About the One Rep Max Calculator
A one rep max calculator estimates the heaviest weight you could lift once for a given exercise, from a set you have already completed at a lower weight for more reps. It lets you programme training percentages without the risk and fatigue of an actual maximal attempt.
The formula
Epley: 1RM = weight · (1 + reps / 30)Weight is what you lifted and reps how many you completed with good form to near failure. The Brzycki formula, weight × 36 / (37 − reps), is an alternative that gives slightly lower estimates at higher rep counts.
How to use this calculator
- 1Enter your Weight Lifted (kg or lb). The field starts at
100, which you can overwrite. - 2Enter your Reps Performed. The field starts at
5, which you can overwrite. - 3Read the result straight away — it recalculates as you type, so there is no button to press. Use Share to copy a link that reopens the page with your exact numbers filled in.
Worked example
| Input | Value |
|---|---|
| Weight Lifted (kg or lb) | 100 |
| Reps Performed | 5 |
Result
Estimated 1RM:
Brzycki: 112.5
Epley: 116.7
Lander: 113.7
Average: 114.3
70%: 80.0 | 80%: 91.4 | 90%: 102.9
Those are the values the page loads with, so you can reproduce this result yourself and then change one field at a time to see what drives the outcome.
Understanding your result
These formulas assume a typical relationship between load and repetitions, which holds well up to about 10 reps and degrades beyond it. A set of 15 depends heavily on muscular endurance and pain tolerance rather than pure strength, so estimates from high-rep sets are much less reliable. Sets of 3 to 6 give the best predictions.
The estimate is also exercise-specific and individual. Squats and deadlifts, which use large muscle groups, tend to allow more reps at a given percentage than bench press or overhead press. Fibre type composition matters too — some athletes are strong at single reps but fade quickly, while others grind out many reps near their max.
Things worth knowing
- Estimate from a set of 3 to 6 reps for the best accuracy. Sets above 10 produce unreliable figures.
- The set must be taken close to failure with good form. A comfortable set understates your maximum.
- Strength programmes typically prescribe 70 to 85 percent of 1RM for hypertrophy and 85 percent plus for strength.
- Recalculate every four to six weeks. Training percentages based on a stale maximum become too easy.
- Never attempt a true one rep max without a spotter and a proper warm-up.
Frequently asked questions
How accurate are one rep max formulas?+
Within about 5 percent when estimated from sets of 3 to 6 reps taken near failure. Accuracy declines sharply above 10 reps, where muscular endurance rather than strength determines the outcome.
Which formula is best?+
Epley and Brzycki are the most widely used and agree closely at low rep counts. Brzycki gives slightly lower estimates at higher reps, which is often the more conservative and useful choice.
Should I test my actual one rep max?+
Rarely, and only with experience, a spotter, and a thorough warm-up. Estimating from submaximal sets carries far less injury risk and fatigue cost, which is why most programmes use it.
How do I use my 1RM to plan training?+
Prescribe working weights as percentages: 65 to 75 percent for volume work, 75 to 85 for hypertrophy, and 85 percent and above for strength. Recalculate every month or so as you get stronger.