Rep Max Calculator (Any Rep Max)
Estimate your X-rep max from your known 1RM using the reverse Epley formula.
What this tool does
This calculator estimates the maximum weight a person can lift for a specified number of repetitions, given their known one-rep max (1RM), using the reverse Epley formula. It takes a 1RM in kilograms and a target rep count as inputs, then outputs the estimated load for that rep range and its percentage of the 1RM. The reverse Epley equation (derived from Boyd Epley's 1985 model) assumes a linear relationship between load and repetitions, and is most commonly applied to compound barbell lifts in the 1–10 rep range.
Formula Used
Disclaimer
This calculator is for educational and informational purposes only. It does not provide medical, nutritional, or training advice. Results are mathematical estimates and may not reflect individual circumstances. Consult a qualified coach, registered dietitian, medical professional, or physiotherapist for personal guidance.
How Rep Max Calculator (Any Rep Max) works
This calculator estimates the maximum weight you can lift for a specified number of repetitions, given your known one-repetition maximum (1RM). It applies the reverse Epley formula: dividing your 1RM by a factor that accounts for the target rep count. For example, if your 1RM is 100 kg and you want to know your estimated 5-rep max, the tool calculates the load you can handle for five consecutive repetitions. The result is expressed in kilograms and as a percentage of your 1RM.
The formula
The calculator uses the reverse Epley equation:
X-rep max = 1RM / (1 + target_reps / 30)
Where:
• 1RM is your one-repetition maximum in kilograms
• target_reps is the number of repetitions for which you want to estimate the maximum load
• The constant 30 is derived from the original Epley formula for predicting 1RM from submaximal loads
The Epley formula was introduced by Boyd Epley in 1985 as a practical tool for strength coaches working with athletes, and the reverse form is the percentage table found in coaching texts such as NSCA's Essentials of Personal Training. Worked through with the defaults: 100 kg ÷ (1 + 5/30) = 100 ÷ 1.1667 = 85.7 kg, which is 85.7% of the 1RM. Because the 1RM is simply divided by a factor, the units carry through: enter a 1RM in pounds and the answer comes back in pounds.
One quirk worth knowing before reading the output too literally: at a target of one repetition the formula returns 96.8% of the entered 1RM rather than 100%. The Epley model was fitted to predict a maximum from submaximal sets, and running it backwards to a single rep leaves that 3.2% gap. Treat one-rep output as the model's artefact, not as a lighter max.
Where this method is most accurate
The reverse Epley formula tends to perform best in the 1–10 repetition range for compound barbell lifts such as the squat, bench press, and deadlift. Accuracy may decline for rep ranges above 12, where muscular endurance and metabolic factors play larger roles than maximal strength; the input accepts up to 30 repetitions so the curve can be inspected, but by then the estimate (half the 1RM at 30 reps) is arithmetic rather than a prediction. Validation work such as Reynolds and colleagues' study found prediction equations track measured performance most closely at low rep counts. The formula also assumes proper lifting technique and consistent rest between sets. Individual variation (influenced by muscle fibre type distribution, training history, and movement efficiency) can shift the predicted load by several percentage points.
What this tool does not do
This calculator provides an estimate based on a mathematical model; it does not measure your actual performance or account for fatigue, form breakdown, or day-to-day readiness. It does not prescribe training loads, program design, or periodisation schemes. The tool does not evaluate injury risk, technique quality, or readiness to lift a given weight. It does not replace assessment by a qualified strength coach or the process of testing rep maxes in the gym under controlled conditions.
Disclaimer
This tool is for educational and informational purposes only. It is not medical, training, or professional advice. The estimates produced by this calculator are mathematical projections and may not reflect individual performance. Consult a qualified coach or healthcare provider before beginning or modifying any strength training program. The authors and publishers disclaim liability for any injury or adverse outcome arising from use of this tool.
Questions
- What is the reverse Epley formula?
- The reverse Epley formula calculates the maximum load for a given number of repetitions from a known 1RM. It divides the 1RM by (1 + reps / 30), where 30 is a constant derived from empirical strength data collected in the 1980s. Note that it does not return the 1RM itself at one repetition: the division gives 96.8% of it, a consequence of inverting a formula that was built to predict maxima from multi-rep sets.
- Why does the formula use the constant 30?
- The constant 30 originates from the original Epley equation (1RM = weight × (1 + reps / 30)), which was calibrated against observed strength performance in trained lifters. This divisor reflects the average rate at which load capacity declines as reps increase.
- Is this formula accurate for high-rep sets?
- The reverse Epley formula is most reliable for 1–10 repetitions. Beyond 12 reps, factors such as local muscular endurance, lactate tolerance, and cardiovascular conditioning become more influential, and the predictions may diverge from actual performance.
- Can I use this calculator for all exercises?
- The formula was developed primarily for multi-joint barbell movements like squats, bench presses, and deadlifts. It may be less accurate for isolation exercises, machine lifts, or movements with different fatigue profiles.
- How do I find my 1RM to use this calculator?
- A 1RM can be tested directly under supervision, or estimated from a submaximal lift using a 1RM prediction formula. Both methods carry inherent error; direct testing is generally more accurate but requires proper warm-up and spotting.
Sources & Methodology
Applies the reverse Epley formula: X-rep max = 1RM / (1 + reps / 30). The Epley equation was introduced by Boyd Epley in 1985 for estimating 1RM from submaximal loads; the reverse form solves for the load at a given rep count from a known 1RM.
- › Epley B. Poundage Chart. Boyd Epley Workout. Lincoln, NE: University of Nebraska; 1985.
- › LeSuer DA, McCormick JH, Mayhew JL, Wasserstein RL, Arnold MD. The accuracy of prediction equations for estimating 1-RM performance in the bench press, squat, and deadlift. J Strength Cond Res. 1997;11(4):211–213.
- › Reynolds JM, Gorman TJ, Robergs RA. Prediction of one repetition maximum strength from multiple repetition maximum testing and anthropometry. J Strength Cond Res. 2006;20(3):584–592.
- › National Strength and Conditioning Association. NSCA's Essentials of Personal Training. 2nd ed. Human Kinetics; 2012.
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