Normal Body Temperature in Adults: The Real Numbers

Everyone learned 98.6 F (37.0 C) in school. So a reading of 97.5 F (36.4 C) worries one house and 99.2 F (37.3 C) panics another. Both are usually normal. Adult temperature is a range that moves through the day. Here are the real numbers doctors use, and what your own reading means.

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 hours, around 4am to 6am. It peaks in the late afternoon or early evening, often close to a degree Fahrenheit (0.5 C) higher.

Exercise, a hot shower, a hot drink, a heavy meal, thick clothing and a warm room all nudge it up briefly. In women who are still having periods, it rises about 0.5 to 1 F (0.3 to 0.6 C) after ovulation.

So a single afternoon reading of 37.3 C (99.1 F) means almost nothing on its own. A pattern over a day or two means something.

The 98.6 figure comes from measurements taken in the 1860s. When researchers reanalyzed records spanning more than 150 years, the modern average landed near 97.9 F (36.6 C). We seem to carry less long-running infection than people did then. If you consistently measure 97.4 F (36.3 C), that is your normal.

Age shifts the whole range down. Adults over 65 often run around 0.5 F (0.3 C) cooler and mount smaller fevers. So a rise of 2 F (1.1 C) above an older person's usual baseline matters, even if it never reaches 100.4 F.

Your reading, number by number#

If you came here with one number in mind, the quick lookup is on the question page: is this temperature a fever. This section is the same table with the reasoning behind each band.

Find your number below. These are oral (mouth) readings in a resting adult.

Some specific readings people look up, in plain terms:

• 36.2 C (97.2 F) and 36.5 C (97.7 F) are normal. Both are common first thing in the morning.
• 97.5 F (36.4 C) and 97.8 F (36.6 C) are normal, not a fever. 97.8 F is close to the modern average.
• 37.2 C (99.0 F) is the top of normal, not a fever.
• 37.3 C (99.1 F) and 37.4 C (99.3 F) are raised but below the fever line.
• 37.8 C (100.0 F) and 37.9 C (100.2 F) sit just under fever. Recheck in an hour.
• 38.0 C (100.4 F) is the start of fever. 101.5 F (38.6 C) is a normal-sized fever for an ordinary infection.

Where you measure changes the number#

Site matters by up to a full degree Fahrenheit. A reading is only useful when you know where it came from.

Forehead scanners drift with sweat, makeup, hair, a cold room or coming in from outside. Ear readings are thrown by wax and a poor probe angle. For oral readings, wait 15 to 30 minutes after eating, drinking or smoking.

What actually raises a temperature, most common first#

Fever is not a malfunction. The brain deliberately resets the target higher, because many germs cope badly with heat.

1. Viral infections. Colds, flu, COVID and stomach bugs cause most adult fevers. They usually peak in one to three days and settle by day five.
2. Bacterial infections. Urine, chest, skin, dental and sinus infections. These tend to keep climbing rather than settle.
3. Recent vaccination. A temperature up to about 38.5 C (101.3 F) for a day or two after a vaccine is a normal immune response.
4. Heat, not illness. Hot weather, hot work, heavy clothing and dehydration raise core temperature without infection.

See our guide to heat stress and hydration at work. 5. Medicines. Some antibiotics and seizure medicines cause drug fever, often a week or two after starting. 6. Inflammatory conditions. Rheumatoid arthritis, inflammatory bowel disease, thyroid overactivity and vasculitis can run a low-grade temperature. 7. Clots. A clot in a leg vein or lung can cause a modest fever with breathlessness or calf pain. 8. Cancers. Uncommon, but lymphoma and kidney cancer are known for fevers with night sweats and weight loss.

Low readings have their own list. Cold surroundings and poor technique explain most. Real low body temperature comes from cold exposure, an underactive thyroid, alcohol, some sedating medicines, and severe infection in frail older adults.

What a thermometer cannot tell you#

This is the part people miss. A number gives you one fact and no more.

• It cannot say whether an infection is viral or bacterial, so it cannot say whether antibiotics would help.
• Height does not track seriousness. Mild viruses reach 39 C (102.2 F). Serious sepsis sometimes causes no fever at all.
• It cannot say where the problem is. Symptoms do that: cough, burning urine, sore throat, a hot swollen limb.
• A normal reading does not rule out infection, especially in older adults and people on steroids or chemotherapy.
• It cannot say whether you are still contagious.

How you feel and what else is happening matter more than the digits. A confused, breathless person at 38.2 C (100.8 F) worries a doctor more than a comfortable person at 39.1 C (102.4 F).

What a doctor does next#

For a fever with an obvious cause in a well-looking adult, often nothing but advice. When the picture is less clear, the usual sequence is:

• Questions about how long, how high, travel, animals, new medicines and contact with illness.
• An examination looking for a source: throat, ears, chest, abdomen, skin, joints.
• Simple tests where needed: a urine dipstick, a full blood count, and inflammation markers such as CRP.
• A chest x-ray, blood cultures or swabs if the fever is high, persistent, or the source is unknown.

A temperature that comes and goes for more than two or three weeks has its own name: fever of unknown origin. It gets a wider search, including imaging and swollen lymph node assessment.

At work#

Occupational medicine treats temperature as two separate questions, and mixing them up causes trouble.

The first is heat, not illness. A worker in a hot plant, in coveralls or a respirator, can reach 38 C (100.4 F) from the job alone. The clues are heavy sweating then none, cramps, headache and a fast pulse in someone who felt well that morning. That is heat strain. It needs cooling, shade and fluids, not fever medicine.

The second is infection, where the question is fitness to be around others. A fever of 38.0 C (100.4 F) with cough, vomiting or diarrhea is a reason to stay home. That matters most in food handling, healthcare and shared vehicles.

Entrance temperature screening is a weak filter. It misses people who are early, medicated or infectious without fever, and flags people who walked in from the sun. It prompts questions rather than answering them.

Night shift flips the daily curve. If you sleep by day, your low point moves. Compare a night worker's reading against their own recent baseline.

What I see most often#

Three patterns come up again and again.

The first is the person who arrives worried about 37.4 C (99.3 F), measured with a forehead scanner, after rushing to the appointment. Sat down for ten minutes and measured by mouth, it reads 36.9 C (98.4 F).

The second is the older adult brought in confused and off their food at 37.6 C (99.7 F). Their usual is 36.2 C (97.2 F). That is a real fever for them, often a urine or chest infection.

The third is the healthy adult who checks six times an hour and watches the number wander by half a degree. Temperature is meant to wander. Check twice a day when unwell and note how you feel. A low-grade temperature that drifts on with ongoing tiredness and weight loss needs a visit, not a longer log.

When this needs a doctor now#

Get emergency care for any of these, whatever the number says:

• A stiff neck, severe headache, or a new rash that does not fade when pressed under a glass.
• Confusion, unusual drowsiness, or a seizure.
• Trouble breathing, chest pain, or blue lips.
• A temperature of 105 F (40.6 C) or above, or below 95 F (35 C) with drowsiness or shivering.

Contact a doctor the same day for a temperature of 104 F (40 C) or above. Call the same day for any fever while on chemotherapy or immune-suppressing medicine. The same goes for fever with severe abdominal pain, back pain with burning urine, or a hot swollen limb. Adults over 65 with a new fever, and pregnant women with a fever, should also call.

Book a routine appointment for a fever lasting more than three days, or one that settles then returns. Book one too for a low-grade temperature that drifts on for weeks with night sweats or weight loss. The free fever checker applies these rules to your exact reading. You can also ask a doctor for a written answer within a day.

The bottom line#

Normal adult temperature is a moving range of about 97 to 99 F (36.1 to 37.2 C), not a single figure. Readings such as 36.2, 36.5 and 37.2 C are normal. 37.3 to 37.9 C is raised but not yet fever. Fever begins at 100.4 F (38.0 C).

It needs a doctor the same day at 104 F (40 C), and is an emergency at 105 F (40.6 C). Where you measure shifts every one of those numbers, so keep one device and watch the trend. The symptoms sitting alongside the reading decide what happens next.