💪 One Rep Max Calculator
Estimate your one-repetition maximum and training percentages for strength training.
What is this tool?
The One Rep Max Calculator is a free online tool that estimates the maximum weight you can lift for a single repetition (1RM) based on the weight and repetitions you can perform. This is one of the most useful tools for strength training, powerlifting, and bodybuilding, as knowing your 1RM allows you to calculate training percentages for structured programs. Testing your actual 1RM by lifting maximum weight can be risky, especially without a spotter or proper warm-up. Instead, you can perform a set with a sub-maximal weight for multiple repetitions, and this calculator uses scientifically validated formulas to estimate your true maximum. The tool provides three different formulas, each with slightly different assumptions, so you can compare and choose the most appropriate estimate for your body type and lifting style. Beyond simply calculating your 1RM, the tool also displays a full percentage table showing the weight you should be able to lift at various rep ranges (95%, 90%, 85%, 80%, 75%, 70%, 65%, and 60%). This is invaluable for following percentage-based training programs like 5/3/1, Texas Method, or Starting Strength, where your working weights are calculated as a percentage of your 1RM.How it works
The calculator implements three well-known prediction formulas, each based on the relationship between weight lifted and number of repetitions performed. The Epley formula estimates 1RM as weight x (1 + 0.0333 x reps), which works well for moderate rep ranges (5-10). The Brzycki formula estimates 1RM as weight x 36 / (37 - reps), which tends to be more accurate for lower rep ranges (1-5). The Lombardi formula uses weight x reps^0.10, which provides a mathematical power function approach. Each formula has limitations: they assume you performed the set to true muscular failure, which is rarely the case. The accuracy also degrades significantly beyond 10-12 repetitions, as the relationship between reps and max strength becomes less linear at higher rep ranges. For the best results, use a weight that you can lift for 3-6 reps with good form and reach near-failure on the last rep. The percentage table is derived by applying the inverse of the Epley formula: for a given percentage of 1RM, the estimated reps are calculated as reps = (1RM / weight - 1) / 0.0333. This produces the standard percentage-to-reps mapping used in strength training programs worldwide. All calculations are performed locally in JavaScript.How to use
- Enter the weight you lifted (in kg or lbs).
- Enter the number of repetitions you performed with that weight (1-12 recommended).
- Click Calculate to see your estimated 1RM from three formulas.
- Review the percentage table to find your training weights.
- Use the appropriate percentage for your training program.
Frequently Asked Questions
Frequently Asked Questions
Which formula is most accurate?
The Brzycki formula tends to be most accurate for low reps (1-5), while the Epley formula works better for moderate reps (5-10). For a conservative estimate, use the lower of the three.
How accurate are 1RM estimates?
When using 3-6 reps to failure, estimates are typically within 5-10% of your actual 1RM. Accuracy decreases with higher rep counts and if you do not reach true failure.
Should I test my actual 1RM instead?
Only if you are an experienced lifter with proper form and a spotter. Actual 1RM testing carries injury risk. For most trainees, estimated 1RM is sufficient for programming.
Can I use this for any exercise?
It works best for compound movements (squat, bench press, deadlift, overhead press). It is less accurate for isolation exercises where technique limitations cap performance before true strength.
Tips & Advice
When using estimated 1RM for training programs, add 5% to the estimate for safety if you are a beginner, as novice lifters often underestimate their true max due to technique inefficiency. For percentage-based programs, the 70-85% range corresponds to typical strength work (5-10 reps), while 85-95% targets maximal strength (1-5 reps). Re-test your estimated 1RM every 4-8 weeks by performing a challenging set and entering the results. Track your progress over time rather than comparing to others, as leverages and muscle insertions vary greatly between individuals.