Your 1RM,
six formulas wide.

Field guide

How to safely estimate your 1RM.

Maxing out, actually loading the bar to a true single, is taxing on the central nervous system, hard to recover from, and (without a spotter) genuinely risky. Estimating your 1RM from a submaximal set is the standard alternative. Done well, the estimate is within a few percent of the real number; done badly, you'll be programming off a value that's 15% too high and burning out within weeks.

The submaximal set protocol

A clean estimate requires a clean test. Five rules:

• Warm up properly. Two to three ramping sets that build to your test load, same way you would warm up for a heavy single in a real session.
• Pick a load you can hit for 3–8 reps. Below 3 is too close to a true 1RM (no safety buffer); above 8 leans on muscular endurance more than max strength and the formulas drift.
• Take the set to honest failure, with good form on every rep. Stop the rep before form breaks ; partial reps don't count, and a grindy form-breaking rep distorts the formula.
• Test only when fresh. Don't estimate 1RM at the end of a heavy training session. Pick a dedicated day, ideally first lift of the workout.
• Use a spotter on bench press. Even submaximal sets can fail unpredictably; spotters are non-negotiable on horizontal pressing.

Worked example

You bench-pressed 225 lb for 5 clean reps to failure. The six formulas estimate (rounded to 0.1 lb):

• Epley: 225 × (1 + 5/30) = 262.5 lb
• Brzycki: 225 × 36/(37 − 5) = 253.1 lb
• Lombardi: 225 × 5^0.10 = 262.0 lb
• Mayhew: 100 × 225 / (52.2 + 41.9 × e^(−0.275)) ≈ 261.8 lb
• O'Conner: 225 × (1 + 5/40) = 253.1 lb
• Wathan: 100 × 225 / (48.8 + 53.8 × e^(−0.375)) ≈ 258.7 lb

Average = 258.5 lb. The conservative O'Conner and Brzycki agree at 253; Epley and Lombardi cluster at 262. The 9-pound spread is typical; programming off the consensus average is the safest call.

Which 1RM formula is most accurate?

The honest answer: none of them is universally best. Every formula was fitted to a specific cohort (untrained college students, NFL combine athletes, powerlifters), and your results depend on which cohort looks most like you. That said, the published validation literature has converged on a few rules of thumb:

Brzycki: best at low reps (1–10)

The Brzycki formula has the tightest fit to direct testing in the 1–10 rep range, where most strength athletes train. It uses a hyperbolic curve that's steepest at low reps, which matches the way real strength curves drop off. Its limitation is the algebraic blowup at R = 37 (the denominator goes to zero) and serious distortion past 12 reps.

Epley: most popular, high-rep tolerant

The Epley formula is a simple linear model: 1RM = W × (1 + R/30). It's the formula every commercial app uses, but it tends to read slightly high at low reps (over-estimating your max for someone with high relative endurance). It stays well-behaved across the full 1–30 range, which is why most calculators default to it.

Lombardi: power-curve fit

The Lombardi formula uses a power curve 1RM = W × R^0.10 that closely tracks empirical data across the 1–30 rep range. It's slightly less popular than Epley but slightly more accurate; the trade-off is that the exponent operation feels less intuitive than a linear correction.

Mayhew, O'Conner, Wathan: the consensus group

The other three formulas are refinements fitted against specific test populations. Mayhew was built around bench-press performance in college football athletes. O'Conner is the most conservative of the six and tends to read 5–10% lower than Epley. Wathan uses a smooth exponential fit that matches direct 1RM testing well across reps.

The pragmatic recommendation

Use the average across all six formulas (the calculator's default). The averaging smooths each formula's specific cohort bias. If you're a low-rep strength athlete training in the 3–6 range, you can lean on Brzycki specifically. If you're testing from a higher-rep set (8+), Lombardi or Wathan track better.

What the percentage chart actually means

The percentage chart is a Prilepin-influenced reference, not a prescription. The "reps at X%" column shows upper bounds for fresh, well-rested sets, not the reps your program prescribes. A typical 80% programmed set is 5–6 reps, not the 8 the chart implies; the gap is the RPE buffer that lets you keep training day-to-day without burning out.

How to use the percentage chart in programming

• Heavy day: 85–95% × 1–3 reps for 3–5 sets. Top singles, doubles, triples.
• Strength day: 75–85% × 3–6 reps for 3–5 sets. Classic 5×5, 5/3/1, double-progression schemes.
• Hypertrophy day: 65–75% × 8–12 reps for 3–5 sets. Bulk muscle work, accessories.
• Volume / pump day: 50–65% × 12–20 reps for 3–5 sets. Endurance, technique, deload.

When to recompute

Re-test (or re-estimate) every 4–8 weeks during a strength block, or any time you've added 5+ lb to a working set across multiple sessions. Stale 1RM percentages mean you're under-loading, which slows progress; chasing percentages from a six-month-old number is a common cause of plateaus.

Disclaimer

These calculations are estimates, not personal-best attempts. A real 1RM test requires a coach or training partner, calibrated equipment, and a planned recovery window. The calculator is a planning tool; it gets you in the right ballpark for setting working weights without the CNS cost and injury risk of a true max attempt.