In general, Border Collies are medium-sized dogs without extreme physical characteristics and with a moderate amount of coat, which means not much hair will be shed. Their double coats vary from slick to lush, and come in many colors, although black and white is the most common. Black tricolor (black/tan/white or sable and white), red (chocolate) and white, and red tricolor (red/tan/white) also occur regularly, with other colors such as blue, lilac, red merle, blue merle, brindle and "Australian red"/gold seen less frequently. Border Collies may also have single-color coats.

Eye color varies from deep brown to amber or blue, and occasionally eyes of differing color occur. (This is usually seen with "merles"). The ears of the Border Collie are also variable — some have fully erect ears, some fully dropped ears, and others semi-erect ears (similar to those of the rough Collie or sight hounds). Although working Border Collie handlers sometimes have superstitions about the appearance of their dogs (handlers may avoid mostly white dogs due to the unfounded idea that sheep will not respect a white or almost all white dog), in general a dog's appearance is considered by the American Border Collie Association to be irrelevant. It is considered much more useful to identify a working Border Collie by its attitude and ability than by its looks.

Dogs bred for showing are more homogeneous in appearance than working Border Collies, since to win in conformation showing they must conform closely to breed club standards that are specific on many points of the structure, coat and colour. Kennel clubs specify, for example, that the Border Collie must have a "keen and intelligent" expression, and that the preferred eye colour is dark brown. In deference to the dog's working origin, scars and broken teeth received in the line of duty are not to be counted against a Border Collie in the show ring.

Height at withers: Males from 19 to 22 in (48 to 56 cm), females from 18 to 21 in (46 to 53 cm).