One Rep Max
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It applies a formula, commonly Epley or Brzycki, that relates the weight lifted and the number of reps completed to a projected single-rep maximum. The fewer reps used in the estimate, the more accurate the projection tends to be.

Each formula was fitted to different sets of lifting data and assumes a slightly different fatigue curve as reps increase. Epley and Brzycki typically agree closely at low rep counts but diverge more as reps climb toward 10-12.

No, formulas lose accuracy above about 10-12 reps because the relationship between reps and strength becomes less linear and more influenced by muscular endurance. For a reliable estimate, test with a set of 3-8 reps close to failure.

The estimate is a useful training reference, but an actual 1RM attempt carries more injury risk and requires a spotter and a proper warm-up progression. Many lifters prefer to program off the estimated number rather than test it directly.

Once you know your 1RM, standard percentage tables let you pick loads for specific goals, such as 70-80% for hypertrophy or 85-95% for strength work. Recalculate periodically since your 1RM changes as you get stronger or detrain.