ReckonBoxHow the one rep max calculator works
A one rep max is the heaviest weight you can lift once with good form on a given exercise. Enter a set you actually completed, say 100 kg for 5 reps, and this tool projects the single across four published equations at once: about 117 by Epley, 113 by Brzycki, 117 by Lombardi and 116 by the NASM variant.
Those four numbers are the point. A calculator that hands you one figure is hiding a disagreement that runs to about 5 kg on a 100 kg working set.
The equations, printed in full
| Formula | Equation | 100 kg x 5 reps |
|---|---|---|
| Epley | weight x (1 + reps/30) | 116.7 |
| Brzycki | weight x 36/(37 - reps) | 112.5 |
| Lombardi | weight x reps^0.10 | 117.5 |
| NASM | (weight x reps / 30.48) + weight | 116.4 |
Strength Level ranks first for this search and prints none of this. The page fetched for this build carries an input form and a repetition-percentage table, with no formula named, no equation shown, no reliable rep range and no stated limitation.
calculator.net does considerably better, naming three of the four and putting typical accuracy within about 10% of a true maximum, with weaker results for beginners. NASM publishes its own variant, which is Epley with 30.48 in place of 30. Barbell Medicine takes a different route again, adding an RPE input from 6.5 to 10, and is refreshingly direct that fatigue, technique, exercise selection, equipment and range of motion all move the relationship between a training set and a real single.
Two things that fall out of the arithmetic
Epley and Brzycki return exactly the same number at 10 reps. Epley gives 1 + 10/30, which is 1.3333. Brzycki gives 36/27, which is also 1.3333. They cross precisely there, and separate on both sides: below 10 reps Epley reads higher, above 10 Brzycki climbs much faster. At 20 reps Brzycki projects 2.12 times your working weight against Epley's 1.67.
Nobody in the ranked set mentions this, which is odd, because it explains the rep cap everyone applies.
Brzycki divides by 37 minus your reps. As reps climb the denominator shrinks, the estimate inflates, and at exactly 37 reps the equation divides by zero and stops existing. That's the arithmetic reason the field caps its advice around 10, though NASM is the only page fetched that states the cap out loud.
Epley has a smaller flaw at the opposite end. Feed it a single rep and it returns 103.3% of a weight you just lifted once, when the honest answer is definitionally 100%. Brzycki returns exactly 100% there. Neither is wrong so much as fitted to a middle range and then extended past it.
Training percentages
Once you have an estimate, the conventional mapping from percentage to rep range looks like this:
| Percentage of 1RM | Typical reps | Usual goal |
|---|---|---|
| 95% | 1 to 2 | Peak strength |
| 90% | 3 to 4 | Strength |
| 85% | 5 to 6 | Strength |
| 80% | 7 to 10 | Hypertrophy |
| 70% | 12 or more | Endurance |
calculator.net publishes a similar set of bands. Treat them as starting points; the load that matters is the one you can move with clean technique on the day, and the estimate above already carries a 10% margin.
Why not just test a true single?
Because the safety case is poor for most people. NASM's guidance points at submaximal testing with good form instead of a maximal attempt, recommends consulting a fitness professional before maximal lifts, and advises working with a spotter. Its own framing is that these calculations give ballpark estimates.
That's the right register for a projection built from one set on one day.
What this calculator does not do
RPE adjustment is absent. Barbell Medicine's approach of folding in a rating of perceived exertion is sound and probably sharpens the estimate, but the page publishes no equation for it, and reproducing an unstated adjustment would be invention.
Exercise-specific coefficients are out too. A bench press and a deadlift don't behave identically across rep ranges, and no source fetched here supplies per-lift factors worth shipping.
There's no maximal testing protocol on this page either. Printing one would be directing readers into an injury risk on a page they arrived at from a search box, and the whole point of a submaximal estimate is that you don't need one. For the bodyweight side of strength work, the lean body mass calculator covers composition, and the protein calculator handles intake.
Frequently asked questions
What is a one rep max? A one rep max is the heaviest weight you can lift once with good form for a given exercise. Rather than testing it directly, this calculator projects it from a set you completed at a lighter weight, which is what NASM recommends: submaximal testing with good form instead of a maximal attempt.
Which 1RM formula is correct? None of them exactly. This tool runs four published equations side by side. Epley is weight x (1 + reps/30), Brzycki is weight x 36/(37 - reps), Lombardi is weight x reps^0.10, and NASM publishes (weight x reps / 30.48) + weight. calculator.net puts typical accuracy within about 10% of a true max, and weaker for beginners.
Why do Epley and Brzycki sometimes give the same answer? They cross at exactly 10 reps. Epley returns 1 + 10/30, which is 1.3333, and Brzycki returns 36/27, which is also 1.3333. Below 10 reps Epley reads higher; above 10 Brzycki climbs faster. At 20 reps Brzycki gives 2.12 times your working weight against Epley 1.67.
Why should I stay under about 10 reps? Because Brzycki divides by 37 minus your reps, so it inflates as reps rise and is undefined at 37 reps exactly. NASM states the cap directly, advising no more than 10 reps to keep the estimate accurate. The other tools apply the cap without explaining it.
Why does Epley say 103% when I lifted a weight once? It is a known quirk of the equation. Epley adds reps/30 to 1, so at 1 rep it returns 103.3% of a weight you just lifted for a single, when the answer is definitionally 100%. Brzycki returns exactly 100% at 1 rep, which is one reason to read several formulas together.
How do I use percentages of my 1RM? Training percentages map roughly to rep ranges: about 95% for heavy singles and doubles, 90% for sets of 3 to 4, 85% for 5 to 6, 80% for hypertrophy work at 7 to 10 reps, and 70% for endurance sets of 12 or more. Those are conventional ranges, and the load that matters is the one you can move with good technique on the day.
What changes the accuracy of the estimate? Barbell Medicine names the variables directly: fatigue, technique, exercise selection, equipment, range of motion and day-to-day performance all move the relationship between a training set and a true single. An estimate from a fresh first set will read differently from one taken at the end of a session.