How the estimate is worked out
A one rep max calculator doesn't measure anything. It takes a set you've done — a weight and a rep count — and runs it through a formula fitted to how strength usually falls off as reps go up. Four are shown here.
Epley: weight × (1 + reps ÷ 30). The most widely used; tends to run slightly high at moderate reps.
Brzycki: weight × 36 ÷ (37 − reps). Tends to run slightly low, and gets unstable above 10 reps.
Lombardi: weight × reps0.10. O'Conner: weight × (1 + 0.025 × reps). Both listed so you can see the spread.
The headline number is the average of Epley and Brzycki, rounded to the nearest 2.5 kg or 5 lb so it's a weight you can actually load. The training-load table applies standard rep-max percentages to that estimate: 90% for around 4 reps, 80% for around 8, 70% for around 12. Those are population averages — your own rep ability at a given percentage will vary a little by lift and by how you've trained.
The estimate is only as good as the set it came from. A set of 3–5 reps taken close to failure is ideal. A set of 12 you could have pushed further will underestimate you, and a grindy set of 15 will overestimate.
FAQs
Is this one rep max calculator free?
Yes. No account, no email, no paywall. It runs in your browser and nothing you enter is stored or sent anywhere. trainerX makes the iPhone app that sets your weights for you; the calculator is free whether or not you use the app.
How accurate is a one rep max calculator?
Within about 5% when the estimate comes from a set of 1–5 reps, drifting to 10% or more from sets of 10 and above. The formulas were fitted to average lifters, so if you are unusually strong at high reps (or unusually weak) they will be off for you. Use the number to plan, then confirm it with a heavy single, double or triple.
Which 1RM formula is best?
None is best for everyone. Epley tends to run slightly high and Brzycki slightly low at moderate reps, so this calculator averages the two for its headline number and shows each formula underneath. Lombardi and O'Conner are listed for comparison. The spread between them is a useful reminder of how approximate any estimate is.
How many reps should I use to estimate my max?
Fewer is better: 2–5 reps, taken close to failure, gives the most reliable estimate. Above 10 reps the relationship between reps and load depends more on endurance than strength, so the formulas lose accuracy. Avoid using a set you stopped well short of failure — the calculator assumes the set was near maximal.
Do I need to test my actual one rep max?
Not unless you compete. For programming, an estimate from a heavy set of 3–5 is enough and much safer than a true max attempt. If you do want to test, do it after a lighter week, with a spotter or safety bars, working up in small jumps.
What percentage of my 1RM should I train at?
It depends on the goal: roughly 80–90% for sets of 3–5 to build strength, 65–80% for sets of 6–12 to build muscle, and 50–65% for higher-rep or technique work. The table above converts your estimate into those loads. A good program moves between ranges rather than living in one.