How to calculate your one rep max
You can estimate a one rep max from any hard set without ever testing a single. Here is how the formulas work, where they disagree, and which number to actually train off.
The short version
Take a set you did close to failure. Multiply the weight by a factor based on how many reps you managed. The two formulas in common use are:
Epley: 1RM = weight × (1 + reps ÷ 30)
Brzycki: 1RM = weight × 36 ÷ (37 − reps)
For a set of 5 at 100 kg, Epley gives 116.7 kg and Brzycki gives 112.5 kg. Both are estimates, and the gap between them is the honest measure of how precise this can be.
Worked example
Say you bench 80 kg for 6 reps, the last one grinding.
Epley: 80 × (1 + 6 ÷ 30) = 80 × 1.2 = 96 kg
Brzycki: 80 × 36 ÷ 31 = 92.9 kg
So your one rep max is somewhere around 93–96 kg. Program off the lower number and you will rarely be wrong in a way that hurts.
Why the formulas disagree
Both are curve fits to population averages, not laws. Below about six reps they agree closely. Above ten, rep performance depends increasingly on conditioning, pain tolerance, and muscle fibre makeup rather than pure strength, so any single-rep prediction from a set of fifteen is close to guesswork.
Epley reads higher at high reps, Brzycki lower. Neither is more correct. Pick one and use it consistently, because the value is in comparing your own numbers over time, not in the absolute figure.
The percentage table you actually need
Most programs are written as percentages of a one rep max. Once you have an estimate:
- 95%: a heavy single or double
- 90%: sets of 2–3
- 85%: sets of 4–5
- 80%: sets of 6–8
- 70%: sets of 10–12
- 60%: warm-ups and technique work
These are approximations too. If 85% does not feel like a five rep set, trust the bar over the arithmetic and adjust your estimate down.
Should you ever test a true max?
If you compete, yes. You need to know what happens under a real single. Otherwise it is rarely worth the fatigue and the risk. An estimate from a set of three to five gives you everything you need to program, costs you nothing extra, and can be updated every time you train.
Do it automatically
Our One Rep Max app runs both formulas, shows the spread, and gives you the full percentage table with plate loading for each step. It is free on iPhone and Android.