Reconstructing the body mass of a fossil animal is an essential step toward understanding its palaeoecological role. Length × width (L×W) of the first lower molar (m1) is frequently used as a proxy for body mass in fossil mammals. However, among rodents, Muroidea have no premolar and an elongated m1, whereas other groups have a premolar and a m1 that is not elongated. This leads to an overestimation of body mass in muroids and/or an underestimation in other rodents. To solve this problem we assembled data of upper and lower tooth row length and body mass in extant rodents, and calculated regression equations for all rodents, rodents with premolars, rodents without premolars and for taxonomic groups at superfamily or family level. Data for complete tooth rows in fossil rodents are scarce, so we took the sum of the lengths of the (three or four) cheek teeth as an approximation of tooth row length. We estimate body mass of the fossil rodents, using the regression equations of the extant taxa.