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One Rep Max Calculator (Wathan Formula)

Enter the weight lifted and reps you completed.

How it is calculated

The estimate uses the Wathan equation: 1RM = 100 × W / (48.8 + 53.8 × e−0.075 × r). W is the weight lifted and r is the number of repetitions completed in the set. The result is in whatever unit you typed (kg or lb); nothing is converted. The tool also shows the set weight as a percentage of the estimated 1RM, W / 1RM × 100, and both numbers are rounded to one decimal. Weight must be above zero and reps at least 1. The reps may be fractional, but the display rounds them to a whole number. The equation is a regression fit, so it works best for sets of roughly 2 to 12 reps taken close to failure, and for higher counts the estimate is less reliable.

Worked example

You lift 80 kg for 8 reps. First the exponential term: e−0.075 × 8 = e−0.6 = 0.5488. Multiply by 53.8 to get 29.53, then add 48.8 for a denominator of 78.33. The numerator is 100 × 80 = 8000. So 1RM = 8000 / 78.33 = 102.1 (1RM), meaning 102.1 kg. Your 80 kg set was 80 / 102.1 = 78.3% of that estimated maximum, which is the load you would reuse in a program written in percentages.

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Common mistakes

  • Entering a set that was stopped well short of failure. The formula assumes the reps were close to your limit, so 8 easy reps with energy left will give a 1RM that is too low.
  • Expecting the 1-rep case to return exactly the weight. For 80 with 1 rep the tool shows 81.0, because the equation is a fitted curve and not an identity.
  • Mixing units. If you enter pounds the result is in pounds; do not compare it with a kilogram table or a plate loading chart in the other unit.
  • Trusting very high rep sets. A 30-rep set gives 146.9 from 80, but endurance and fatigue dominate there, so the true maximum may be quite different.
  • Treating the estimate as a safe test weight. It is a guide for programming percentages, and it is unwise to attempt a single at that load without a spotter and a proper warm-up.

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.

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Comments (1)

  • Editorial team

    Estimates like this assume the set was taken close to failure, and they get less dependable as reps climb, so a set of 3 to 8 usually suits better than 25. Enter one rep and the formula returns about 1.3% above the weight lifted, a sign it was fitted to multi-rep sets. Treat the number as a starting point for planning, not a max to attempt. Which lift and rep range do you use?

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