One-Rep Max (1RM) Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/5/2026
The 1RM (theoretical maximum weight for one rep) is estimated with the Epley formula: weight lifted × (1 + reps ÷ 30). For 100 kg lifted for 5 reps, the estimated 1RM is about 116.7 kg.
Explanation
The 1RM (one-rep max) is the maximum weight a person can lift for a single complete repetition of a given exercise. Measuring it directly requires a genuine maximal effort, which is risky without proper supervision — most lifters therefore prefer estimating it from a submaximal test (a weight lifted for several reps), using a formula like Epley's, developed by strength coach Boyd Epley in the 1980s and the most widely used since. This estimate is most reliable between 1 and 10 reps; beyond that, accuracy degrades since muscular endurance plays a bigger role than pure strength. The 1RM serves as a reference for programming training as a percentage of maximum load (for example, training at 70% of 1RM for a hypertrophy goal), without needing to retest the actual maximum every session.
Example: 100 kg lifted for 5 reps
Inputs
Weight lifted: 100 kg. Reps: 5.
Calculation
1RM = 100 × (1 + 5 ÷ 30) = 100 × 1.1667 ≈ 116.7.
Result
The estimated one-rep max is about 116.7 kg.
Frequently asked questions
Is this estimate accurate?
It's an estimate, not a direct measurement: for sets of 3 to 6 reps, the Epley formula typically lands within about 5% of the actual measured 1RM, but the individual gap varies with technique, exercise, and training experience. Only a direct test (under qualified supervision) gives an exact value.
Why does accuracy decrease beyond 10 reps?
Because beyond about ten reps, performance depends more and more on muscular endurance (fatigue resistance) and less and less on the pure maximal strength that 1RM is trying to estimate — the two physical qualities don't progress in strict proportion to each other. For a reliable estimate, aim for a test between 1 and 10 reps.
How do I use the 1RM to program training?
Once the 1RM is estimated, many strength training programs set working loads as a percentage of that value (for example 60-70% for hypertrophy, 80-90% for maximal strength). This lets you adjust training intensity without retesting the maximum every session, which would carry an unnecessary injury risk.