Males tend to have darker, more conspicuous markings than the females. The dark-eyed junco is long and has a wingspan of . Body mass can vary from . Among standard measurements, the wing chord is , the tail is , the bill is and the tarsus is . Juveniles often have pale streaks and may even be mistaken for vesper sparrows ("Pooecetes gramineus") until they acquire adult plumage at 2 to 3 months, but junco fledglings' heads are generally quite uniform in color already, and initially their bills still have conspicuous yellowish edges to the gape, remains of the fleshy wattles that guide the parents when they feed the nestlings.