Mayhew One-Rep Max Calculator from Weight and Repetitions

Enter the lifted weight and the number of completed repetitions, and the calculator applies the Mayhew equation to return an estimated one-repetition maximum in the same unit. It works with kilograms or pounds because the equation preserves the unit you provide. The calculation is fully offline and deterministic, making it suitable for quick training checks, repeatable software workflows, and comparisons based on consistent input data.

Enter a representative set

Choose a completed working set that reflects your normal technique and a genuine effort, then enter its lifted weight and repetition count. The weight must be a positive number, while repetitions must be a positive whole number. You may use kilograms or pounds; the result keeps exactly the same unit because the Mayhew equation scales the input rather than converting it. For example, entering a weight measured in kilograms produces an estimate in kilograms. Use the total external load consistently, including the bar when that is how you record training. A set stopped far from fatigue may understate your potential because the formula assumes the repetition performance carries useful information about maximum strength. Conversely, repetitions completed with shortened range of motion, assistance, or substantially different technique may make the estimate less comparable to a strict single. Record inputs honestly and use the same conventions across sessions so changes in the estimate reflect training rather than changes in measurement.

Understand the Mayhew calculation

The calculator uses the Mayhew equation: one-repetition maximum equals 100 times the lifted weight divided by 52.2 plus 41.9 times e raised to negative 0.055 times the repetitions. This exponential relationship is why the estimated multiplier changes with repetition count rather than increasing by one fixed percentage for every additional repetition. The returned estimate is rounded to two decimal places for a stable, readable result, while the calculation itself uses the supplied numeric weight. The process contains no random values, current dates, network requests, account history, or hidden profile adjustments. Identical valid inputs therefore produce identical outputs. The formula does not need to know whether the weight represents kilograms or pounds, but it also cannot label or convert that unit for you. If you enter 100 pounds, interpret the estimate as pounds; if you enter 100 kilograms, interpret it as kilograms. Results from other one-rep-max equations may differ because their fitted relationships and assumptions are not identical.

Use the estimate responsibly

Treat the result as a planning estimate, not a guarantee that a particular maximal attempt will succeed. Readiness changes with sleep, fatigue, exercise order, equipment, technique, rest time, and the specific movement. A useful workflow is to calculate from comparable sets over time, watch the trend, and select conservative training loads from that trend rather than immediately loading the exact estimate. If you test a true maximum, use appropriate safety equipment, competent spotting, and progression suited to the exercise and your experience. The calculator rejects missing, nonnumeric, nonfinite, zero, and negative weights, as well as repetition counts that are not positive integers. That strict validation prevents an apparently precise result from being produced from unusable data. It does not assess pain, injury, medical conditions, or exercise suitability. Stop if a movement causes pain and seek qualified professional guidance when health or rehabilitation decisions are involved. In automated logs, store the source set beside the estimate so future comparisons remain auditable and meaningful.