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Free Strength Tool

1 Rep Max Calculator

Estimate your one-rep max (1RM) for any lift using four proven formulas. Get your training percentages for strength, hypertrophy, and power work, no sign-up required.

Four validated formulas Training percentages included Free, no sign-up

Enter Your Set

Use a recent working set taken close to failure for the most accurate estimate.

Enter the weight you lifted in kg.

Between 1 and 20. Most accurate between 2 and 10 reps.

Optional. If you had reps left in the tank, we'll adjust the estimate upward.

How to Use This Calculator

  1. Test with a real working set

    Use a weight you lifted recently with good form. The calculator works best when you used genuine effort and stopped within 1 to 3 reps of failure. Grinded singles with breakdown in form give bad inputs.

  2. Keep reps in the accurate range

    Formulas are most reliable between 2 and 10 reps. Below 2 reps, you are already at or near your true 1RM. Above 10 reps, estimates become increasingly unreliable as fatigue and technical breakdown distort the load-rep relationship.

  3. Factor in RPE if you can

    If you know you had reps left in the tank (RPE 7 to 9), use the RPE dropdown. This adjusts your effective reps upward for a more honest 1RM estimate.

  4. Use the training percentages as a starting point

    Individual response to percentages varies. If 80% feels heavier than 8 reps, retest your 1RM.

The Science Behind 1RM Formulas

Predicting a one-rep max from a submaximal set has been studied since the 1950s. Modern formulas emerged in the 1980s and 90s and are used in most strength programs and powerlifting coaching today.

The Four Formulas

1RM Formulas

Epley (1985): 1RM = weight × (1 + reps / 30)

Brzycki (1993): 1RM = weight × 36 / (37 − reps)

Lombardi: 1RM = weight × reps0.10

O'Conner: 1RM = weight × (1 + reps / 40)

Epley is the most widely used. Reliable between 1 and 10 reps. Slightly overestimates at higher rep counts.

Brzycki tends to be more accurate for lower reps (1 to 5) but breaks down above 10 reps. Favored in powerlifting circles.

Lombardi is the most conservative and performs better at higher rep counts (8+), but underestimates at low reps.

O'Conner is the most beginner-friendly estimate. Generally the most conservative of the four.

Why This Calculator Averages Four Formulas

No single formula is perfect across all rep ranges. By averaging four validated equations, you get a more balanced estimate that is not skewed by the bias of any individual formula. Research comparing 1RM prediction equations has consistently found that accuracy depends on the lift, the lifter's training history, and the rep range used. A formula that fits bench press well may underestimate squat, and vice versa.

Limitations

1RM formulas are estimates, not measurements. They assume a linear relationship between reps and intensity that does not hold at the extremes. True 1RM testing (worked up to a heavy single with full warm-up and spotters) is more accurate but also more fatiguing and carries higher injury risk. For most lifters, estimated 1RMs from 3 to 8 rep sets are sufficient for programming purposes. Never use a calculated 1RM as your first attempt in a powerlifting meet.

When to Retest Your 1RM

Recalculate your 1RM every 4 to 8 weeks, or whenever a working set feels noticeably easier than expected. If you hit more reps than the calculator predicts at a given percentage, your strength has outpaced your last estimate. If you cannot hit the predicted reps, you may have lost strength (deload, stress, illness) or your original estimate was inflated.

Frequently Asked Questions

Is this calculator accurate?
For sets in the 2 to 10 rep range, estimates from the four averaged formulas are typically within 5 to 10% of a tested 1RM. Accuracy is best with 3 to 8 rep sets taken close to failure. Above 10 reps the relationship between reps and load becomes less reliable, and confidence drops accordingly.
Should I actually test my true 1RM in the gym?
True 1RM testing is more accurate but also more fatiguing and carries higher injury risk, especially without experienced spotters and a full warm-up. For most lifters, an estimated 1RM from a 3 to 8 rep working set is accurate enough for programming. Save tested maxes for competition prep or occasional benchmark sessions.
Why do the four formulas give different results?
Each formula was derived from a different data set and fits different rep ranges best. Brzycki is usually strongest at 1 to 5 reps, Epley is balanced across 1 to 10, Lombardi handles higher reps better, and O’Conner is the most conservative. Averaging all four reduces the bias of any single equation.
Can I use this for any exercise?
Yes, but the estimate is most reliable for compound barbell lifts like the squat, bench press, deadlift, and overhead press. For isolation movements or machines, the load-rep relationship is noisier and the predicted 1RM should be treated as a rough guide rather than a true max.
What if I used a weight I could do for more than 10 reps?
You can still enter the set, but treat the result as a ballpark. Above 10 reps, fatigue and technical breakdown distort the relationship between load and reps, so formula error grows quickly. Retest with a heavier weight in the 3 to 8 rep range when you can.
How often should I recalculate my 1RM?
Every 4 to 8 weeks is a sensible cadence, or sooner if a working set feels noticeably easier than expected. If you beat the predicted reps at a given percentage, your strength has outpaced your last estimate. If you cannot hit the predicted reps, you may have lost strength or your previous number was inflated.

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