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15 Rep Max Calculator
Estimate your 1RM from a recent set of 15 repetitions.
Why Calculate from 15 Reps?
High rep ranges (15+) test muscular endurance. Estimating a 1RM from this many reps is less precise due to metabolic fatigue accumulation (the 'burn'), but still provides a useful baseline for tracking endurance progress.
Using this calculator allows you to predict your max strength safely. Just enter the weight you lifted for 15 reps above.
Frequently Asked Questions
What is a 1RM (One-Repetition Maximum)?
Your one-repetition maximum (1RM) is the maximum amount of weight you can lift for a single repetition of a given exercise. It's a common measure of strength in weight training.
What is the best rep range for estimating 1RM?
For the most reliable estimate, we recommend testing with a weight you can lift for 3 to 5 reps. Calculations based on lower rep ranges are significantly more accurate because they rely closer to your true neuromuscular limit. As you exceed 10 reps, factors like metabolic fatigue (the 'burn') can skew the results, making the 1RM prediction less accurate.
What percentage of 1RM should I use for muscle growth?
To build Strength, train with 85-100% of your 1RM (1-5 reps). For Hypertrophy (Muscle Size), the optimal range is typically 70-85% of your 1RM (6-12 reps). For Endurance, use lighter loads of 50-70% (15+ reps). The 'Rep Max Breakdown' table in this calculator provides these exact numbers for you.
How accurate is this calculator?
This calculator uses the well-known Brzycki formula to estimate your 1RM. While it is a widely accepted standard in sports science, it is still an estimate. Individual factors like muscle fiber type, fatigue, and limb length can influence your true 1RM. For the most accurate result, perform a true 1RM test under safe conditions with spotters.
Why should I calculate my 1RM?
Knowing your 1RM helps you tailor your training program effectively. Many periodized strength and hypertrophy programs (like 5/3/1 or Smolov) are based on specific percentages of your 1RM. This data allows you to train at the exact intensity needed to achieve your specific fitness goals without guessing.
Which exercises does this work for?
The formula works best for compound lifts like Bench Press, Squat, Deadlift, and Overhead Press. It can be used for isolation exercises or machine weights, but accuracy may vary as isolation movements often allow for higher rep ranges where the formula becomes less precise.