If you've ever pulled a "one-rep max" estimate from a spreadsheet or calculator app, there's a good chance one of two formulas was running under the hood: Epley (1985) or

Brzycki (1993). They look similar on paper but give different answers under fatigue. Which matters when you're programming percentages.

## The formulas

Epley:

1RM = weight × (1 + reps / 30)