Standard deviation (SD) in poker quantifies how dispersed results are around the average winrate. In NL hold’em cash games, typical SD is 80-120 BB/100 hands, meaning even winning players can experience downswings of hundreds of BBs through pure variance. High SD indicates a more aggressive playing style or more all-in situations, while low SD may suggest more passive play or requiring higher equity before committing stacks. Understanding SD is fundamental for correct bankroll sizing: 30-50 buy-ins are recommended for 95% ruin protection based on estimated SD and winrate.