Your one rep max (1RM) is the heaviest weight you can lift for a single repetition of an exercise. It's the standard benchmark for strength, and most training programs prescribe weights as a percentage of it. Testing a true 1RM is taxing and can be risky, so most lifters estimate it instead.
This one rep max calculator does exactly that: enter a weight and the reps you completed, and it estimates your 1RM in seconds. It works in kilograms or pounds and updates live, so you can plan your next session without grinding out a max single.
How the one-rep max calculator works
This calculator uses the Epley formula, one of the most widely used 1RM estimators:
1RM = weight × (1 + reps / 30)
You provide two inputs: the weight you lifted and the number of reps you completed with good form (ideally to near-failure). The formula adds roughly 3.3% to the lifted weight for each rep beyond the first, then scales it up to predict a true single. At exactly 1 rep, it returns the weight itself.
The estimate is most accurate in the lower rep ranges — about 1 to 10 reps. Beyond that, fatigue and technique vary too much for any formula to stay reliable.
The calculator also converts your estimated 1RM into useful training percentages so you can program your working sets: roughly 90% for 4 reps, 80% for 8 reps, and 75% for 10 reps. These give you target loads for hypertrophy and strength work without re-testing your max each time.
Worked example
Suppose you back-squat 100 kg for 5 reps. Plug those into the Epley formula:
1RM = 100 × (1 + 5/30) = 100 × (1 + 0.1667) = 100 × 1.1667 ≈ 116.7 kg.
So your estimated one rep max is about 117 kg. From there the training percentages give you working loads: ~90% ≈ 105 kg for sets of 4, ~80% ≈ 93 kg for sets of 8, and ~75% ≈ 88 kg for sets of 10. Use these as starting points and adjust to how the bar feels.
Things to keep in mind
- The Epley formula is an estimate, not a tested max — your real 1RM may be higher or lower depending on the lift and your experience.
- Accuracy drops as reps climb; it's most reliable around 1 to 10 reps and overestimates for high-rep sets.
- Results assume the reps were performed with full range of motion, good technique, and close to failure. Easy sets will under-predict your max.
- 1RM varies by exercise and by day (sleep, nutrition, fatigue). Treat the number as a planning tool, not a guarantee — and only attempt heavy singles with a spotter or safety setup.
Frequently asked questions
What is a one rep max (1RM)?
Your one rep max is the maximum weight you can lift for a single, full repetition of an exercise with proper form. It's the standard measure of maximal strength and the reference point most programs use to assign training loads as a percentage.
How does this one rep max calculator work?
It uses the Epley formula: 1RM = weight × (1 + reps/30). You enter the weight you lifted and how many reps you completed, and it estimates the single-rep max. It then shows training percentages like 90% for 4 reps and 80% for 8.
How accurate is an estimated 1RM?
Estimates are most accurate in lower rep ranges, roughly 1 to 10 reps. The fewer reps you use (while still being challenging), the closer the estimate. High-rep sets introduce more error, so the result is a reliable guide rather than an exact figure.
Do I have to test my actual one rep max?
No. The point of this calculator is to estimate your 1RM from a normal set without attempting a true max single, which is fatiguing and riskier. Lift a challenging weight for a few reps and let the formula do the rest.
What weight should I use for sets of 8 or 10?
As a starting point, use about 80% of your estimated 1RM for sets of 8 and about 75% for sets of 10 (roughly 90% for 4 reps). The calculator shows these percentages automatically so you can program working sets quickly.
Does the calculator work in pounds and kilograms?
Yes. You can enter your lifted weight in either kilograms or pounds, and the estimated 1RM and training percentages display in the same unit. The result updates live as you change the weight or reps.