1RM
calculator

Estimate your one-rep max from a set you can already do. Five formulas side-by-side — Epley, Brzycki, Lombardi, O'Conner, Mayhew. Imperial or metric, with the rep-range honesty most calculators skip.

Your one-rep max is the most weight you can lift for a single rep. This calculator estimates it from a heavier-than-warmup set you can already complete, so you never have to grind a true max to know roughly where it sits. Enter the load and the reps, and five published formulas give their estimates at once. The headline number is their average; the five cells show how much they disagree.

How the math works

Each formula takes the weight you lifted and the number of reps, and projects the load that would leave you at one rep. They were fit to different data sets, so they weight reps slightly differently. All five multiply the lifted weight by a rep-based factor, which is why the estimate scales the same whether you work in pounds or kilos.

At a single rep, Brzycki and Lombardi return the lifted weight exactly; the others sit a few percent above it. That gap is the point — a 1-rep set is the max, so any spread there is the formula showing its assumptions rather than measuring anything.

Worked example

A lifter benches 225 lb (102 kg) for 5 reps. The five formulas estimate the one-rep max as:

• Epley: 225 × (1 + 5/30) = 263 lb (119 kg)
• Brzycki: 225 × 36 / (37 − 5) = 253 lb (115 kg)
• Lombardi: 225 × 5^0.10 = 264 lb (120 kg)
• O'Conner: 225 × (1 + 0.025 × 5) = 253 lb (115 kg)
• Mayhew: (100 × 225) / (52.2 + 41.9 × e^(−0.055×5)) = 268 lb (121 kg)

Average of the five: 260 lb (118 kg). The spread from lowest to highest is 15 lb (7 kg) — about 6% at 5 reps, which is close agreement. Program off the average and you're inside that band; program off the single highest formula and you've quietly added a few percent to every working set.

When this calculator is wrong

1RM estimates are reliable up to about 6 reps, marginal from 7 to 10, and close to meaningless past 10. In the 3–6 rep range the five formulas agree with each other and with measured 1RMs; from 7 to 10 reps the spread between them widens to 5–10% and they drift from the real number by a similar amount. Past 10 reps you're measuring muscular endurance, not maximal strength, and no formula fixes that. The counter-case: for sub-maximal work in the 50–70% range — accessory lifts, RPE-based training, back-off sets — even a noisy estimate is fine, because the load isn't near the ceiling. The error only bites when the program calls for 85%-plus.

Two more limits worth naming:

• Mayhew is a bench-press formula. It was derived on bench-press performance, so it's the odd one out for squats and deadlifts. Generic calculators apply all five formulas to every lift without saying so. On lower-body lifts, weight Mayhew's estimate lightly.
• The estimate can't see your day. Central nervous system readiness, sleep, and how the warm-ups felt move a true max by more than the difference between formulas. Treat the number as a planning figure, not a guarantee for a given session.

What to do with the result

Use the average as your training max and build percentages from it. The strip in the calculator shows the common bands: 70–80% for volume, 85%+ for strength work near the top. Round to what your plates allow rather than chasing an exact number the formula never really promised.

Re-estimate every few weeks from a fresh rep-set rather than testing a true max often — a max attempt costs recovery that a submaximal set doesn't. When you do want the real number for a meet or a benchmark, work up to it over a session with a spotter, using the estimate to pick your openers.

Common questions

How accurate is a 1RM calculator?

Close, inside its range. From 3 to 6 reps the formulas track measured one-rep maxes well and agree with each other within a few percent. From 7 to 10 reps that agreement loosens to 5–10%. Past 10 reps the estimate reflects endurance more than max strength, and the number stops meaning much.

Which 1RM formula is most accurate?

No single one wins across every lift and rep range, which is why we show five and average them. Epley and Brzycki are the most widely used and sit close together at low reps. Mayhew was built specifically for the bench press, so it's the most lift-specific of the set. Averaging keeps any one formula's bias from dominating.

How many reps should I use to estimate my 1RM?

Three to six. That's where all five formulas were most accurate against measured maxes. A hard set of 5 is a good default — heavy enough to be near-maximal, light enough that you're not risking a grind. Avoid sets past 10; the estimate degrades quickly.

What's the difference between Epley and Brzycki?

They weight reps differently. Epley adds a fixed fraction per rep (reps/30), so it keeps climbing at higher reps. Brzycki divides by (37 − reps), which rises faster as reps approach the high teens and is undefined at 37. At 1–6 reps they land within a couple of percent of each other; the gap widens as reps go up.

Can I use this for squat and deadlift, not just bench?

Yes, with one caveat. Epley, Brzycki, Lombardi, and O'Conner are general strength formulas and apply across lifts. Mayhew was validated on the bench press, so for squats and deadlifts lean on the other four and treat Mayhew as the outlier. The average still gives you a usable training max.

Should I just test my true 1RM instead?

Sometimes. For a meet, a benchmark, or setting openers, a real max under a spotter is the accurate answer. For everyday programming it costs recovery a submaximal set doesn't, so most lifters estimate from a rep-set most of the time and test rarely.