One Rep Max Calculator
Calculate lift strength estimates
One Rep Max Calculator tool
One Rep Max Calculator
One Rep Max Calculator: key facts
- What it does
- Calculate lift strength estimates
- Category
- Health & Fitness Calculators
- Cost
- Free, with no account, sign-up, or install.
- Your data
- Runs entirely in your browser — the files and text you enter are never uploaded to a server.
- Last reviewed
- . Report an incorrect result.
What this tool is for
Your one-rep max — the most weight you can lift for a single repetition — is the benchmark strength training programs are built around, but testing it directly is risky and tiring. This calculator estimates it from a lighter set you have already done, using the weight lifted and the number of reps.
Lifters use it to set training percentages, track progress without maxing out, and program their next cycle. It computes two well-known estimates side by side — the Epley and Brzycki formulas — so you can see how they compare on the same lift rather than trusting a single number.
How to use the One Rep Max Calculator
- Enter the weight you lifted for the set.
- Enter the number of reps you completed with that weight.
- Read the Epley estimated one-rep max, shown prominently.
- Compare it against the Brzycki estimate shown alongside, and Copy the result.
The Brzycki equation
A one-rep max is the heaviest single repetition you could complete with good form, and testing it directly is both time-consuming and risky. Prediction equations exist to infer it from a set you have already done, which is safer and gives a number you can programme from.
This calculator uses Brzycki's formula, which divides the weight lifted by a linear function of the repetition count. The structure implies each additional repetition reduces the percentage of maximum you are working at by a fixed 2.78%, which is a reasonable approximation across low and moderate rep ranges.
Accuracy depends heavily on the set you enter. The estimate is most reliable from sets of about two to ten repetitions taken close to failure. Feed it a comfortable set with several repetitions left in reserve and it will underestimate; feed it a set of twenty and the linear assumption breaks down, because endurance rather than strength becomes the limiting factor.
- The default entry — 80 kg for 5 repetitions — estimates a one-rep max of 90.01 kg.
- The same 80 kg for 8 repetitions estimates 99.33 kg, and for 3 repetitions 84.71 kg.
- Working backwards, that 90 kg maximum suggests roughly 76.5 kg for a set of 5 at 85% — the kind of figure a programme is written from.
What the One Rep Max Calculator gets right
- It calculates two established formulas — Epley and Brzycki — at once, so you see the range rather than relying on one estimate.
- It lets you gauge your max from a submaximal set, avoiding the injury risk of a true one-rep attempt.
- The estimate works for any lift and any weight unit, since it depends only on reps and load.
- It runs instantly in your browser with no sign-up.
Using the estimate, and lifting safely
Prediction equations differ from one another, and Brzycki is one of several in common use — Epley, Lombardi, and O'Conner among them. They agree closely at low repetitions and diverge as the count rises, so a figure from a set of five is far more trustworthy than one from a set of fifteen. Treat the output as a working estimate to programme from and refine, not a number to attempt.
Reliability also varies by exercise. Compound barbell lifts with long ranges of motion — squat, bench press, deadlift — behave reasonably predictably. Isolation movements and machines often do not, and a true maximum on an exercise you have never loaded heavily is far less certain than the arithmetic suggests.
If you do attempt a heavy single, do it properly: warm up thoroughly with ascending sets, use a competent spotter or safety pins, do not test when tired or unwell, and stop if form breaks down. The risk of injury is concentrated at true maximal loads, which is the main argument for estimating rather than testing. Anyone new to resistance training, returning from injury, or managing a joint, heart, or blood pressure condition should get qualified guidance before working near maximum.
Frequently Asked Questions
How is one-rep max estimated from multiple reps?
Formulas like Epley (weight × (1 + reps/30)) and Brzycki (weight ÷ (1.0278 − 0.0278 × reps)) use the relationship between the load you lifted and how many reps you managed to project the weight you could lift just once. Fewer reps give a more accurate estimate.
Why do Epley and Brzycki give different numbers?
They are different mathematical models fitted to strength data, so they diverge slightly, especially at higher rep counts. Showing both gives you a realistic range rather than a single false-precision figure — your true max likely sits near them.
How many reps should I use for the best estimate?
Estimates are most accurate from sets of around 2 to 6 reps. Beyond about 10 reps, fatigue and technique introduce more error, so the projection becomes less reliable. Use a challenging but clean set in the lower rep range.
Should I actually test my true one-rep max?
Estimating is safer and sufficient for most training purposes. A true max attempt carries injury risk and requires proper warm-up, technique, and ideally a spotter, so many lifters rely on calculated estimates for programming instead.
