How the 1RM calculator works
Your one-rep max is the heaviest weight you can lift for a single rep with good form. Testing it directly is tiring and carries more injury risk than a normal set, so most lifters estimate it from a heavier set of a few reps. The calculator uses three well-known formulas and shows their average as your headline number.
- Epley: 1RM = weight × (1 + reps ÷ 30). Tends to read slightly high at higher reps.
- Brzycki: 1RM = weight × 36 ÷ (37 − reps). Very close to Epley under 10 reps.
- Lombardi: 1RM = weight × reps0.10. Gives more conservative results for low-rep sets.
If you enter a single rep, the calculator simply returns that weight, because you already lifted your max.
Using the percentage chart
Most strength programs prescribe weights as a percentage of your 1RM. As a rough guide, 85–95% is used for heavy sets of 1–5 reps to build maximal strength, 70–85% for sets of 6–12 reps to build muscle, and 50–65% for warm-ups, technique work and speed sets. The rep numbers in the chart are typical ranges, and your own numbers may run a little higher or lower.
Tips for an accurate estimate
- Use a set where you stopped with 0–1 reps left in the tank. Easy sets underestimate your max.
- Keep it under 10 reps. Above that, fatigue and conditioning skew the estimate.
- Use the same lift. A 1RM from a squat set doesn't tell you anything about your deadlift.
- Re-test every 4–8 weeks so your training percentages keep up with your progress.
Frequently asked questions
How accurate is a 1RM calculator?
For sets of 2–6 reps taken close to failure, estimates are usually within a few percent of a tested max. Accuracy drops above 10 reps, because endurance starts to matter as much as strength.
Is it safe to test a true one-rep max?
For experienced lifters with a spotter or safety pins, occasional 1RM tests are common. For beginners, an estimated max from a 3–5 rep set gives you almost the same information with far less risk.
Why do the formulas give different numbers?
Each one was fitted to different groups of lifters and rep ranges. They agree closely at low reps and drift apart as reps go up, which is why the calculator averages them.
How often should I recalculate my 1RM?
Every 4–8 weeks, or at the end of each training block. Beginners get stronger quickly, so updating your max keeps your training percentages challenging.
