One-rep max calculator

How a 1RM calculator works

A one-rep max (1RM) calculator estimates the maximum weight you could lift for a single rep, based on a set you've already completed. The premise: there's a fairly stable relationship between the weight you can lift for, say, 5 reps and the weight you could lift once. Researchers have proposed roughly a dozen formulas over the decades. Most of them produce very similar results in the 1 to 10 rep range, and start to diverge sharply above 10.

That divergence is why most calculators (this one included) recommend using a set of 1 to 8 reps for the most accurate estimate. Sets of 12+ reps depend heavily on muscular endurance and on mental tolerance for discomfort, both of which vary more from person to person than pure strength does.

The six formulas, explained

Epley

1RM = w × (1 + r/30). The most common formula in gym software. Published in 1985, it's a simple linear approximation that works well in the 2–10 rep range and assumes each additional rep beyond 1 reduces the load by roughly 3.3%. Tends to slightly over-estimate at very high reps.

Brzycki

1RM = w × 36 / (37 − r). Published in 1993 by Matt Brzycki. The formula has a hard ceiling at 37 reps (the denominator goes to zero), so it's only valid below that. Brzycki is widely considered more accurate than Epley for lower-rep sets (1–5) and slightly more conservative for higher reps.

Lombardi

1RM = w × r0.10. A power-law formula — elegant mathematically, but tends to under-predict at very low reps (it returns the same weight you lifted when r = 1). Best used as a sanity check rather than a primary estimate.

Mayhew

1RM = w × 100 / (52.2 + 41.9 × e−0.055r). Derived from research on the NFL-225 bench press test (reps with 225 lb). Generally produces results between Epley and Brzycki. Particularly well-validated for the bench press in a population of trained athletes.

O'Conner

1RM = w × (1 + 0.025r). The most conservative of the common formulas — it assumes each rep beyond 1 only "costs" 2.5%. For most lifters, O'Conner under-estimates their true 1RM, which makes it useful when you want a deliberately cautious number (e.g., seeding a program for a beginner).

Wathan

1RM = w × 100 / (48.8 + 53.8 × e−0.075r). Another exponential-decay model, validated against bench, squat, and deadlift in college football players. Comparable accuracy to Mayhew across most rep ranges.

Should I just use the average?

Yes — and that's what we display by default. Averaging across all six formulas smooths out the edge cases (Lombardi's low-rep under-estimate, Epley's high-rep over-estimate) and lands you on a number that's almost always within 5 lb of your actual max. For programming purposes — picking weights for the next four weeks — the average is more than precise enough.

What if I actually tested my 1RM?

Use that number. A tested max is always more accurate than any formula. Calculators exist for the 51 weeks of the year when you aren't grinding through an attempt at a true single.

Using the training percentages

The percentage table below your estimated 1RM is a quick reference for picking working weights. Common uses:

• 95% — heavy single, top-set @ RPE 9–10
• 90% — heavy double or triple, typical "top set"
• 85% — solid working triple or set of five
• 80% — back-off sets of five
• 75% — volume work, sets of 6–8
• 70% — accumulation block, sets of 8–10, or warm-up

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