Rep-max formulas agree closely at low reps and fall apart at high ones — within about five kilos of each other at five reps on a hundred-kilo lift, but seventy-seven kilos apart at twenty.
Each formula is a curve fitted to population data, and the curves are anchored near the heavy end where the measurements were taken. Extrapolating them out to twenty reps takes each one further from its data and in a different direction.
Even at five reps it is a population average, and an individual can sit several kilos either side depending on fibre type and how practised they are at grinding. It is also lift-specific: the fit is far better for a squat or a bench press than for an isolation movement. And the most-quoted formula, Epley, is not calibrated at a single rep at all — it reads about 3% high there.
Estimate with the rule, then check it against the calculator that models it properly.
Open Gym Companion →Rep-max formulas agree closely at low reps and fall apart at high ones — within about five kilos of each other at five reps on a hundred-kilo lift, but seventy-seven kilos apart at twenty. Each formula is a curve fitted to population data, and the curves are anchored near the heavy end where the measurements were taken.