Generally, doctors consider a temperature of 100.4°F (38°C) or above to be a fever.
However, several factors can influence the threshold, including:
Individual differencesThe typical body temperature for an adult is around 98.6°F (37°C), although this can vary slightly from person to person. Some people are naturally a little hotter or colder than this.
Any temperature that is persistently higher than usual for a given individual could be a sign of a fever, particularly if they he other symptoms that suggest an infection, such as:
chillsfatiguebody achesheadachecoughstuffy or runny noseburning sensation when urinatingdiarrheaskin discolorationAgeThe temperature that doctors consider a fever for adults and children is often the same, at 100.4°F (38°C).
However, some guidelines state that the threshold for children over 3 months old is slightly higher, at 101.3°F (38.5°C) or more.
Some guidelines also set the threshold a little lower for older adults.
Differences by methodThere are several methods of taking a person’s body temperature with a thermometer, including:
rectallyorallyin the earin the armpiton the foreheadBody temperature measurements can vary by as much as 0.9°F to 1.8°F (0.5°C to 1°C) depending on the anatomic site.
A 2021 review notes that what experts consider a fever in each case varies slightly. A medical textbook, Harrison’s Principles of Internal Medicine, defines it as follows:
MethodHigh temperatureRectal99.5°F to 100.9°F (37.5°C to 38.3°C)Oralmore than 99°F (37.2°C) in the morning or 99.9°F (37.7°C) in late afternoonArmpitmore than 99°F (37.2°C)A rectal temperature is the most accurate measure of core body temperature, but other methods are less invasive and more convenient in many cases.