A one-rep max (1RM) is the heaviest weight you can lift once with full range of motion. This one rep max calculator estimates it from a set you have already done: enter the weight and the reps, and it returns your 1RM with the equation that produced it, the equation's source and how far the seven published equations disagree. It then converts that number into training loads rounded to real plates.
Every estimate is computed by an equation, and the calculator names it. The default is Brzycki (1993), the only common equation this site has verified against its original paper; it is valid for sets under 10 reps. The other six (Epley, Lombardi, Mayhew, O'Conner, Wathen and Lander) run on the same input so you can see the spread. If you want to know which 1RM formula to use for your lift and rep range, the formulas page prints each one with its source and validation evidence.
Two rules keep the estimate inside published limits. Reynolds, Gordon and Robergs (2006, J Strength Cond Res 20(3):584–592) found that no more than 10 reps should be entered into these equations and that a 5-rep set predicted best, so results from 11 to 20 reps carry a warning and anything above 20 is not estimated. And at 1 rep the calculator returns the weight you lifted, because five of the seven equations would otherwise return a number higher than the lift itself.
An estimated 1RM converts to training loads through percentages of your 1RM. The table under the result uses the NSCA Training Load Chart (reps 1–10 and 12; the chart has no 11-rep column, so the calculator does not invent one) and rounds each load to the nearest weight you can put on a 20 kg or 45 lb bar, ties rounding down. The training max line is 90% of the estimate, the figure Wendler's 5/3/1 article bases its calculations on, with 85% as an option.
An estimate has an error margin, and it differs by lift. LeSuer et al. (1997, J Strength Cond Res) reported that all seven equations underestimated the deadlift, that all but two differed from the measured bench press 1RM, and that all but one differed from the measured squat 1RM. The calculator shows the lift note beside the result. The spread across the seven equations is the quickest read of how accurate a 1RM estimate is for your input. Engine-generated example: 100 kg × 5 reps gives estimates from 112.50 kg (O'Conner) to 119.01 kg (Mayhew), a spread of 6.51 kg; the same 100 kg × 10 reps spreads from 125.00 kg to 134.75 kg (Wathen), 9.75 kg.
If the number matters (a meet opener, a programme entry) you can confirm it by testing your 1RM directly with the NSCA protocol instead of estimating it.
Written and reviewed by Rohith (@rohith_the_coach). Calculators built by Babji Kittu (@babji_kittu). Published 12 Oct 2026. Every constant on this page is listed with its source on the formula sources and constants page.