Your one-rep max is the heaviest weight you can lift once with good form. It matters because most training programmes prescribe loads as a percentage of it: sets at 70 per cent for volume, 85 per cent for strength, and so on.
Testing it directly is time-consuming, requires a spotter, and carries injury risk. Estimating it from a set you can already do is safer and accurate enough for programming.
Several formulas exist. Epley and Brzycki are the most widely used, and they agree closely at low repetitions and diverge as reps increase.
The reason is straightforward: predicting a single maximal effort from a set of five is reliable, and predicting it from a set of fifteen is not. Muscular endurance and maximal strength are different qualities, and the relationship between them varies between people.
Use a set of 3 to 6 repetitions for the most reliable estimate. Above ten reps, treat the result as a rough indication.
The set you enter should be genuinely close to failure, with perhaps one repetition left in reserve. A comfortable set of eight underestimates your maximum considerably.
Form must be consistent. A partial squat produces a higher number and a useless one.
Test when rested, not at the end of a session. Fatigue from earlier work shifts the result down.
Common training ranges as a percentage of one-rep max:
Strength: 85 per cent and above, 1 to 5 repetitions.
Hypertrophy: 65 to 80 per cent, 6 to 12 repetitions.
Endurance: below 65 per cent, 12 or more repetitions.
These ranges overlap considerably, and muscle growth occurs across a wider span than was once thought. Training close to failure matters more than hitting a precise percentage.
Your maximum changes as you train, so an estimate from three months ago will underprescribe your loads.
Re-estimating every four to six weeks keeps the percentages meaningful. A simpler approach for many people is to track the weight and reps themselves and progress when a set feels easier, which requires no calculation at all.
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Educational only, not medical advice.