One Rep Max Calculator

How to use the one rep max calculator

Enter the weight you lifted and how many reps you got with it, and the tool estimates your one-rep max — the most you could lift for a single rep — without you ever having to attempt that single rep. The weight is just a number, so it works the same in pounds or kilograms; use the lb/kg toggle to label it, but the math never converts between the two. The defaults (100 for 5 reps) are there so you can see the tool work before you swap in a set of your own.

Use a real working set, taken to failure or close to it. The estimate assumes the reps you entered are roughly all you had — that the set was hard. If you stopped with three reps left in the tank, the calculator reads that as an easier load than it was and lands low; if your form broke down before true failure, it lands high. The honest input is a set where the last rep was a genuine grind, not a number you picked because it looked tidy.

Read the estimated 1RM off the Epley line first — that is the primary number, and on the defaults a set of 100 for 5 reps comes out to 116.67. Next to it sits the Brzycki estimate, 112.5 on the same set, shown as a second opinion: the two formulas weight reps slightly differently, so they rarely agree to the decimal, and the small gap between them is a fair sense of the uncertainty in any rep-based estimate. When the set is a true single, both formulas simply return the weight, because one rep already is your one-rep max.

Below the estimate is the percentage-of-1RM table, and that table is the part you actually program from. It maps each percentage of your max to the reps you can typically expect there — 100% for about 1 rep, 95% for 2, 90% for 4, 85% for 6, 80% for 8, 75% for 10, 70% for 12 — and shows the load each percentage works out to. The loads are simply your 1RM times the percentage, so on a 116.67 max, 90% is about 105. To run a 5×5 you would look up the 80–85% band and train somewhere around 93 to 99.

Treat the output as a planning estimate, not a verdict. Rep-max formulas are at their best in the low-rep range — roughly 1 to 6 reps — and drift upward as the reps climb, because high-rep performance varies so much from person to person and lift to lift. If you want the cleanest estimate, feed the tool a heavy set of 3 to 6 rather than a light set of 12, and re-test every few weeks as your training moves the real number.

The formula

Two standard rep-max formulas turn a submaximal set — a weight and the reps you got with it — into an estimated one-rep max. The tool shows both: Epley as the primary estimate, Brzycki as a comparison:

Epley:   1RM = weight × (1 + reps / 30)
Brzycki: 1RM = weight × 36 / (37 − reps)

Worked example with the defaults — 100 for 5 reps: Epley gives 100 × (1 + 5/30) = 116.67, and Brzycki gives 100 × 36/(37 − 5) = 112.5. The two land close but not identical, which is expected. At a single rep both formulas collapse to the weight itself — 100 for 1 rep returns 100 either way — because one rep already is the one-rep max. The percentage table then scales that estimate: 90% of the 116.67 Epley max is about 105.

Accuracy is best at low reps and drifts as the reps climb: above about 10 reps the formulas tend to overestimate, because how many reps someone gets at a given percentage varies a lot by person and by exercise, and that variation is exactly what a single formula cannot capture. The percentage table is how lifters pick working loads — a 5×5 is usually run around 80–85% of the estimated max, lighter back-off work down near 70%. And the safety case for estimating at all: do not test a true 1RM untrained. The estimate is the lower-risk way to get the number, which is the whole point of the tool.

Frequently asked questions

Related tools