The tail is usually shorter than the head and body and is never much longer. A series of adult C. laucha from the Mato Grosso of Brazil weighed about 30-38 grams. The pelage usually is not very thick, but C. lepidus has soft fur. Coloration of the upper parts is buff, tawny, dark brown, grayish buff, or grayish. The underparts are usually grayish or white.
Some species have a small white patch behind each ear.
The tail is dark above and paler below in some forms and all white in others. The body form is mouselike. The ear is prominent, the hands and feet are narrow, and the tail is moderately haired. The low-crowned cheek teeth distinguish Calomys from related genera. Injuries of the tail in the form of a missing part or damaged skin are fairly common in Calomys and are probably caused by predators.