One-Rep Max Calculator

How to use this tool

Enter weight. Type the weight you lifted (in kg or lb).
Enter reps. Enter how many reps you completed (1–20).
View estimate. View the average of five formula estimates, individual formula values, an inverse-Epley rep-target table, and percentage-based working weights calculated from the average.

How 1RM is calculated

Because directly testing a true one-rep max carries injury risk, several formulas estimate it from a sub-maximal effort (weight × reps). This calculator uses five commonly cited formulas and reports their average:

Epley: w × (1 + reps ÷ 30)
Brzycki: w × 36 ÷ (37 − reps)
Lombardi: w × reps^0.10
O'Conner: w × (1 + 0.025 × reps)
Wathan: 100w ÷ (48.8 + 53.8 × e^{−0.075 × reps})

How to use 1RM for programming

Once you know your estimated 1RM, you can set training weights as percentages of it. Common programming percentages:

90–100%: max strength, 1–3 reps
80–85%: strength, 3–5 reps
70–75%: hypertrophy/strength, 6–8 reps
60–70%: hypertrophy, 8–12 reps
50–60%: endurance, 15+ reps

Formula accuracy comparison

Using a sample input of 100 lbs × 5 reps:

Formula	Estimated 1RM	Notes
Epley	116.7 lbs	Most widely cited; slight overestimate at higher reps
Brzycki	112.5 lbs	Conservative; preferred for reps ≤10
Lombardi	117.5 lbs	Tends to overestimate for lighter loads
O'Conner	112.5 lbs	Simple linear formula; less accurate at extremes
Wathan	116.6 lbs	Exponential; shown rounded to one decimal

Epley is the most widely used in gyms and coaching software. Brzycki is most accurate for low-rep sets (≤5 reps). All formulas become less accurate above 10 reps.

Testing your 1RM safely

Direct 1RM testing carries real injury risk. If you do test directly, follow this protocol:

Warm-up: 10 reps at 50%, 5 reps at 70%, 3 reps at 85%, 1 rep at 90%.
Rest: 3–5 minutes between near-maximal attempts.
Spotter: use a qualified spotter for squat and bench press.
Bracing: use a belt if trained to do so; no belt for beginners.
Stop: if form breaks down or the lift feels dangerous, terminate immediately.

For most training purposes, using a 3–5 rep near-maximum effort with the estimator above is safer and almost as informative.

1RM accuracy limits

These formulas assume the rep set is taken to full failure (the last rep is a genuine maximum effort). If you stop a set with reps in reserve, the formula will underestimate your 1RM. Most formulas are validated for 1–10 reps; beyond 10 reps the estimates diverge significantly because metabolic fatigue becomes a larger factor than pure strength.

Frequently asked questions

What is a one-rep max (1RM)?

A one-rep max is the maximum weight you can lift for a single full repetition of an exercise with proper form. It is the standard measure of maximum strength in powerlifting and strength training.

Why do the formulas give different results?

Each formula was developed from different datasets and rep ranges. The Epley and Brzycki formulas are most accurate for 2–10 reps; Wathan tends to be better at higher rep counts. The average of multiple formulas is generally the most reliable estimate.

Should I actually attempt my estimated 1RM?

Not unless you are trained in 1RM testing, have a spotter, and have warmed up properly. These are estimates. Actual maximal attempts carry injury risk. Use the estimate for programming purposes - e.g., to set percentages for your training sets.

How are the working weights by percentage calculated?

Each working weight is the displayed average 1RM multiplied by the listed percentage. The separate rep-target table uses the inverse Epley formula, so the two tables answer different training questions.