World of Lifting
Gym Math
Free strength calculators.
One Rep Max
Epley formula estimate
Enter weight and reps above
What is a one-rep max?
A one-rep max (1RM) is the heaviest weight you can lift for a single, full-range rep with good form. Most strength programs are built around this number: work sets get prescribed as a percentage of 1RM, like “5x5 at 75%,” so a good estimate means you load the bar right instead of guessing.
Testing a true 1RM is hard on your joints and nervous system, and the injury risk goes up if your form breaks down under a near-max load. That’s why most lifters estimate from a comfortable working set instead, using a formula like the one above.
How the Epley formula works
This calculator uses the Epley formula: 1RM = weight × (1 + reps ÷ 30). It’s one of the most common estimation formulas around because it’s simple and holds up reasonably well for sets of 1 to 10 reps.
Other formulas exist too, like Brzycki and Lombardi, and they’ll spit out slightly different numbers. None is the “right” one. They’re just different curve-fits to lifting data. The gap stays small for sets under 6 reps and widens past 10, which is why this calculator caps reps at 20.
| Formula | 100kg × 5 reps estimates |
|---|---|
| Epley | 116.7 kg |
| Brzycki | 112.5 kg |
| Lombardi | 117.5 kg |
Using your 1RM to program training
Once you’ve got a 1RM, tested or estimated, most programs express working weights as a percentage of it. The rep max table above shows roughly what percentage each rep count corresponds to, so you can go from a target rep range to a loading number, or work backwards from a weight to figure out how many reps it’ll take.
Re-test or re-estimate every month or two as your strength changes. A stale 1RM throws off every percentage built on top of it.
FAQ
How accurate is a 1RM calculator?
For sets of 1-5 reps taken close to failure, these formulas usually land within 2-5% of a true tested max. The higher the rep count, the worse the estimate gets. A set of 12+ reps tells you more about your muscular endurance than your strength, so it’s a poor stand-in for a 1-rep effort.
What rep range gives the most accurate estimate?
One to five reps, taken close to technical failure. If you grind out the last rep or your form falls apart before you hit your target count, the estimate comes out too high.
Should I test my actual 1RM, or just estimate it?
For everyday training, an estimate from a hard top set works just as well and is a lot safer. Save a true 1RM attempt for meets, peaking blocks, or when you actually need to verify your numbers, and only do it fresh, warmed up, and with a spotter.
Why do different 1RM calculators give different numbers?
Different calculators use different formulas, like Epley, Brzycki, and Lombardi, each fit to its own set of lifting data. At low rep counts the differences are small and won’t change how you train. Pick one formula, stick with it, and track your trend instead of comparing numbers across different calculators.
How do I know what percentage of my 1RM to lift?
Depends on your goal. Heavy, low-rep work (85%+) builds maximal strength. Moderate loads (65-85%) build a mix of strength and size. Lighter loads (under 65%) are mostly for technique, deloads, or hypertrophy volume. If you’re following a structured program, it’ll tell you the percentages, and this calculator turns those into actual numbers on the bar.
More Gym Math Tools
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Plate Loader
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Want a program built around these numbers? Browse the WOL Library for percentage-based and RPE-based training plans.