Estimates the most you could lift once, from a weight you managed for several repetitions. Epley gives weight × (1 + reps ÷ 30) and Brzycki gives weight × 36 ÷ (37 − reps). Neither asks you to attempt a true single.
The most you can lift for a single repetition is your one-rep max. Attempting it outright is risky, so it is estimated from a weight you managed for several repetitions.
is the weight lifted and the repetitions. The two formulas are built differently, so their answers differ a little.
Suppose 80 kg goes up for 8 repetitions. Epley gives kg and Brzycki gives kg, so about 100 kg is a fair working figure. Turning it round, 86.9 kg is the weight to expect for 5 repetitions.
Both formulas are built for sets of up to roughly ten. At twenty or thirty repetitions, endurance takes over from strength and the question changes. The more repetitions involved, the looser the estimate.
Brzycki's denominator hits zero at 37 repetitions and flips sign beyond it, so the limit here is 36.
This is an estimate, not a promise. Use a spotter when attempting a heavy single.