In more detail
Common estimates include Epley, weight × (1 + reps / 30), Brzycki, weight × 36 / (37 − reps), and Lombardi, weight × reps^0.10. They agree reasonably well for sets of about 2 to 10 reps and drift apart beyond that, because muscular endurance varies a lot between people and exercises. Training programmes often prescribe loads as a percentage of 1RM, so an estimate is enough to plan sessions.
Why it matters
Testing a true one-rep max is tiring and carries more injury risk; it needs a proper warm-up, good technique and spotters or safety bars. Estimating from a submaximal set avoids most of that. Since formulas can differ by several kilograms, use the same one each time to track progress, and treat the number as a guide rather than a target to chase.
Example
100 kg for 5 reps
Epley: 100 × (1 + 5 / 30) = 116.7 kg
Brzycki: 100 × 36 / (37 − 5) = 112.5 kg
Lombardi: 100 × 5^0.10 = 117.5 kg
80 % of the Epley estimate = 93.3 kg