How is a one rep max calculated?
It is not calculated so much as extrapolated. You lift a submaximal weight for as many hard repetitions as you can, and an equation projects what a single would have been. Two are in common use and both are implemented here.
1. Epley
1RM = weight × (1 + reps ÷ 30)
2. Brzycki
1RM = weight × 36 ÷ (37 − reps)
3. Each training percentage
target = 1RM × percent ÷ 100, then loaded from the plates you have
For 100 kg lifted five times, Epley gives 116.7 kg and Brzycki gives 112.5 kg. Neither is the right answer. They are two approximations of a number you did not measure.
How accurate is a 1RM calculator?
Accurate enough to programme from, and not accurate enough to treat as a measurement. The published testing is clear about where it holds: Mayhew and colleagues found prediction accuracy was better when fewer than ten repetitions to fatigue were used, and reported separately that accuracy decreases as repetitions rise.
The easiest way to see that is to watch the two equations pull apart. Every figure below comes from this calculator, using 100 kg as the lifted weight.
Estimated 1RM from 100 kg, by repetitions performed.| Reps | Epley | Brzycki | Gap |
|---|
| 3 | 110.0 | 105.9 | 4.1 |
|---|
| 5 | 116.7 | 112.5 | 4.2 |
|---|
| 8 | 126.7 | 124.1 | 2.5 |
|---|
| 10 | 133.3 | 133.3 | 0.0 |
|---|
| 12 | 140.0 | 144.0 | 4.0 |
|---|
| 15 | 150.0 | 163.6 | 13.6 |
|---|
At ten reps they agree exactly, and not by rounding: Epley works out to 1 + 10/30 and Brzycki to 36/27, which are both four thirds. That is a coincidence of two curve fits crossing, not a sign either is right. Past it they diverge fast, and at fifteen repetitions they sit 13.6 kg apart, which is more than many lifters add to a squat in a year. That is why this calculator declines above twenty repetitions entirely.
Where these equations came from
Worth stating plainly, because most sites present them as settled science. Epley comes from a 1985 training manual, the Boyd Epley Workout. Brzycki was published in 1993 in the Journal of Physical Education, Recreation and Dance. Neither began life as a peer-reviewed study, and neither is derived from anything about muscle physiology. They are curve fits that happened to work well enough to stick.
What has been peer-reviewed is the work checking how well they perform, which is what the sources at the foot of this page cite. That distinction is the reason this page shows you both answers and the gap between them instead of picking one and presenting it as the figure.
Why the table shows plates as well as percentages
Because a percentage is not a weight you can load. If your estimated max is 116.7 kg then 85% is 99.2 kg, and no combination of standard plates makes 99.2 kg. The nearest you can build is 97.5 kg, which the table shows along with how far short it falls.
Anywhere else this is two separate tools and a piece of paper. If you only need the loading for one weight, the barbell plate calculator does that on its own, and both share the same plate inventory picker so a gym missing its 15s is handled either way.