Calculation details
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What does this calculator estimate?
A one-rep max calculator estimates the maximum weight you can lift once, based on a lighter set to failure. Enter the weight and reps to get your estimated 1RM.
- Epley: 1RM = weight × (1 + reps ÷ 30)
- Most accurate for reps under 10
- Use for programming, not everyday testing
What a 1RM estimate is for
The one-rep max is the most a person can lift once. Because testing a true max is risky, formulas like Epley's estimate it from a submaximal set — letting you program percentages without heavy one-rep attempts.
Limitations to watch for
Epley's formula is most accurate for 2–10 reps; very high rep sets (15+) overestimate. The estimate assumes you took the set close to failure with good form. A real 1RM depends on fatigue, technique, and the lift itself (squat vs bench).
How to use it in practice
Test a weight you can lift for 3–8 reps with clean form, near failure. Use the estimate to set training percentages (e.g., 65–85% for working sets). Re-test periodically as strength improves. Never attempt a heavy 1RM without a spotter and warm-up.
['Enter the weight lifted.', 'Enter the number of reps performed (ideally under 10).', "The tool applies Epley's formula and shows 95% of the max."]
How this calculator works
Formula
Estimated one-rep max (Epley formula): 1RM = weight × (1 + reps ÷ 30). 95% of max ≈ 1RM × 0.95.
Worked example
Lifting 185 lb for 5 reps: estimated 1RM = 185 × (1 + 5 ÷ 30) = 185 × 1.1667 ≈ 216 lb; 95% ≈ 205 lb.
Assumptions to verify
- The set was performed to near failure with good form.
- The lift is a compound barbell movement.
- The trainee is experienced enough to train near maximal loads.
Frequently asked questions
How is one-rep max estimated?
The Epley formula: 1RM = weight × (1 + reps ÷ 30). 185 lb for 5 reps estimates a 216 lb max.
What is the Epley formula?
A widely used prediction: estimated max = lifted weight × (1 + reps ÷ 30). It's simple and accurate for low-rep sets.
How many reps should I use for the test?
3–8 reps is the sweet spot. Very high rep sets make the estimate unreliable and the set itself fatiguing.
Why not just test a real 1RM?
True max attempts carry injury risk and need heavy warm-ups and a spotter. Submaximal estimates give you the programming number safely.
What is the 95% figure?
A common working top-end percentage (95% of max) used for heavy training days. Many programs cap working sets around there.
Does this work for every lift?
Best for compound lifts like squat, bench, and deadlift. Isolation exercises and machines may estimate differently.
How often should I retest?
Every 4–8 weeks as strength changes. Use the same formula each time so progress is comparable.
Cite this tool
BoringToolsKit. “One Rep Max Calculator.” boringtoolskit.com/one-rep-max-calculator/ (reviewed 2026-08-25). Free to reference in articles, syllabi, and answer posts with a link.
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