Do You Add a Degree or Subtract a Degree When Taking an Underarm Temperature? states that when taking an underarm temperature, also known as an axillary temperature, a degree must be added to the temperature on the thermometer. The degree must be added because the underarm is outside the body rather than inside, making the underarm's temperature naturally lower. states that a normal axillary temperature is between 96.6 and 98 degrees Fahrenheit or between 35.9 and 36.7 degrees Celsius. KidsHealth explains that body temperatures vary throughout the day, depending on the time of day and the body's level of activity. When an infection or illness is present, however, the hypothalamus causes a body's temperature to increase to fight off the unwanted germs.