How the Epley Formula Estimates Your One-Rep Max

Testing a true one-rep max requires near-maximal effort and real injury risk without proper supervision, so most lifters estimate it instead from a lighter, higher-rep set using a validated formula.

The Epley formula

1RM = weight × (1 + reps/30)

Worked example

Lift 80kg for 5 reps: 1RM = 80 × (1 + 5/30) = 80 × 1.1667 ≈ 93.3kg estimated one-rep max.

Why the formula includes reps/30 specifically

The relationship between reps performed and percentage of 1RM used isn't perfectly linear in reality, but the Epley formula's specific constant (dividing reps by 30) was fit to match observed strength-endurance data reasonably well across a useful range of rep counts — it's an empirical approximation, not a law of physics, similar to the U.S. Navy body fat formula discussed elsewhere on this site.

Why the estimate gets less reliable at high reps

The formula assumes a fairly consistent relationship between reps and relative intensity, which holds reasonably well under about 10 reps. Push it further — say, 20 reps at a given weight — and the formula tends to overestimate true 1RM, because very high-rep sets are increasingly limited by muscular endurance and fatigue resistance rather than pure maximal strength, two related but distinct qualities that the simple formula doesn't separately account for.

Comparing two lifters at different rep ranges

Lifter A does 100kg for 3 reps: 1RM ≈ 100 × (1+3/30) = 110kg. Lifter B does 80kg for 10 reps: 1RM ≈ 80 × (1+10/30) = 80 × 1.333 ≈ 106.7kg. Despite lifting less absolute weight, Lifter B's estimate comes reasonably close to Lifter A's — illustrating how the formula weighs both the weight lifted and the number of reps performed, rather than just crowning whoever moved the heaviest single load.

From 1RM to training percentages

Once you have an estimated 1RM, most strength programs prescribe working weights as a percentage of it — for example, "5 sets of 5 at 80% of 1RM." With a 110kg estimated max, that's 110 × 0.80 = 88kg for that day's working sets — which is exactly why accurately estimating 1RM matters even if you never actually attempt a true single-rep max in training.

Common mistakes to avoid

• Using the formula on sets taken to complete failure at high reps (15+), where its accuracy degrades meaningfully
• Treating the estimated number as if it were a tested, exact maximum rather than a reasonable approximation with some real error margin
• Forgetting that fatigue, sleep, and nutrition on the day of the submaximal set all affect the estimate's accuracy, the same way they'd affect an actual max attempt

Estimate your own 1RM with the one-rep max calculator, or compare strength across body weights with the Wilks score calculator.