1RM calculator
Estimate your one-rep max from any set. Epley formula, the one Zefro uses.
How it works
Your one-rep max is the most you could lift once. Zefro estimates it with the Epley formula: weight × (1 + reps ÷ 30). It is most accurate between 2 and 10 reps.
How to read your result
The result is an estimate of the heaviest weight you could lift once. It is worked out from a lighter set you actually did, not measured. The formula is Epley: weight × (1 + reps ÷ 30). It works best on a set taken close to your limit.
Fewer reps give a better guess. In a study of 70 adults who tested their 1, 5, 10 and 20 rep maximums on the bench press and leg press, the 5-rep maximum predicted the one-rep maximum best, and the authors advise using no more than 10 reps in an equation like this. This page uses whatever reps you type, so keep it to 10 or fewer. Even then, people differ: across 269 studies, people lifting 80% of their max averaged about 9 bench press reps, and the typical gap between one person and the average was about 2.5 reps (a general figure for all lifts).
Use the number to pick working weights and to follow your progress. Compare like with like: same exercise, similar reps. If you stop a set well before your limit, the estimate comes out low. In one study of trained lifters doing knee extensions, those who stopped where they judged they were at their limit could typically have done about 2 more reps, though the uncertainty was wide (0 to 4). Zefro uses the same formula but does not trust sets of more than 12 reps.
A worked example
Take 100 kg for 5 reps. Step 1: reps ÷ 30 = 5 ÷ 30 = 0.1667. Step 2: add 1 = 1.1667. Step 3: weight × that value = 100 × 1.1667 = 116.67 kg. The calculator rounds to a whole number and shows 117 kg. In pounds it works the same way: 225 lb × (1 + 5 ÷ 30) = 225 × 1.1667 = 262.5 lb, shown as 263 lb.
Another set: 60 kg for 8 reps. 8 ÷ 30 = 0.2667, plus 1 = 1.2667, 60 × 1.2667 = about 76 kg. Even a single rep adds a little: 100 × (1 + 1 ÷ 30) = 103.33 kg, shown as 103 kg. If you really lifted 100 kg once, your max is at least 100 kg and you do not need the formula. Other equations give slightly different answers: the bench press equation that the 70-adult study fitted to 5-rep maximums (1.1307 × 5RM + 0.6999) gives 1.1307 × 100 + 0.6999 = 113.77 kg for 100 kg, about 3 kg under Epley's 116.67 kg for the same set.
How accurate is the Epley formula?
It is an estimate, so expect some error. In one study of 20 trained lifters doing bench press sets to failure at 70% of their max, Epley had the lowest standard error of the four methods compared. It is more accurate with few reps, and people get different numbers of reps at the same weight.
How many reps should I use?
Fewer is better. In a study of 70 adults, a 5-rep set predicted the max better than 10 or 20 reps, and the authors advise no more than 10. This page does not limit what you type. The Zefro app does not trust sets of more than 12 reps.
Does it work the same for every exercise?
Not exactly. In a review of 269 studies, people got more reps on the leg press than on the bench press at the same percent of their max: about 13 against 9 at 80%. A formula with fixed numbers cannot account for that. Treat estimates for different exercises with extra care and compare each exercise with its own past results.
Is a real one-rep max test more accurate?
A tested max is a direct measure, and repeat tests agree closely: across 32 studies the median agreement score (ICC) was 0.97 on a scale up to 1, yet results still moved about 4% from test to test (median coefficient of variation 4.2%). The estimate is a quick way to get near that number from a lighter set.