1RM calculator — accurate one-rep max estimation, all formulas

How the math works

Drop a load and the reps you completed with it into the calculator. Each rep-to-1RM formula extrapolates a maximum from a sub-maximal set, but they don’t agree on the same answer — they make different assumptions about how strength tapers from low reps to high reps. Showing all seven plus a single averaged result is a hedge against any one model’s bias.

The seven formulas:

• Epley — load × (1 + reps / 30). The original; over-estimates above ~10 reps.
• Brzycki — load × (36 / (37 − reps)). Tighter at low reps, drifts low above 10.
• Lander — (100 × load) / (101.3 − 2.67123 × reps). Built from college football data; well-behaved 3–10 reps.
• Lombardi — load × reps^0.10. The flattest projection; runs low at low reps, high at high reps.
• Mayhew — (100 × load) / (52.2 + 41.9 × e^(−0.055 × reps)). Bench-press-derived; conservative.
• O’Conner — load × (1 + 0.025 × reps). The simplest linear model; tends to over-estimate.
• Wathen — (100 × load) / (48.8 + 53.8 × e^(−0.075 × reps)). Smooth across the rep range; most consistent across populations.

The averaged value tells you “what most reasonable models predict given the input you gave me”. The breakdown panel below lets you sanity-check whether the formulas agree (tight estimate) or diverge (noisy estimate, treat with caution).

Worked example

Say you squat 100 kg for 5 reps, one rep shy of failure. Here’s what each formula returns:

Formula	Estimated 1RM
Brzycki	112.5 kg
O’Conner	112.5 kg
Lander	113.7 kg
Wathen	116.6 kg
Epley	116.7 kg
Lombardi	117.5 kg
Mayhew	119.0 kg
Average	115.5 kg

The spread is 112.5 to 119.0 kg, a ~6.5 kg band. That tightness is the signal: at 5 reps the models broadly agree, so 115 kg is a trustworthy working number. Push the same set to 12 reps and the band would blow out past 20 kg, at which point the average stops meaning much. Stay in the 3–8 rep window and the estimate holds up.

When to use it

Best results when the input set was performed at 3–8 reps, near (but not at) failure, on a compound lift you’ve trained recently. Outside that window the predictions get noisy; ignore the averaged number when the formulas disagree by more than ±10 kg.

What it doesn’t tell you

• Whether you can actually lift the predicted max today. Daily readiness, recovery state, and your actual maximum velocity at MVT all affect the real-world max. The estimate is the most likely 1RM under average conditions.
• Where on the rep curve you are individually. New lifters’ rep curves differ from elite lifters’. The formulas average across populations; your personal curve might consistently sit higher or lower.

For a more individualised estimate, build a load–velocity profile. The velocity-derived 1RM updates per session and is calibrated to your specific strength curve. The book walks through why velocity beats rep-based formulas for tracking max strength session to session.

FAQ

Which 1RM formula is the most accurate?

No single formula wins for everyone. Each one was fit to a different population and rep range, so the most accurate one depends on your lift, training age, and how many reps you used. This calculator shows all seven and averages them, because the mean cancels out individual model bias better than betting on one formula.

How many reps should I use for the best 1RM estimate?

Three to eight, taken one rep shy of failure. Below three reps a small load error swings the estimate hard. Above eight, perceived effort and conditioning cap the set before true strength does, and the formulas drift apart. Five reps is the sweet spot for most lifters.

Why do the 1RM formulas disagree with each other?

They make different assumptions about how strength tapers as reps climb. Some are linear (O’Conner, Epley), some exponential (Mayhew, Wathen), some power curves (Lombardi). Inside the rep range a formula was built for, it agrees with the others. Outside that range it diverges. A wide spread is your cue to retest at lower reps.