What's a Normal Body Temperature, and Why Does It Depend Where You Measure?

There’s no single normal body temperature. The 37 °C (98.6 °F) most people learned comes from the 19th century, and modern measurements put the average adult’s oral temperature a little lower, at about 36.6 °C (97.9 °F). Your temperature also differs by where it’s measured, the time of day, and from one person to the next, so a reading only makes sense alongside its site and your own usual values.

This article is general information, not medical advice.

What Studies Find

• The average is lower than 37 °C. Obermeyer and colleagues analysed 243,506 temperatures from 35,488 healthy outpatients and found a mean oral temperature of 36.6 °C, with 95% of readings between 35.7 °C and 37.3 °C. Temperature was lower in older people and higher in women, and varied over the day.
• It may have fallen over time. Protsiv and colleagues compared US records from the 1860s to the 2010s and found that mean body temperature has dropped by about 0.03 °C per birth decade, possibly from lower rates of chronic infection and changes in metabolism.

Each Site Reads Differently

Core temperature, inside the body, is what matters, and each site approximates it differently:

Site	Compared with an oral reading
Rectal	Usually higher; the closest to core temperature
Ear (tympanic)	Often a little higher; sensitive to technique and earwax
Forehead (temporal artery)	Varies with sweat and the room’s temperature
Mouth (oral)	The usual reference; affected by recent drinks and breathing through the mouth
Armpit (axillary)	Usually lower, and the least reliable

A systematic review by Sund-Levander and colleagues found the normal range was different at each site. And a meta-analysis by Niven and colleagues found that thermometers at the ear, forehead, mouth, and armpit often differed from core temperature by more than 0.5 °C, and missed about a third of fevers.

What Counts as a Fever

A temperature of 38.0 °C (100.4 °F) or higher is the threshold most commonly used for a fever in adults. Because older adults run cooler, a smaller rise can matter for them. A fever with a stiff neck, confusion, a rash, trouble breathing, or one that lasts several days needs medical attention.

Taking a Temperature You Can Compare

1. Use the same thermometer at the same site each time.
2. For an oral reading, wait 15 to 30 minutes after eating, drinking, or smoking, and keep your mouth closed with the tip under your tongue.
3. Follow the thermometer’s own instructions for position and time.
4. Compare readings taken at about the same time of day, since temperature is lowest in the early morning and highest in the late afternoon.

Tracking your cycle uses a separate measurement, basal body temperature, taken on waking; see What Can Basal Body Temperature Tell You About Your Cycle?

Following It in Kaizora

Kaizora keeps a metric for each site on the Vitals page: Oral Temperature, Tympanic Temperature, Temporal Artery Temperature, Axillary Temperature, and Rectal Temperature, so readings from different sites are never averaged together. Body Temperature is for readings whose site isn’t known, as some apps export them. Log a reading in °C or °F; it’s stored exactly, whichever you choose.

The Short Version

• The average adult’s oral temperature is about 36.6 °C, not 37 °C, and varies from person to person.
• Rectal reads highest and armpit lowest; compare readings from the same site.
• 38.0 °C (100.4 °F) or more is the usual threshold for a fever.

Sources

• Obermeyer Z, Samra JK, Mullainathan S, “Individual differences in normal body temperature: longitudinal big data analysis of patient records,” BMJ 359 (2017): j5468. doi:10.1136/bmj.j5468
• Protsiv M et al., “Decreasing human body temperature in the United States since the Industrial Revolution,” eLife 9 (2020): e49555. doi:10.7554/eLife.49555
• Sund-Levander M, Forsberg C, Wahren LK, “Normal oral, rectal, tympanic and axillary body temperature in adult men and women: a systematic literature review,” Scandinavian Journal of Caring Sciences 16, no. 2 (2002): 122–128. doi:10.1046/j.1471-6712.2002.00069.x
• Niven DJ et al., “Accuracy of peripheral thermometers for estimating temperature: a systematic review and meta-analysis,” Annals of Internal Medicine 163, no. 10 (2015): 768–777. doi:10.7326/M15-1150