How to Calculate One Rep Max
Three formulas, one idea: a hard submaximal set contains enough information to predict your max.
The three standard formulas: Epley (weight x (1 + reps/30)), Brzycki (weight x 36/(37 - reps)), Lombardi (weight x reps^0.1). They agree within a few percent for sets under 10 reps near failure, and all degrade past 12 reps.
Epley: the default
1RM = weight x (1 + reps / 30). For 200 x 8: 200 x (1 + 8/30) = 200 x 1.2667 = 253.3. Epley is linear in reps, easy to do mentally (each rep adds about 3.33 percent), and the most cited formula in programming literature.
Brzycki: the conservative
1RM = weight x 36 / (37 - reps). For 200 x 8: 200 x 36/29 = 248.3, a few pounds under Epley. Coaches who program heavy singles often prefer Brzycki because underestimating slightly is safer than overestimating when the bar is heaviest.
Lombardi: the curve
1RM = weight x reps^0.1. For 200 x 8: 200 x 8^0.1 = 200 x 1.231 = 246.3. Lombardi models diminishing returns per rep with an exponent, which fits high-rep data slightly better but is harder to compute by hand.
When the math breaks
All three assume the set ended near failure with consistent form. A set with reps in reserve underpredicts. Sets past 12 reps overpredict because endurance, not strength, is the limiter. And no formula accounts for technique differences between your 5-rep groove and your 1-rep groove.
Skip the arithmetic
Let the calculator run all three formulas on your numbers.
1RM formula questions
Which 1RM formula is most accurate?
Epley and Brzycki are the best validated, agreeing within a few percent for sets of 1 to 10 reps. Brzycki's slightly conservative numbers suit heavy programming.
Can I calculate 1RM from 20 reps?
Not reliably. Past about 12 reps, muscular endurance rather than maximal strength limits the set, and all standard formulas overpredict the true max.
Do I need to go to failure?
Near failure gives the best estimate: stop 0 to 2 reps shy of technical failure. A comfortable set with reps left over will underpredict your max.
Why do formulas disagree?
Each formula fits the rep-max relationship with different math. They agree within a few percent under 10 reps and diverge as reps climb, which is another reason to estimate from low-rep sets.