1RM Formulas Explained: Six Ways to Estimate Your One Rep Max

What is a 1RM?

Your One Rep Max (1RM) is the heaviest weight you can lift for a single clean repetition with proper form. It is the single most important number in percentage-based strength training — every program that prescribes "work up to 80%" or "do 5 sets at 70%" is scaled from your 1RM.

1RM is specific to the lift: your squat 1RM tells you nothing about your bench, and vice versa. It's also specific to the day: your true max varies by 2–5% between good and bad training days. That's why estimation — based on a heavy set of 3 or 5 — is often a more stable reference than an actual max-out attempt.

Why Estimate Instead of Test?

Testing a true 1RM is useful, but expensive:

• Requires a full warm-up protocol (typically 20–30 minutes)
• Best with a spotter, especially for squat and bench
• Drains your CNS for 3–5 days — no serious training during that window
• Most valid when you're fresh, fed, and mentally peaked (rare on a Tuesday evening)

Estimation lets you infer your 1RM from a submaximal heavy set — say, 5 reps at the heaviest weight you can cleanly handle — and plug it into a formula. Accuracy is within ±2–5% of your true max, which is well inside the variability of max-out attempts themselves.

The Six Formulas

These are the six most-used 1RM estimation formulas. Each was derived from a different dataset and gives slightly different numbers. Our 1RM calculator averages all six to smooth out individual formula bias.

Epley (1985)

1RM = w × (1 + r/30) — simple, widely used, tends to overshoot slightly at higher reps. Good default for 1–10 rep estimates.

Brzycki (1993)

1RM = w × 36 / (37 − r) — the most commonly cited formula in the strength research. Slightly lower estimates than Epley. Best accuracy in the 1–10 rep range.

Lombardi (1989)

1RM = w × r^0.1 — very close to Brzycki in the 1–5 range, diverges a bit at higher reps. Used in some older NSCA textbooks.

Mayhew (1992)

1RM = 100 × w / (52.2 + 41.9 × e^−0.055r) — developed specifically for bench press from trained college football players. Tends to predict slightly higher 1RMs than most.

O'Conner (1989)

1RM = w × (1 + r/40) — essentially Epley with a gentler curve. Gives the lowest estimates of the six. Conservative default.

Wathan (1994)

1RM = 100 × w / (48.8 + 53.8 × e^−0.075r) — very similar in shape to Mayhew, calibrated on a mixed-lift dataset. Commonly appears alongside Brzycki in academic papers.

In all six: w = weight lifted, r = reps performed.

Accuracy & Rep Range

All six formulas are most accurate in the 1–5 rep range. Above 5 reps, individual variation in muscle-fiber composition starts to matter more than the formula, and estimates spread wider.

Why 3–5 reps is the sweet spot: you're working at roughly 85–90% of 1RM, which is heavy enough to engage the same neural and muscular patterns as a true max but light enough to complete safely without a spotter. The formulas were calibrated primarily on this range.

Using Your 1RM in Percentage Programs

Once you have an estimated 1RM, almost every percentage-based program is derived from it. Here's a standard percentage chart derived from the calculator output:

Some programs use a Training Max (85–90% of true 1RM) as the reference instead. Same math, different reference number.

When to Test vs When to Estimate

Test a true 1RM when:

• You're 4–8 weeks into a peaking cycle and want to verify it worked
• You're prepping for a powerlifting meet and need a realistic opener
• You haven't gauged your actual max in over a year

Estimate when:

• You're programming day-to-day percentages
• You don't have a spotter
• You're training alone or in a commercial gym
• You're in a training block that already drains your CNS (Smolov Jr., RSC)

Common Mistakes

Using a PR from 6 months ago. If you haven't touched that weight since, it's not your 1RM anymore. Estimate from recent work.

Estimating from 12+ rep sets. The formulas break down above 10 reps. You're measuring endurance, not max strength.

Testing partial-range "records." Half-depth squats or half-range bench are not 1RMs. Test full range or don't test.

Ignoring bar speed. If your estimate set had your last rep moving at 2 inches per second with a rounded back, it's not clean. Redo with a lighter weight.

Treating one formula as gospel. No single formula wins on every lifter and every rep range. Average them, or pick the one that matches your recent lifting feel.

Not recalibrating. Your 1RM changes every 4–12 weeks. If your programming percentages still reference an estimate from January, you're not really training at 85% anymore.

Frequently Asked Questions