Repetition Max Predictor Calculator

Example data table

Formula used

This calculator predicts a one‑rep maximum (1RM) from a working set. Common equations include:

• Epley: 1RM = w × (1 + reps / 30)
• Brzycki: 1RM = w × 36 / (37 − reps)
• Lombardi: 1RM = w × reps^0.10
• Mayhew: 1RM = (100 × w) / (52.2 + 41.9 × e^(−0.055 × reps))
• O'Conner: 1RM = w × (1 + reps / 40)
• Wathan: 1RM = (100 × w) / (48.8 + 53.8 × e^(−0.075 × reps))

If you enter RPE, adjusted reps = reps + (10 − RPE). This approximates extra reps you could do before failure.

Why 1RM prediction matters for training loads

Estimated one‑rep max helps translate a submax set into a working benchmark. Coaches commonly prescribe percentages for strength blocks, for example 70–85% for volume and 85–95% for intensification. A calculated estimate lets you adjust loads without frequent true max attempts, reducing risk and fatigue. For example, if your average prediction is 150 kg, 80% targets about 120 kg for sets of five. Tracking the estimate weekly highlights progress even when rep ranges change. Exporting results to CSV supports simple spreadsheets, while the PDF snapshot is handy for coaching reviews. Always pair numbers with technique notes: bar speed, depth, and grip width. Consistent conditions produce cleaner trends, making this calculator more valuable than any single test day. If pain appears or form breaks down, reduce load and consult a qualified professional before chasing higher estimates during heavy cycles.

How repetitions influence accuracy across formulas

Prediction error typically increases as reps rise, because endurance and technique drift change set performance. Many lifters see tighter estimates between 1–10 reps. When you exceed that range, equations can diverge by several percent. Comparing multiple predictors and the average can stabilize decisions for higher‑rep sets.

Using effort to adjust for reps in reserve

Effort scales such as RPE provide context beyond reps alone. If you completed 5 reps at RPE 9, you likely had about one rep in reserve. This tool approximates that by adding (10 − RPE) to reps, then reruns the formulas. It is useful when sets are not taken to failure.

Rounding and plate math for practical loading

Small decimal outputs are less useful than gym‑ready numbers. Rounding to 0.5, 1, 2.5, or 5 units aligns predictions with common plates and dumbbell jumps. For barbell work, a 2.5-unit step often matches realistic progression. Keep rounding consistent so trend comparisons remain meaningful.

Interpreting results for programming and safety

Use the selected estimate to plan top sets and back‑off work, but treat it as a guide, not a guarantee. A day‑to‑day swing of 2–5% is normal due to sleep, stress, and warm‑up quality. If formulas disagree widely, repeat the set later or use the average to stay conservative.

FAQs

Which formula should I choose?

If you are unsure, select Average to balance different predictors. For low reps, Epley and Brzycki are commonly used. Track which option matches your real-world performance over several sessions.

Does this work for all lifts?

It works best for compound lifts with consistent technique, such as squat, bench press, and deadlift. Variations with limited range, momentum, or unstable equipment can distort predictions.

Why does RPE change the result?

RPE estimates how close you were to failure. The calculator converts it to a rough reps-in-reserve adjustment, then recalculates. This helps when you stop early but still want a max estimate.

What rep range is most reliable?

Predictions are typically strongest from 1–10 reps. As reps rise, muscular endurance and pacing affect outcomes, and formulas may spread apart. Use the average and be conservative at higher reps.

How often should I update my estimate?

Updating weekly or every training block is usually enough. Use the same lift, similar warm-up, and similar standards each time. Consistency makes the trend more useful than a single high score.

Can I use this for percentage-based programs?

Yes. Take the selected 1RM estimate and multiply by your target percentage to set training loads. Re-check if bar speed slows unusually or if fatigue accumulates across sessions.