Your one rep max (1RM) is the maximum weight you can lift for exactly one repetition with good form for any given exercise. Knowing your 1RM is essential for structuring strength training programs — most programming prescribes work as a percentage of 1RM (e.g., '5 sets of 5 at 80% 1RM'). This guide explains how to calculate your estimated 1RM from submaximal lifts without actually attempting a dangerous true max effort.