One rep max calculator
Your one-rep max (1RM) is the most you can lift for a single rep, and you can estimate it without ever testing it. FitCalcs takes a set you can already do, a weight and the reps you got, and averages the Epley and Brzycki formulas. It then gives a percentage table so you can pick training loads: most strength work sits at 80 to 90 percent of your 1RM. The estimate is most accurate at ten reps or fewer. This is general information, and you should never test a true 1RM without a spotter and a proper warm-up.
How this is worked out
Your one-rep max (1RM) is estimated from a set you can already do, using two well-known formulas. FitCalcs shows the average of both:
Epley: 1RM = weight x (1 + reps ÷ 30)
Brzycki: 1RM = weight x 36 ÷ (37 − reps)
Both are most accurate at 10 reps or fewer; above that, estimates drift apart and read high. Use the percentage table to set training loads. The two formulas were published by Epley (1985) and Brzycki (1993).
Informational only. Never test a true 1RM without a spotter and a proper warm-up. Editorially reviewed by FitCalcs, with each figure citing its source.
Common questions
How do I estimate my one-rep max without testing it?
You take a set you can already do, then work backwards from the weight and reps. FitCalcs uses a weight and the number of clean reps you managed with it and averages the Epley and Brzycki formulas to estimate the most you could lift once. It saves you attempting a true maximal single, which carries more injury risk.
What are the Epley and Brzycki formulas?
They are two standard equations that turn a weight-and-reps set into a one-rep max. Epley is weight times (1 plus reps divided by 30). Brzycki is weight times 36 divided by (37 minus reps). FitCalcs averages the two, which smooths out the small differences between them. They were published by Epley in 1985 and Brzycki in 1993.
How many reps give the most accurate estimate?
Ten reps or fewer. Both formulas are most reliable in that range, and above about ten reps the estimates drift apart and tend to read high, because endurance and pacing start to matter more than pure strength. For the best estimate, use a set where you stopped within a rep or two of failure at ten reps or less.
What percentage of my one-rep max should I train at?
Most strength work sits at about 80 to 90 percent of your one-rep max, which the percentage table on this page works out for you. Lighter loads build muscle and technique, heavier loads build maximal strength. Never test a true one-rep max without a proper warm-up and a spotter or safety bars.
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Editor, FitCalcs
Oliver Mackman is the named editor of FitCalcs and a director of Best Business Loans Ltd (Companies House 16833937), the company that operates the site. He is accountable for its editorial standards, sourcing and corrections, and requires every result to cite the underlying equation and the named UK source behind it, with health tools carrying the appropriate not-medical-advice caveats.
Last reviewed: 12 June 2026