Falco biarmicus is a medium-sized falcon, ranging from 35 to 50 cm long with a wingspan of 90 to 110 cm. The females are heavier, weighing from 700 to 900 g, whereas the males typically weigh from 500 to 600 g. The backs of adult lanner falcons are slate gray, juveniles are brown; both adults and juveniles have off-white or reddish-brown undersides streaked with gray. Northern subspecies have undersides spotted with black; southern subspecies lack spotted undersides. The head is reddish-brown or white with a black 'moustach' stripe. Females typically have darker coloration than males.

Range mass: 500 to 900 g.

Range length: 35 to 50 cm.

Range wingspan: 90 to 110 cm.

Sexual Dimorphism: female larger; sexes colored or patterned differently