About the One-Rep Max Calculator
This one-rep max calculator estimates the heaviest weight you could lift for a single repetition, based on a weight and rep count from a recent set, using the two most widely cited prediction formulas — Epley and Brzycki — alongside a percentage table for programming training weights. It saves attempting a genuinely maximal single lift, which carries real injury risk without a spotter and proper technique, by estimating it from a safer, sub-maximal set instead.
How to use the One-Rep Max Calculator
- 1
Enter the weight you lifted for a working set.
- 2
Enter how many reps you completed with that weight, taken close to failure for an accurate estimate.
- 3
Read the estimated one-rep max from both formulas and their average.
- 4
Check the training percentage table to convert that max into practical weights for different rep ranges.
What people use it for
Programming a strength training block
Convert an estimated one-rep max into specific working weights for a percentage-based programme, such as 5x5 at 80% or a deload week at 60%.
Tracking strength progress over time
Recalculate an estimated max periodically from a comparable working set to see genuine strength gains without needing to test a real maximal lift every time.
Setting a safe starting weight for a new lift
Estimate a rough one-rep max for a new exercise from a light, comfortable set before working up, rather than guessing at an unfamiliar movement.
The Epley and Brzycki formulas
Epley's formula estimates one-rep max as the lifted weight multiplied by one plus reps divided by 30. Brzycki's formula instead multiplies the lifted weight by 36 divided by 37 minus reps. Both are regression equations fitted to real lifting data from different studies, and while they broadly agree at low rep counts, they diverge more as reps increase — Brzycki tends to predict a slightly lower max at higher rep counts than Epley does, which is why averaging the two, as this tool does, gives a reasonable middle estimate rather than favouring either formula's particular bias.
Why the formulas break down above about 12 reps
Both formulas assume the set was taken close to muscular failure and that strength, not endurance, is the limiting factor — an assumption that holds reasonably well for sets of roughly 2 to 10 reps. Beyond about 12 reps, muscular endurance and metabolic fatigue start to matter as much as raw strength, and the formulas' predictions become progressively less reliable, often overestimating what could genuinely be lifted for a single rep.
Tips
- Use a set taken close to genuine failure for the most accurate estimate — a set stopped well short of failure understates your real capacity.
- Estimates from a 3 to 6 rep set tend to be the most reliable; very low or very high rep counts push the formulas towards their limits.
- Retest periodically rather than relying on one estimate indefinitely — strength changes, and an old estimate can be meaningfully out of date after a few months of training.