The short answer
A 1RM calculator estimates the heaviest weight you could lift once, from a set of 1 to 12 repetitions. This one uses the Epley and Brzycki equations and shows the mean of both, then turns it into a table of training weights per percentage of 1RM using the NSCA repetition table: strength work at 85% and above, muscle growth between 67 and 85%, muscular endurance below 67%. It runs in your browser and stores nothing.
Fill in a set you took close to failure and press the button. Your estimated 1RM and the training weights per percentage show up here.
Programme every client in percentages, automatically
RungGym builds a 3 to 6 week block per client with loads that rise week by week, filters out exercises the client's complaints advise against and lets the client log every set on their phone, with a rest timer and personal records. The coach reads the sheet before the next session instead of asking for numbers.
How the numbers are calculated
1. Two equations, one estimate
Both equations were derived from sets of 1 to 10 repetitions and are among the most used in research and in the gym. Epley grows linearly with the repetitions, Brzycki grows faster near 10. The page shows the mean of the two and both values next to it.
Epley: 1RM = weight × (1 + reps ÷ 30)
Brzycki: 1RM = weight × 36 ÷ (37 − reps)
Estimate: (Epley + Brzycki) ÷ 2, rounded to 0.5 kg2. Repetitions per percentage of 1RM
The NSCA table links each percentage of 1RM to the number of repetitions most people can perform at it. The weights in the result table are your estimate multiplied by that percentage.
| % of 1RM | Repetitions |
|---|---|
| 100% | 1 |
| 95% | 2 |
| 93% | 3 |
| 90% | 4 |
| 87% | 5 |
| 85% | 6 |
| 83% | 7 |
| 80% | 8 |
| 77% | 9 |
| 75% | 10 |
| 70% | 11 |
| 67% | 12 |
| 65% | 15 |
3. Zones per training goal
The intensity zones follow the ACSM position stand on resistance training and the NSCA chapter on programme design. They are the same zones the RungGym training engine uses for its sets, repetitions and rest.
| Goal | % of 1RM | Repetitions |
|---|---|---|
| Maximal strength | 85–100% | 1 to 6 |
| Muscle growth (hypertrophy) | 67–85% | 6 to 12 |
| Muscular endurance | 50–67% | 12 to 20+ |
Limits of an estimate
- The equations are validated for 1 to 10 repetitions; from 12 upwards the error grows quickly, so the form stops at 12.
- Trained lifters tend to get more repetitions at a given percentage on leg exercises than on upper body exercises, so one table never fits every lift perfectly.
- The set has to be taken close to failure with good form. A set stopped early underestimates the 1RM.
- Not a substitute for a tested maximum when a programme really depends on it, and never a reason to attempt a true max without a spotter and a warm-up.
Sources
- Epley B. Poundage chart. Boyd Epley Workout. Lincoln, NE: Body Enterprises; 1985.
- Brzycki M. Strength testing: predicting a one-rep max from reps-to-fatigue. J Phys Educ Recreat Dance. 1993;64(1):88-90. doi:10.1080/07303084.1993.10606684 (opens in a new tab)
- Haff GG, Triplett NT, eds. Essentials of Strength Training and Conditioning. 4th ed. Champaign, IL: Human Kinetics; 2016. Chapter 17, table 17.7.
- American College of Sports Medicine. Progression models in resistance training for healthy adults. Med Sci Sports Exerc. 2009;41(3):687-708. doi:10.1249/MSS.0b013e3181915670 (opens in a new tab)
Questions people ask
How accurate is a 1RM calculator?
For sets of 1 to 5 repetitions most equations land within a few percent of a tested maximum; the error grows with the repetitions and differs per exercise. Use the estimate to set training weights, and treat a tested max as the only real number.
Which set should I use?
A recent set with good form that you could not have continued for more than one extra repetition. A warm-up set or a set stopped early gives a number that is too low.
Why does the table stop at 65%?
Below 65% of 1RM the number of possible repetitions rises so fast that a table stops being useful; endurance work is programmed by repetitions and rest, not by percentage.
Does RungGym use this?
The training engine works with the same intensity zones and repetition ranges, and the client logs the weight of every set in the app, so the next block starts from what was really lifted.
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