One Rep Max Calculator
Your estimated 1RM from any hard set, plus the training percentages built on it.
The formulas
Epley: 1RM = w × (1 + reps/30)
Brzycki: 1RM = w × 36 / (37 - reps)
100 kg for 5 reps estimates 116.7 kg (Epley) or 112.5 kg (Brzycki); this page shows both and averages them. The two agree closely up to about 6 reps and drift apart beyond 10, which is why sets of 1 to 6 give the most trustworthy estimate.
Using it in training
Most programs prescribe percentages of 1RM, so an estimated max saves you from actually grinding singles: 90% for heavy triples, 80% for fives, 70% for volume work. Estimates run slightly optimistic on exercises with a big technique component (deadlift, snatch) and for lifters with mostly slow-twitch fiber, so treat the first session at a new percentage as a test, not a contract.