Body temperature isn’t a set number — here’s what is considered normal and how to know if you have a fever

GettyImages 1144947713
Normal body temperature generally ranges from 97°F to 99°F, while a fever is at least 100.4°F. Boris Zhitkov/Getty Images
  • Normal body temperature can range between 97°F and 99°F — 98.6°F isn’t the only normal reading.
  • A range is more accurate because body temperature is affected by many factors, including time of day.
  • A fever is considered any temperature over 100.4°F, which is your body’s way of fighting an infection.
  • Visit Insider’s Health Reference library for more advice.

For over a century, normal human body temperature has been considered 98.6°F (37°C). This number was established in 1871 by German physician Carl Wunderlich, who determined the average by testing millions of patients with what was a new instrument at the time: the thermometer. 

Since then, however, researchers have observed that normal body temperature varies from person to person and depends on gender, age, and time of day, among other factors. 

Here’s what you need to know about your body temperature, when it’s considered higher than normal, and how to take it properly.

A normal body temperature can range from 97°F to 99°F

The truth is, there is no one exact “normal” body temperature, and depending on your age, the time of day, and how active you are, it’s more accurate to describe a normal body temperature as a range.

“Temperature can vary between individuals, where some members of a family are consistently warmer than others,” says Charles Brantly, MD at Central Health. “This is not necessarily a bad thing. The normal range for the vast majority of people is between 97°F (36.1°C) and 99°F (37.2°C).”

While normal body temperatures for adults range between 97°F and 99°F, the spectrum is slightly different for children and older adults.

“On average, children tend to be slightly warmer than adults, and those over [the age of] 65 are cooler,” says Brantly. “This is generally a reflection of a faster metabolism in those of a younger age … Exercise, hydration status, and clothing will all affect your day time temperature as well.”

Other factors that may affect your body temperature

While core body temperature for men and women is roughly the same, women generally have a lower skin temperature because of their higher percentage of body fat. Brantly says that women can also have varying temperatures during their monthly menstrual cycle.

Chawapon Kidhirunkul, MD at BDMS Wellness Clinic, also says that time of day can impact your temperature. “Our temperature drops at night during sleep and increases over the day,” Kidhirunkul says. “The lowest temperature is at around 4 a.m., and the highest peak at 5 p.m.” 

This rise in temperature is due to increased cortisol — the stress hormone — in the body as we move through the day. Kidhirunkul adds that another factor can be food, which usually increases body temperature slightly after a meal. 

So the next time you reach for a thermometer, keep in mind that it is natural for your temperature to fluctuate between 97°F and 99°F, depending on your particular circumstances.

What temperature indicates a fever 

High body temperature is one of the first symptoms of illness, and a fever is an indication that your body is fighting off an infection, like the flu virus. According to Cleveland Clinic, temperatures above 100.4°F are considered a fever for adults. 

But for children, Cleveland Clinic says fever indications can vary depending on how you take the reading: 

  • 99.5°F or higher for oral temperature
  • 100.4°F or higher for rectal temperature 
  • 99°F or higher for underarm temperature

Fevers are often associated with other symptoms like chills, headaches, tiredness, body aches, and sweating. 

If you’re feeling feverish, Brantly advises measuring your temperature and recording it several times a day, as thermometers aren’t always accurate and your temperature may rise and fall throughout the day. 

While most fevers usually resolve themselves within a week, there are certain steps you can take to relieve discomfort, such as staying hydrated, dressing in lightweight clothing, and getting plenty of rest.

How to take your temperature 

There are several options when taking your temperature, such as armpit, forehead, mouth, ear, and rectal methods, according to Kidhirunkul. 

For the most inexpensive and easiest ways to get a temperature reading, a digital thermometer can take a temperature three ways

  • Oral: The thermometer is placed under the tongue — a method best for adults and young children.
  • Rectum: For babies or adults who cannot hold the thermometer under their tongue, this method involves gently inserting the thermometer into the rectum. 
  • Armpit: The axillary method is another option for anyone who cannot tolerate oral temperature. The thermometer is placed under the armpit. 

The most common and easiest method of taking your temperature is in the mouth, however, Kidhirunkul says rectal temperatures are the most accurate. When it comes to a professional setting, Brantly notes that most hospitals and clinics rely on ear thermometers, though these are more expensive.

Insider’s takeaway 

Normal body temperature can range between 97°F and 99°F. Many factors can affect body temperature, from your age to the time of day. Overall, if you have a temperature higher than 100.4°F, it’s likely that you have a fever.