competent spotter and a lift whose technique you are confident in under load. If you are new to the movement, returning from injury, or working around a joint that already hurts, use the estimate - and have the injury looked at by a clinician or physiotherapist rather than training through it.

Take it further with AI

Copy this into ChatGPT or Claude with your own numbers filled in. It hands over the figures this calculator worked out, so the answer is built on real arithmetic instead of a guess.

I used the One-Rep Max Calculator at https://www.bfcbrilliance.com/tools/one-rep-max-calculator.

What I entered:
- Weight lifted (lb or kg): ___
- Reps completed (reps): ___

What it calculated:
- Estimated 1RM: ___
- Epley says: ___
- Brzycki says: ___
- How far apart they are: ___
- 85% of the estimate: ___

Use those figures as given — they are already worked out, so please don't recalculate or estimate your own. Help me turn them into a plan: what to buy or do, in what order, roughly what it should cost, and the mistakes people most often make with this job.

Keep this general and do not give medical advice — where it matters, tell me what to raise with a clinician.

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