One-Rep Max Calculator
Estimate your one-rep max (1RM) for a lift from a weight and rep count.
Cómo funciona
One-rep max (1RM) is the maximum weight you could lift for a single complete repetition of an exercise. Rather than requiring a risky maximal attempt, this calculator estimates it from a weight and rep count you can safely and reliably perform, using the Epley formula: 1RM = weight × (1 + reps / 30).
For example, if you can lift 200 lbs for 5 reps, the Epley formula estimates your 1RM as 200 × (1 + 5/30) = 200 × 1.167 ≈ 233 lbs. The formula works by extrapolating from the observed relationship between weight and how many reps can be performed at that weight — generally, lighter weights allow more reps, and this pattern is fairly predictable within a normal training rep range.
1RM estimates are most accurate for rep ranges roughly under 10 — as rep counts climb much higher (15, 20, or more), muscular endurance and fatigue management start to influence the set more than pure strength, causing formulas like Epley's to systematically overestimate true 1RM at those higher rep ranges.
Beyond curiosity, 1RM estimates are widely used to plan training programs: many strength programs prescribe working weights as a percentage of 1RM (e.g., "work at 80% of 1RM for sets of 5"), letting a lifter scale a program to their own strength level without needing to test a true maximal lift, which carries meaningfully higher injury risk.
Several other 1RM formulas exist besides Epley — Brzycki, Lombardi, and others — each derived from slightly different assumptions and typically agreeing closely with each other in the middle rep ranges (roughly 3-8 reps) but diverging more at the extremes, which is why 1RM estimates should be treated as a useful approximation rather than an exact figure.
For safety, most trainers recommend testing or approaching a true 1RM only occasionally, if ever, under proper supervision with a spotter — training and progress tracking are generally better served by working within safer, moderate rep ranges and using an estimated 1RM to gauge relative progress over time.
Tracking estimated 1RM over weeks or months (rather than testing a true maximal lift each time) is a practical way to monitor strength progress: if the weight-and-reps combination you can perform steadily increases, your estimated 1RM rises correspondingly, giving a consistent, lower-risk proxy for actual strength gains over a training cycle.
How accurate is this?
It's most accurate for rep ranges under about 10 — very high-rep sets tend to overestimate true 1RM.
What is the Epley formula exactly?
1RM = weight × (1 + reps ÷ 30) — it estimates your theoretical single-rep maximum from a known weight and rep count you can currently perform.
Why does accuracy drop at high rep counts?
At higher reps (15-20+), muscular endurance and fatigue management play a bigger role in the set than pure strength, causing the formula to overestimate the true 1RM more than it would at lower rep ranges.
Why would I want to know my estimated 1RM?
Many strength training programs prescribe working weights as a percentage of 1RM, so knowing an estimate lets you calculate appropriate weights for a program without needing to test a risky true maximal lift.
Is it safe to test a true 1RM in the gym?
It carries meaningfully higher injury risk than submaximal training, and is generally recommended only occasionally, if at all, and ideally with a spotter and proper technique supervision.
Are there other 1RM formulas besides Epley?
Yes — Brzycki and Lombardi are two other commonly used formulas; they generally agree closely with Epley in moderate rep ranges but diverge more at very low or very high rep counts.
Should I use my estimated 1RM as an exact target?
Treat it as a useful approximation for programming purposes rather than an exact figure — actual maximal strength can vary based on factors like fatigue, technique, and day-to-day readiness.
What rep range gives the most reliable 1RM estimate?
Roughly 1-10 reps tends to give the most reliable estimates, since the underlying formulas are calibrated against that range and diverge more outside it.
How can I use estimated 1RM to track progress over time?
Recalculate it periodically from your current working weights and reps — a steadily rising estimated 1RM indicates real strength gains without needing to repeatedly test a risky true maximal lift.
Does body weight or muscle group affect how accurate the Epley formula is?
The formula's general accuracy pattern (best at low-to-moderate reps) holds across most exercises and body weights, though individual variation in fatigue resistance can still shift results slightly for any given person.