How to Estimate Your One Rep Max

Quick Answer

A one rep max is the heaviest load you could lift once with good form, and it is the number strength programs are written against. This 1RM calculator, or max lift calculator, estimates it from a submaximal set you actually completed, running the Epley, Brzycki, Lombardi, and O'Conner formulas at once and averaging them. The Epley formula multiplies the weight by 1 plus reps divided by 30, so 100 kg for 5 reps estimates about 117 kg; the brzycki formula divides by a falling linear term and reads slightly lower. The result also converts straight into the training percentage table, from 95 percent for doubles down to 65 percent for sets of fifteen, so progressive overload needs no arithmetic.

How the Estimate Works

There is a stable relationship between how heavy a load is relative to your maximum and how many times you can move it. Roughly 90 percent of a max allows four reps, 80 percent allows eight, 70 percent allows twelve. Every estimation formula is a curve fitted to that relationship, run backwards: given the weight and the reps, what maximum would produce that pairing. This is why the reliability of the answer depends entirely on the set being taken close to failure. A comfortable set of eight describes a different curve than a brutal one.

Epley Versus Brzycki, and Why Four Formulas

How the estimation formulas are derived is the same story four times: each is a curve fitted to the reps-versus-load relationship, then run backwards. Epley uses a linear rep term, Brzycki divides by a falling linear term, and Lombardi and O'Conner fit different curves again. At three to five reps they agree within a couple of percent, so the choice is irrelevant. At twelve or more they can diverge by ten percent or more, because a long set is limited by muscular endurance rather than by peak force, and endurance varies enormously between lifters. Showing the spread rather than one number is the honest presentation: when the formulas disagree, that disagreement is information about how far you extrapolated.

Turning a Max Into Working Weights

Programs are written in percentages because they scale to any lifter. What percentage of a max suits each training goal is well established: strength work generally sits at 85 percent and above for one to five reps, hypertrophy at 70 to 85 percent for six to twelve, and muscular endurance below 70 percent. The percentage table in the result does the conversion for you, so five sets of five at 80 percent becomes a bar weight rather than a calculation. Re-estimate every few weeks as the submaximal set gets easier and the whole table moves with it.

Examples

100 kg for 5 reps. For example, Epley gives 117 kg and Brzycki 113 kg, an average near 115, so a program calling for 80 percent means about 92 kg.

60 kg for 3 reps. For example, the four formulas land between 64 and 66 kg, a spread of under 4 percent, which is the tightest range you can get from an estimate.

80 kg for 12 reps. For example, Brzycki reads 115 kg and Epley 112 kg, but a 15 rep set from the same lifter might imply 120, which is the drift a high-rep estimate carries.

Common Mistakes