One Rep Max Calculator
Enter the weight you lifted and how many reps you completed to estimate your one-rep max (1RM), the heaviest weight you could lift for a single rep. This calculator uses the Epley formula, the most widely used 1RM estimator.
How to calculate one rep max
The Epley formula estimates your true one-rep max from a lighter set taken to or near failure. Divide the number of reps by 30, add 1, then multiply by the weight you lifted. It works best for sets of about 10 reps or fewer; very high-rep sets make any 1RM formula less reliable, since fatigue and technique play a bigger role than raw strength.
1RM = weight x (1 + reps / 30)
Worked example
A lifter completes 8 reps at 185 lb.
- Reps ratio: 8 / 30 = 0.2667
- 1RM = 185 x (1 + 0.2667) = 185 x 1.2667 = 234.3
- 90% of 1RM = 234.3 x 0.9 = 210.9
- 80% of 1RM = 234.3 x 0.8 = 187.5
Result: An estimated one-rep max of about 234.3 lb, with 90% at 210.9 lb and 80% at 187.5 lb.
Frequently asked questions
What is a one-rep max (1RM)?
Your one-rep max is the heaviest weight you can lift for a single, complete repetition with good form. It is the standard reference point for strength programming, letting lifters set training weights as a percentage of their max rather than guessing at an absolute number.
Why use the Epley formula instead of testing my real max?
Testing a true 1RM requires a proper warm-up, spotters, and carries injury risk, especially for beginners. The Epley formula estimates it safely from a lighter set (ideally 3 to 10 reps) you already performed, so you get a useful number without the risk of a maximal single lift.
How accurate is a 1RM estimate?
It is generally accurate to within a few percent for sets of 10 reps or fewer taken close to failure. Accuracy drops for higher rep ranges (15+) because fatigue and muscular endurance start to matter more than the pure strength the formula is modeling. Retest periodically as you get stronger.
What are the 90% and 80% figures for?
Strength programs commonly prescribe working weights as a percentage of 1RM: heavy strength work often sits around 85 to 95%, while hypertrophy (muscle-building) work often sits around 65 to 80%. Showing 90% and 80% alongside your estimated max gives you ready-to-use training weights without extra math.
Does the Epley formula work for every exercise?
It works best for major compound lifts like the bench press, squat, and deadlift, where the research behind it was developed. It is less reliable for isolation exercises or movements with a very different fatigue profile. Use it as a planning guide rather than an exact prediction for every lift.