An adult dog's normal body temperature can range from 101 - 102.5° Fahrenheit (38.3 - 39.2° Celsius). A temperature of less than 99° F (37.2° C), or above 103.5° F (39.7° C), is considered to be dangerous and requires immediate veterinary care.
A 2014 study which compared axillary (armpit) temperatures with rectal temperatures in dogs and cats found a large gradient between the two, and concluded that, "axillary temperature should not be used as a substitute for rectal temperature."
Digital thermometers can be placed in a dog's armpit to provide an approximate body temperature, though axillary (armpit) temperatures are not considered to be as accurate as rectal or ear measurements.