What is a One Rep Max (1RM)?
In weight training, a One Repetition Maximum (1RM) is the maximum amount of weight that a person can possibly lift for one complete repetition of an exercise through a full range of motion. It is the gold standard metric used by powerlifters, Olympic weightlifters, and strength coaches to determine an individual's absolute strength.
Why Calculate Instead of Testing?
Actually performing a true 1RM test under a heavy barbell (such as a Squat or Bench Press) is incredibly taxing on the central nervous system and carries a high risk of injury—especially for novice and intermediate lifters whose form may break down under maximal loads.
Instead of risking injury, you can perform a safer sub-maximal lift (for example, lifting a moderate weight for 5 to 8 repetitions until technical failure) and plug those numbers into a mathematical formula to highly accurately estimate your 1RM.
The Formulas Used
Because different people have different muscle fiber type compositions (fast twitch vs slow twitch), no single formula is 100% accurate for everyone. Our calculator takes an average of the two most universally trusted clinical strength formulas:
The Brzycki Formula (1993)
Developed by Matt Brzycki, this formula is highly accurate for rep ranges under 10.
1RM = Weight × (36 / (37 - Reps))
The Epley Formula (1985)
Developed by Boyd Epley, this formula is widely used in collegiate strength and conditioning programs.
1RM = Weight × (1 + (0.0333 × Reps))
How to Use Your Training Percentages
Once you have your 1RM, you can program your workouts based on percentages. If your goal is maximal strength, you should train in the 85% to 95% range for 1-5 reps. If your goal is muscle hypertrophy (size), the 70% to 80% range for 8-12 reps is optimal.