One Rep Max Calculator
Your one rep max (1RM) is the heaviest weight you can lift for a single, full repetition of an exercise. It is the reference number that most strength programs use to set daily training loads, because working percentages are calculated directly from it. Lifters also call this a PR calculator: your one rep max is your projected personal record for a single rep.
For most lifters, estimating a 1RM from a lighter set is better than actually testing it. An estimate carries no risk of a failed max attempt, spares the central nervous system the fatigue of a true single, and stays accurate to within roughly 2 to 5 percent when the set used is 10 reps or fewer.
How the formulas work
This calculator estimates your max from three established formulas. Epley predicts 1RM = w x (1 + r/30), Brzycki uses 1RM = w x 36 / (37 - r), and Lombardi uses 1RM = w x r^0.10, where w is the weight lifted and r is the number of reps completed.
The three formulas diverge slightly because each was fitted to different data, so this tool reports the average of all three to smooth out that spread. Accuracy is highest at low rep counts and degrades above 10 reps, where fatigue and pacing start to distort the relationship between reps and true maximum strength.
How to use your 1RM
Once you have a 1RM, you set training weights as a percentage of it. Sets aimed at maximal strength typically sit at 85 to 95 percent for 1 to 5 reps, hypertrophy work sits at 65 to 80 percent for 6 to 12 reps, and muscular endurance work sits at 50 to 65 percent for higher reps.
These ranges are guidelines, not hard limits. Recalculate your working weights whenever your estimated 1RM changes so that your percentages track your current strength rather than an outdated number.
Frequently asked questions
How accurate are one rep max calculators?
They are accurate to within roughly 2 to 5 percent when you use a set of 10 reps or fewer. Accuracy falls off as rep count rises, because high-rep sets are limited by conditioning and pacing as much as by maximal strength.
Should I actually test my 1RM?
Most lifters do not need to. A true 1RM test carries a real risk of a failed rep under a heavy load and produces significant fatigue, while an estimate from a submaximal set gives you a usable number safely. Competitive powerlifters and athletes who must know an exact max are the main exceptions.
How many reps should I use for the estimate?
Use a set taken close to failure in the 3 to 6 rep range for the most reliable result. Fewer reps leave less room for formula error, and sets above 10 reps introduce enough variance that the estimate becomes noticeably less trustworthy.
How often should I recalculate my 1RM?
Recalculate every 4 to 6 weeks, or whenever you hit a rep record on a working set. Keeping the number current ensures your training percentages match your present strength instead of drifting out of date.
Is a PR calculator the same as a 1RM calculator?
Yes. PR stands for personal record, and in the context of strength training that means the heaviest weight you can lift for one rep, which is exactly what a 1RM calculator projects. This tool estimates that number from a submaximal set rather than requiring you to test it directly.
What is the Epley formula for one rep max?
1RM = weight x (1 + reps / 30). It is the most widely used 1RM formula because it is simple to calculate and stays accurate to within a few percent for sets of 10 reps or fewer.
How does the NASM one rep max formula differ?
NASM's published calculator uses 1RM = (weight x reps / 30.48) + weight, which is an Epley variant with a slightly different divisor. For typical rep counts the two formulas produce results that differ by well under half a percent, so either one is a reasonable estimate.