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How to Calculate Your One Rep Max (Free 1RM Calculator)

Want to know your one rep max without grinding out a true max attempt? Here's how powerlifters actually estimate their 1RM — formulas, free 1RM calculator, and the rep-max conversion every serious lifter uses.

Pasha Mor

Founder, 1RM.fit · July 5, 2026

Your one rep max is the heaviest weight you can lift for a single rep with good form. Most lifters never test a true 1RM — it's risky, exhausting, and rarely necessary. The smarter move is to estimate it from your working sets using a one rep max calculator. This guide shows you exactly how.

I'm Pasha, founder of 1RM.fit. We have a free 1RM calculator built into the app — every set you log automatically estimates your one rep max for every lift, no math required. Here's how the calculation actually works.

The Best One Rep Max Formula (Brzycki)

The most accurate 1RM calculator formula is Brzycki's:

1RM = Weight × (36 / (37 - Reps))

Example: you bench 100 kg for 5 reps. Your estimated one rep max is:

100 × (36 / (37 - 5)) = 100 × 1.125 = 112.5 kg

Brzycki is accurate within ±2.5% in the 3-10 rep range. Above 10 reps, accuracy drops — too much endurance bias. Below 3 reps, accuracy is also questionable because rep dropoff isn't linear.

Other One Rep Max Calculator Formulas

Epley

1RM = Weight × (1 + Reps / 30). Slightly higher estimates than Brzycki. Common in casual fitness apps.

Lombardi

1RM = Weight × Reps^0.10. More accurate in the very-low rep range (2-4 reps) than Brzycki.

O'Conner

1RM = Weight × (1 + 0.025 × Reps). Most conservative formula. Good if you want a safer estimate.

Most modern 1RM calculator tools (including ours) use Brzycki for sets in the 3-10 rep range and Lombardi for low-rep work.

Barbell bench press — the lift most commonly used to estimate one rep max — The bench press, squat, and deadlift are the three lifts where one rep max tracking matters most for strength programming.

Quick Rep Max Conversion Table

Use this table to estimate your one rep max from any working set. Find your rep count, multiply your working weight by the % shown.

1 rep: 100% (your actual 1RM)
2 reps: ÷ 0.95 (e.g. 100 kg × 2 ≈ 105 kg 1RM)
3 reps: ÷ 0.92
5 reps: ÷ 0.87
6 reps: ÷ 0.85
8 reps: ÷ 0.80
10 reps: ÷ 0.75
12 reps: ÷ 0.70

Why Estimate Instead of Testing

Safer. True 1RM attempts have a real injury risk. Estimating from rep work has zero.
More accurate over time. Daily 1RM swings ±5-10% based on sleep, food, stress. Estimates from your normal training weights are more stable.
Better for programming. Programs like 5/3/1 prescribe percentages of your 1RM. An estimate gives you working weights without needing a max-out day every cycle.

When You Should Test a True One Rep Max

You're competing in powerlifting and need a real meet 1RM
You're ending a long training cycle and want a benchmark
Your estimated 1RM has been frozen for 12+ weeks despite training hard

Outside those situations, just keep training in the 5-10 rep range and let the calculator track your strength.

Your estimated 1RM moving up week after week is more useful than a true 1RM you tested once last spring.

Track Your One Rep Max Automatically

The 1RM.fit app uses Brzycki and Lombardi automatically — every set you log feeds your estimated one rep max for that lift in real time. You see your bench, squat, deadlift, and OHP 1RM as graphs, with the formula behind every number transparent.

Download 1RM.fit free. No paid one rep max calculator needed.

Want a deeper look at the apps that handle this well? Read our best weight lifting app comparison or the best workout program apps guide.

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