With the probable extinctions of the imperial woodpecker and the ivory-billed woodpecker, this species, at long and a weight of , stands as the largest woodpecker in the world. Among standard measurements, the wing chord is , the tail is , the bill is and the tarsus is . This unique-looking woodpecker has several obvious distinctive features: a very long, strong chisel-tipped bill, an elongated neck and a long tail. A slight crest maybe occasionally evident. This species plumage is almost entirely dark grey or blackish slate-grey overlaid with small white spots. The throat is paler grey and males have small red moustache. Normally, the nominate subspecies is the darkest, most slaty gray race. "M. p. harterti" has a more pale throat with a greater amount of whitish feather tips forming small spot and is slightly paler below than the nominate, sometimes appearing almost whitish on the belly. The size and structure readily distinguishes this bird from almost any other species, including other woodpeckers. Occasionally, at first glance, the great slaty woodpecker is mistaken for a hornbill but, obviously, such a resemblance is slight at best.