Estimated lean (fat-free) body mass, using the Boer formula.
The Boer formula: LBM = 0.407×weight(kg) + 0.267×height(cm) − 19.2 for men, or 0.252×weight(kg) + 0.473×height(cm) − 48.3 for women.
Fat mass is simply your total weight minus the estimated lean body mass; fat percentage then divides that fat mass by your total weight.
The Body Fat Calculator uses the US Navy circumference method (neck/waist/hip measurements) to estimate body fat % directly, while this tool uses the Boer formula (height and weight only) to estimate lean mass first, then derives fat mass and percentage from that.