What a one-rep max tells you
Your one-rep max, or 1RM, is the heaviest weight you could lift for a single clean repetition of an exercise. Testing it directly is risky and tiring, so most lifters estimate it instead from a set they have already completed. This calculator does that using two trusted formulas, Epley and Brzycki, and then works out common training percentages so you can plan your sessions around real numbers.
Two examples
Lift 100 for 5 reps and the Epley formula estimates a 1RM of about 117, while Brzycki puts it near 113. The two rarely match exactly, which is normal, so treat the pair as a sensible range rather than a single figure.
Now lift 60 for 10 reps. The estimate climbs to about 80, because more reps at a given weight imply a higher true max. This is why rep-based estimates get less accurate as the reps climb: past about 10 to 12 reps, endurance starts to matter as much as raw strength.
How to use the percentages
- Most strength programs are written as a percentage of 1RM, so the 90% and 80% figures give you working weights straight away.
- Test with a weight you can move for 2 to 6 reps. That range gives the most reliable estimate.
- Warm up thoroughly and use a spotter for heavy sets. An estimate is safer than a true max attempt, but form still matters.
Frequently asked questions
Which formula is more accurate?
Both are widely used and usually land within a few percent of each other. Epley tends to read slightly higher at higher reps. Use the range they give rather than one exact number.
How many reps should I test with?
Between 2 and 6 reps gives the most reliable estimate. Accuracy drops as the reps rise, because endurance starts to influence the result.
Is it in kilograms or pounds?
Either. The result comes out in whatever unit you enter, since the maths is just a ratio.
Is estimating safer than testing?
Yes. A true 1RM attempt carries real injury risk. Estimating from a moderate set avoids that while still guiding your training loads.