What a one rep max is
Your one rep max (1RM) is the heaviest weight you can lift once with full range of motion and form you would defend. Nearly every strength program is written against it: "5×5 at 80%" means nothing until you know what 100% is.
There are two ways to get the number. You can test it, which takes a full session, a spotter, and joints that are ready for limit loads. Or you can estimate it from a recent hard set, which takes ten seconds and gets within a few percent. For programming purposes, the estimate is almost always enough.
The two formulas
This calculator runs your set through both of the standard equations:
- Epley (1985): 1RM = weight × (1 + reps ÷ 30)
- Brzycki (1993): 1RM = weight × 36 ÷ (37 − reps)
Worked example: 100 kg for 5 reps. Epley says 100 × (1 + 5/30) = 116.7 kg. Brzycki says 100 × 36/32 = 112.5 kg. The average, about 115 kg, is the headline result.
A quirk worth knowing: the two formulas agree exactly at 10 reps, where both multiply the weight by 1.33. Below 10 reps Epley runs a little higher; above 10, Brzycki does. The gap between them is a useful reminder that every 1RM estimate carries a margin of error. Presenting one number to a decimal place would be false precision.
Accuracy falls off past 10 reps
Both equations assume a tight relationship between how many reps you can do and how much you can lift once. That relationship holds well from 2 to 6 reps and reasonably well up to 10. Beyond that, muscular endurance takes over as the limiting factor. A 20-rep squat set says more about your conditioning and pain tolerance than about your maximal strength, which is why this calculator stops accepting inputs at 12 reps.
The best input is a set of 3–6 reps, taken within a rep or two of failure, on the same lift, from the last week or two. Estimates are also lift-specific: your bench 1RM ratio does not transfer to your deadlift. Run each lift through its own set.
How to use the percentage table
The table under your result converts the estimate into working weights. As a rough map: sets at 85% and above build maximal strength in the 2–6 rep range, while 65–80% covers most muscle-building work in the 8–15 rep range. The bands overlap on purpose. If a program prescribes "4×8 at 75%", read the 75% row and load the bar.
Treat percentages as targets, not laws. On a day when 75% moves like 85%, cut the load. Strength fluctuates with sleep, stress, and food — a max is a moving target, and eating enough matters more than lifters like to admit. The protein calculator gives you a daily gram target for recovery, and the TDEE calculator tells you whether your calories support the training at all.
Before you attempt a real max
Two safety rules are non-negotiable. First, do not max in your first year of training. Connective tissue adapts more slowly than muscle, and near-limit loads punish technique that has not settled yet. Second, never max alone. Use a power rack with safety pins, or a spotter who knows the plan — this matters most on bench press, where a failed rep pins the bar to your chest.
Warm up in steps: empty bar, then roughly 50%, 70%, 85%, and 95% of the target before the attempt, with full rest between sets. And if the calculator says 140 kg but 130 moved slow and ugly, believe the bar. The estimate is a prediction, not a promise.
