One-Rep Max Calculator

The One-Rep Max Calculator estimates the heaviest weight you could lift for a single rep (your 1RM), based on a lighter weight and the number of clean reps you performed with it. It's the standard way lifters, coaches and programming templates figure out working percentages without actually attempting a maximal single, which carries real injury risk and requires a full taper to test safely.

How it works

Enter the weight you lifted and how many reps you completed with good form, choose kilograms or pounds, and the calculator estimates your 1RM using five widely used regression formulas:

• Epley — 1RM = weight × (1 + reps / 30)
• Brzycki — 1RM = weight × 36 / (37 − reps)
• Lombardi — 1RM = weight × reps^0.10
• O'Conner — 1RM = weight × (1 + 0.025 × reps)
• Wathan — 1RM = (100 × weight) / (48.8 + 53.8 × e^(−0.075 × reps))

Each formula was derived from slightly different populations and rep ranges, so they rarely agree exactly. Rather than picking one, the calculator shows all five side by side along with their average, which is generally a more stable estimate than any single formula alone. Results are rounded to the nearest plate-friendly increment — 1 kg or 2.5 lb — so the numbers you get are ones you can actually load on a bar.

Why reps matter

Every 1RM formula is a regression fit against real lifts in the low-rep range, and all of them get less reliable the higher the rep count goes. Past roughly 10–12 reps, the underlying relationship between reps and strength starts to break down — fatigue, technique breakdown and muscular endurance increasingly dominate over the way that the neuromuscular system expresses near-maximal strength. The calculator accepts up to 15 reps, but treat estimates in the 12–15 range as a rough ballpark rather than a number to program from. The most accurate readings come from sets of 1–5 reps performed with a real rep in the tank, not to complete failure.

Percentage-of-1RM chart

Once your 1RM is estimated, the tool also displays a percentage chart from 50% to 100% of that number, alongside the rep range each percentage typically supports. This is the table most strength programs (5/3/1, linear periodization, powerlifting peaking blocks, etc.) reference when they say "work up to 80% for a triple" — you don't need to redo the math for every session, just read the weight off the chart.

Warm-up set progression

Jumping straight to a heavy working weight is a common way to get hurt. The warm-up table below the percentage chart lays out a five-set ramp — starting with the empty bar and climbing through 50%, 70%, 85% and 90% of your estimated 1RM — so you arrive at your working sets primed instead of cold.

How accurate is a 1RM estimate?

Formula-based estimates are typically within about 5–10% of a true tested max for reps in the 1–5 range, but they're still estimates, not measurements — technique, leverages, fatigue and training experience all shift the real number. Use the estimate as a training guide, and retest periodically as you get stronger.

Which formula should I trust most?

None of them is definitively "correct" for every lifter. Epley and Brzycki are the two most commonly cited in strength-and-conditioning literature; the average of all five smooths out the formula-to-formula noise and is a reasonable default if you don't have a preference.

Does this tool store my data?

No — everything runs in your browser. Nothing you enter is saved or sent anywhere.