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One-Rep Max (O'Conner) Calculator

Enter weight and reps to estimate your one-rep max.

Result

96.00 kg
86.40 kg
76.80 kg

How it is calculated

The tool applies the O'Conner formula to a set you have already done: 1RM = weight × (1 + reps ÷ 40), where weight is the load lifted and reps is the number of clean repetitions completed. It then shows 90% and 80% of that estimate for planning training loads. Results are labelled in kilograms and rounded to two decimals, so enter the weight in kg (pounds will be mislabelled). The weight must be above zero and reps must be at least 1. Because the formula is linear, a single rep still adds 2.5%: 80 kg for 1 rep returns 82.00 kg, so the estimate is only meaningful for sets of roughly 2 or more reps. It is a rough estimate and differs between lifts and between people.

Worked example

You lift 80 kg for 8 repetitions. Divide the reps by 40: 8 ÷ 40 = 0.2. Add 1 to get the multiplier, 1.2. Multiply the weight by it: 80 × 1.2 = 96.00 kg as the estimated one-rep max. Training percentages follow directly: 90% of 96 is 86.40 kg and 80% is 76.80 kg. So a session prescribed at 80% of max would use about 76.8 kg on the bar.

Try this example →

Common mistakes

  • Entering the weight in pounds; the output is labelled kg and the percentages will be wrong for a kilogram plate set.
  • Entering a 1-rep max as the input and expecting the same number back; the formula returns 82.00 kg for 80 kg x 1, a 2.5% overestimate.
  • Counting sloppy or assisted reps; the estimate assumes every rep was a full, controlled repetition.
  • Using a set taken to a very high rep count (15 or more), where linear formulas drift further from the real maximum.
  • Forgetting that the 90% and 80% lines are percentages of the estimate, not extra sets of the same weight, and loading the bar without rounding to the plates available.

About this calculator

The O'Conner formula estimates a one-rep max (1RM) — the maximum weight a person can lift for a single repetition — from a lighter weight lifted for multiple reps, using 1RM = weight × (1 + 0.025 × reps). It's one of several linear estimation formulas, alongside Epley and Brzycki, each built from different regression data, and it tends to produce slightly more conservative estimates than Epley's formula at higher rep counts.

Strength coaches and lifters use 1RM estimates to set training percentages for programs based on a percentage of max, like 5/3/1 or linear periodization, without needing to actually attempt a maximal, injury-risking single lift. Because true 1RM testing is fatiguing and carries injury risk, especially for untrained lifters, formula-based estimates from a submaximal set are the standard, safer way to track strength progress over time.

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Comments (1)

  • Editorial team

    Estimates from heavy, low-rep sets are the most trustworthy, because the formula is linear and keeps adding 2.5 percent per rep. A set of 20 with 100 kg would be projected at 150 kg, which almost certainly overstates what you could lift once. Use sets of about 10 reps or fewer, enter kilograms, and treat the 90 and 80 percent lines as starting points for planning, not prescriptions. Would you like other formulas such as Epley or Brzycki added?

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