How to Calculate Your One-Rep Max (1RM): Formulas Explained
You don't need to grind out a dangerous single rep at your absolute limit to know your one-rep max. A handful of well-tested formulas estimate it accurately from a lighter, safer set — here's how they work and how to use the number once you have it.
Contents
What a 1RM Actually Measures
Your one-rep max (1RM) is the heaviest weight you can lift for exactly one complete, well-executed repetition of a given lift. It's the standard reference point strength coaches use to program training loads, because nearly every serious lifting program is built around percentages of it — "work up to 3 sets of 5 at 80% of your max" only means something once you know what 100% actually is.
A tested 1RM comes from literally attempting a maximal single, usually after a careful warm-up and several progressively heavier attempts. An estimated 1RM comes from a formula applied to a lighter set — say, 5 or 8 reps — performed close to failure. For almost everyone outside competitive powerlifting, the estimate is both safer and practically just as useful.
The Formulas 1RM Calculators Use
Several formulas exist, all built from the same basic observation: the more reps you can do with a given weight, the further that weight is below your true max. They just model the relationship slightly differently.
| Formula | Equation | Notes |
|---|---|---|
| Epley | Weight × (1 + Reps ÷ 30) | Most widely used; trends slightly higher at higher rep counts |
| Brzycki | Weight × 36 ÷ (37 − Reps) | Tends to be a touch more conservative than Epley |
| Lander | (100 × Weight) ÷ (101.3 − 2.67123 × Reps) | Common in exercise-science literature |
| O'Conner | Weight × (1 + 0.025 × Reps) | Simpler linear model, more conservative at high reps |
All four give broadly similar numbers in the 2-8 rep range where they're most reliable, and start to diverge more the higher the rep count climbs — which is exactly why most calculators cap their reliable range around 10-12 reps.
🧮 Calculate Your 1RM
Enter your weight and reps to get an instant estimate across multiple formulas.
Open One Rep Max Calculator →Worked Example
Epley Formula — 100kg × 5 reps
Run the same 100kg × 5 set through Brzycki and you'd get roughly 112.5kg — close to Epley's number but a little lower, which is typical of how the formulas compare in this rep range.
Using the 1RM Percentage Chart
Once you have an estimated max, training programs usually prescribe a percentage of it rather than an absolute weight, so the number stays useful as you get stronger or switch programs.
| % of 1RM | Typical Rep Range | Common Use |
|---|---|---|
| 90-100% | 1-3 reps | Max-strength and peaking work |
| 80-90% | 4-6 reps | Strength building |
| 70-80% | 7-10 reps | Strength-hypertrophy blend |
| 60-70% | 12-15 reps | Hypertrophy / muscle growth |
| Below 60% | 15+ reps | Endurance and technique work |
Why Estimating Beats Maxing Out
A genuine maximal single puts a lot of strain on joints, connective tissue and technique all at once, with no real margin if something goes wrong — especially without an experienced spotter. An estimated 1RM from a controlled, submaximal set removes almost all of that risk while still giving you a number accurate enough to program training percentages around.
Summary
Your one-rep max is best estimated, not tested, for the vast majority of lifters — a lighter set of 2-8 reps run through a formula like Epley or Brzycki gets you within about 5-10% of your true max safely. Use the resulting number to set training percentages, and recalculate it every month or two as your strength improves.
This article provides general fitness information only and is not a substitute for guidance from a qualified strength coach, especially around maximal testing.
Frequently Asked Questions
What is a 1RM?
1RM stands for one-rep max — the heaviest weight you can lift for a single, complete repetition of a given exercise with good form. It's used to gauge strength, program training percentages and track progress over time.
How accurate is an estimated 1RM calculator?
Formula-based estimates are generally accurate within about 5-10% of your true 1RM when based on a set of 2-8 reps performed close to failure. Accuracy drops the further you get from that rep range, which is why most calculators recommend testing with sets under about 10 reps.
Which 1RM formula is most accurate?
No single formula is universally most accurate — Epley tends to estimate slightly higher for higher rep sets, while Brzycki tends to be more conservative. Many lifters average two or three formulas together, or simply use whichever has tracked closest to their actual tested max in the past.
Do I need to actually test my 1RM?
Not usually. A true maximal single carries real injury risk, especially without a spotter or coach, and an estimated 1RM from a submaximal set is accurate enough for programming training percentages for most lifters. Competitive powerlifters are the main group who genuinely need a tested max.
How often should I retest or recalculate my 1RM?
Every 4-8 weeks is a reasonable interval for recalculating your estimated 1RM from a recent top set, which keeps your training percentages current as you get stronger without requiring constant maximal testing.