Normal Body Temperature in Adults: The Real Numbers

Everyone learned 98.6 F in school, so a thermometer reading of 97.5 causes worry in one house and 99.2 causes panic in another. Both are usually normal. Here are the real numbers doctors use, including exactly where fever begins.

Normal is a range that moves#

Healthy adult temperature runs roughly 97 to 99 F (36.1 to 37.2 C) by mouth. It is lowest in the early morning and can sit close to a degree Fahrenheit higher by evening. Exercise, a hot shower, a big meal and ovulation all nudge it up for a while. So a single afternoon reading of 37.3 C means almost nothing; a pattern does.

The 98.6 figure comes from a German study in the 1860s. When Stanford researchers reanalyzed temperatures across 150 years, the modern average landed near 97.9 F (36.6 C), likely because we carry less chronic infection and inflammation than the 19th century did. If you consistently measure 97.4, that is your normal, not a problem.

Where the thermometer goes changes the number#

Site matters by up to a degree. Armpit readings run about 0.5 to 1 F lower than oral; ear and rectal readings run about 0.5 to 1 F higher; forehead scanners sit near oral but wobble with sweat and room temperature. Whatever device you own, use the same one the same way each time, so your comparisons mean something.

The fever line, exactly#

Two groups read the scale differently. Adults over 65 often mount smaller fevers, so a rise of 2 F above their usual baseline counts even under 100.4. And anyone on chemotherapy or immune-suppressing medicines should call about any fever at all, same day. The free fever checker applies these rules to your exact reading in a few seconds.

When the number is not the story#

A fever with a stiff neck, confusion, trouble breathing, chest pain, or a rash that stays when pressed under a glass is an emergency regardless of how high it is. A low-grade temperature that drifts up and down for weeks alongside night sweats, weight loss or swollen lymph nodes is a different problem from any infection, and it needs a doctor visit rather than a thermometer log. When you are unsure which situation is yours, ask a doctor and a written answer comes back within a day.