One Rep Max: Why Gym Calculators Give Different Numbers

1RM (one rep max) calculators vary dramatically because different formulas estimate differently. A lift that calculates to 300 lbs on one calculator might show 285 lbs on another.

The reason: Different mathematical models based on different data sets.

Common 1RM Formulas

Brzycki Formula:

1RM=Weight1.0278−0.0278×Reps1RM=1.0278−0.0278×RepsWeight

Example: 200 lbs × 5 reps = 1RM of 226 lbs

Epley Formula:

1RM=Weight×(1+0.0333×Reps)1RM=Weight×(1+0.0333×Reps)

Example: 200 lbs × 5 reps = 1RM of 233 lbs

Lombardi Formula:

1RM=Weight×Reps0.101RM=Weight×Reps0.10

Example: 200 lbs × 5 reps = 1RM of 219 lbs

Same lift, 14-lb difference between formulas.

Why the Variance Exists

Different formulas fit different data populations:

Brzycki: Based on younger lifters

Epley: Based on older/weaker populations

Lombardi: Based on powerlifters

Your actual 1RM could be any of these—until you test it.

Best Practice

Don't rely on calculated 1RM:

Use estimates only for form/safety adjustments

Test actual 1RM every 4-6 weeks

Average the formula results for better estimate

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