One Rep Max Calculator: How to Calculate Your 1RM

Your one-repetition maximum (1RM) is the heaviest weight you can lift once with good form, and it is the reference that percentage-based programs are built on. You can estimate it from a set of several reps with a formula instead of testing it, which is what a One Rep Max Calculator does. This guide covers the formulas, how accurate they are, how to test safely if you want a true max, and how to turn the number into training loads.

What a 1RM Is Used For

Strength programs prescribe loads as a percentage of 1RM so that a "heavy triple" or a "set of eight" means a specific weight. The National Strength and Conditioning Association's load guidance maps intensities to goals: roughly 85% of 1RM and above, for sets of six or fewer, emphasizes maximal strength; about 67–85% for six to twelve reps emphasizes hypertrophy; 67% and below for twelve or more reps emphasizes muscular endurance. Knowing your 1RM also lets you track progress across months without maxing out every week.

The Estimation Formulas

Each formula takes the weight you lifted and the number of reps you completed in one set and projects what a single rep would be. The calculator offers four:

Epley: 1RM = weight × (1 + reps ÷ 30)
Brzycki: 1RM = weight × 36 ÷ (37 − reps)
McGlothin (also published as Lander): 1RM = weight × 100 ÷ (101.3 − 2.67123 × reps)
Lombardi: 1RM = weight × reps^0.10

Example. You bench 90 kg for 5 reps. Epley gives 90 × (1 + 5 ÷ 30) = 105 kg; Brzycki gives 90 × 36 ÷ 32 = 101 kg; McGlothin gives 90 × 100 ÷ 87.94 = 102 kg; Lombardi gives 90 × 5^0.1 = 106 kg. The spread of a few kilograms between formulas is typical, and it grows with the rep count.

The formulas were developed for sets of roughly 2–10 reps, and studies in the Journal of Strength and Conditioning Research find they are more accurate when the set is taken to fatigue in fewer than 10 reps. Which formula is "best" depends on the population tested: in one study of untrained women, Epley's predictions did not differ significantly from measured 1RM while Brzycki's and Lander's did. For practical purposes, use a hard set of 3–6 reps and treat the result as an estimate within a few percent.

Using the Calculator

Enter the weight lifted in kilograms, the reps completed, and pick a formula (Epley is the default and the most widely used). The calculator also asks for training experience and scales the estimate by 1.0 for advanced lifters (3+ years), 0.95 for intermediates (1–3 years) and 0.90 for beginners (under a year). The idea is that newer lifters tend to fall short of a formula's projection on a true single because their technique and confidence under maximal load are less developed; if you'd rather see the raw formula result, choose "advanced".

Use a recent set performed with full range of motion and good form, ideally 3–6 reps close to failure. Sets of 12 or more produce unreliable estimates. Re-estimate every few weeks of consistent training, or whenever your working weights have clearly moved, so your percentages stay current.

Testing a True 1RM Safely

A direct test is more accurate than any formula but carries more risk, so it belongs in a planned session when you are rested.

1. Warm up generally, then build with sets of decreasing reps: for example 8 reps at about 50% of your estimated max, 3–5 at 70%, 2–3 at 80%, then singles at around 90%.

2. Rest 3–5 minutes between heavy attempts.

3. Attempt your estimated 1RM. If it moves well, add a small increment (2.5–5 kg) and try again; stop after three to five attempts or at the first grind that compromises form.

4. For bench press and squat, use spotters or a rack with safety bars set at the right height. Never test alone.

Skip testing when you are fatigued, sleep-deprived, injured or at the end of a hard training block. If you have a heart condition, high blood pressure or a joint problem, check with a clinician before maximal lifting; the numbers here are training guidance, not medical advice.

Turning a 1RM Into Training Loads

With a 1RM of 150 kg on the squat, a strength session might be 3 × 3 at 85% (127.5 kg), while a hypertrophy session might be 4 × 8 at 70% (105 kg), using the NSCA ranges above. Many programs work from a "training max" set at about 90% of the true 1RM, so that prescribed percentages stay achievable on ordinary days and progress continues without missed lifts.

Adequate protein supports strength training; the protein intake calculator scales a daily target to body weight, training level and goal, and the Macro Calculator sets protein alongside fat and carbs.

Related guides

Macro Calculator: How to Calculate Your Macros

Sources

NSCA — Using intensity based on sets and repetitions: a brief overview of load setting and programming strategy