One Rep Max Calculator (Wathan Formula)

About this calculator

A one-rep max (1RM) is the heaviest weight a lifter can lift for a single repetition, and rather than testing it directly — which carries injury risk — it's commonly estimated from a lighter set taken to or near failure. The Wathan formula is one such estimation equation, derived from regression on lifting data, expressed as 1RM = weight × 100 / (48.8 + 53.8 × e^(-0.075 × reps)), which tends to produce slightly different estimates than the more commonly cited Epley or Brzycki formulas, especially at higher rep counts.

1RM estimation is core to strength training program design: coaches and lifters use a percentage of estimated 1RM (say, 75% for an 8-rep hypertrophy set) to set training loads without requiring a maximal, fatiguing test lift every time programming changes. Because different formulas diverge more as reps increase, having access to a specific named formula like Wathan's — rather than just “an” estimate — matters to lifters and coaches who want consistency with the specific method their program or research reference uses.