Epley Formula: How to Calculate Your 1RM

What Is the Epley Formula?

The Epley formula is a mathematical equation that estimates your one-rep max (1RM) from the weight you can lift for multiple repetitions. It was developed by Boyd Epley in the 1980s while working with the University of Nebraska athletic program.

The formula assumes a linear relationship between the number of reps you can perform and the percentage of your max. This makes it simple to calculate and understand, which is one reason it became the most widely used 1RM estimation formula.

The Formula

Where:

• weight = the weight lifted (in kg or lb)
• reps = the number of repetitions completed

How the Epley Formula Works

Let's walk through a step-by-step example. Say you bench press 100kg for 5 reps:

Your estimated 1RM is 116.7kg.

Here are more examples with 100kg:

When to Use the Epley Formula

The Epley formula is ideal in these situations:

• Percentage-based programming: Most training programs prescribe loads as percentages of 1RM (e.g., "5x5 at 75%"). Epley gives you the reference point.
• Progress tracking: Compare your estimated 1RM over weeks and months without the injury risk of frequent true max attempts.
• Mid-range rep sets: Epley is especially accurate for sets of 6-10 reps, making it perfect for typical hypertrophy and strength work.
• Quick mental math: The formula is simple enough to calculate in your head. Just divide reps by 30 and multiply.

Accuracy and Limitations

The Epley formula performs well within its intended range, but has known limitations:

Key Limitations

• Linear assumption: The formula assumes a straight-line relationship between reps and %1RM, but the real relationship is slightly curved.
• No upper bound: Unlike Brzycki (which becomes undefined at 37 reps), Epley keeps producing values at any rep count, which can give unrealistic estimates at high reps.
• Exercise-dependent: Developed primarily with compound lifts (squat, bench, deadlift). May be less accurate for isolation movements or Olympic lifts.
• Individual variation: Fiber type composition, training history, and fatigue tolerance all affect accuracy.

Epley vs Brzycki

The Brzycki formula (1RM = weight x 36 / (37 - reps)) is the other most popular 1RM formula. Here's how they compare using 100kg:

• At low reps (1-5), both formulas give nearly identical results.
• At mid reps (6-10), Epley gives slightly higher estimates.
• At high reps (12+), the difference becomes significant - Epley estimates are noticeably higher.
• For conservative estimates (e.g., planning competition attempts), Brzycki is preferred.

Practical Applications

Here's how to use the Epley formula effectively in your training:

1. Set up training percentages. Estimate your 1RM, then calculate working weights. For example, if your Epley 1RM is 120kg and your program calls for 5x5 at 80%, work with 96kg.
2. Track strength over time. Log your best set each week and calculate the estimated 1RM. Trending upward means you're getting stronger without needing to test true maxes.
3. Use fresh sets only. Estimate from your top set when you're rested (3-5 min rest). Fatigued sets give inflated rep counts and unreliable estimates.
4. Stick with 3-6 rep sets. This range gives the best signal-to-noise ratio for 1RM estimation. Heavy enough to reflect strength, not so many reps that endurance confounds the result.