Normal body temperature and how to take it correctly

In short

• A reading above 100.4°F (38°C) usually indicates a fever.
• Normal temperature varies by person, age, activity and time of day.
• One reading is not a diagnosis; how you look and feel matters more than the number.
• A digital thermometer under the tongue or in the armpit is a standard choice.
• Babies under 3 months with a fever need medical advice right away.

What counts as normal

The familiar 98.6°F (37°C) is an average, not a rule for every person. MedlinePlus gives a normal range of about 97°F to 99°F (36.1°C to 37.2°C). Temperature varies with age, activity and time of day, and in adults it is typically lowest in the early morning.

It helps to know your own baseline. Take it a few times on different days when you are well, with the same thermometer and in the same place. When you are sick, you can then see how far the number really departs from your usual.

What counts as a fever

A reading above 100.4°F (38°C) usually indicates a fever, most often from an infection or illness. A fever is not a disease; it usually signals that the body is fighting something. A thermometer is optional, because you can also have a fever when the reading is below the threshold but you feel hot, cold or shivery.

The overall picture counts for more than the figure: whether you are alert, breathing comfortably, drinking, and able to get up and look after yourself. An adult at 101°F who is drinking and talking is usually better off than someone at a lower reading who is confused and cannot stand.

Choosing a thermometer

• Digital thermometer: the standard choice for adults and children, used under the tongue or in the armpit and read when it beeps.
• Other types: NHS advice for adults says not to use glass or infrared thermometers or forehead strips, so follow the leaflet that came with your device.
• Mercury thermometers: they pose hazards from breakage and disposal, and WHO notes that safe mercury-free alternatives exist.

Using one thermometer for the whole household keeps readings comparable. Comparing yesterday's reading from one device with today's from another invites false alarms.

How to take a reading

1. WaitRest indoors for a few minutes first. If a child has just had a bath or been wrapped in a blanket, wait a few minutes before checking.
2. Check the deviceMake sure the tip is clean and the battery works, and read the leaflet for your model.
3. ArmpitPlace the tip inside the top of the armpit and close the arm over it, holding it there for as long as the leaflet says.
4. MouthPlace the tip under the tongue and wait for the beep.
5. Record itWrite down the value, the time and where you measured it. If you doubt a reading, retake it after a short wait.

Mistakes that skew the result

• Checking right after a hot drink, a workout or time in the cold.
• A damp armpit or an arm held loosely against the body.
• Switching thermometers or places from one reading to the next.
• Judging by the back of your hand and concluding there is no fever.
• Checking every few minutes, which raises worry and adds little.

A few readings a day at roughly the same times, plus one more if you clearly feel worse, are enough. Jot down symptoms, what you took and how much you drank, because a few days of notes tell your clinician more than one number remembered from memory.

Home care for a fever

The aim is to keep you comfortable, not to chase the number. Rest, drink plenty of fluids and check that your urine is light yellow. For a very high fever or for discomfort, a pharmacist or provider may suggest over-the-counter acetaminophen or ibuprofen as directed on the label. Stay home and away from others until the fever has gone.

Children with fevers should not take aspirin. NHS advises against aspirin for anyone under 16, and it also advises against giving paracetamol to babies under 2 months or ibuprofen under 3 months.

If a mercury thermometer breaks

EPA advises everyone, including pets, to leave the room, opening windows to the outside and closing doors to other rooms. Never use a vacuum or a broom, because they spread the mercury and raise exposure, and never wash it down a drain. Wearing gloves, remove glass onto a paper towel, gather beads with a squeegee or cardboard, and seal everything in bags. Then contact your local health department or poison control at 1-800-222-1222 for disposal, and ventilate the area to the outside for at least 24 hours.

Red flags and when to get help

Red flags: call 911 for difficulty breathing or breathing very fast, a stiff neck with sensitivity to light or severe headache, a rash that does not fade under pressure, a first seizure, confusion or being very drowsy and hard to wake, or blue, grey or pale skin or lips. Do not wait to retake the temperature.

For babies under 3 months, contact a clinician right away for a temperature of 100.4°F (38°C) or higher, even if the baby looks well. For other children, seek urgent advice if the fever has lasted 5 days or more, the child is dehydrated, is not eating or is not acting like themselves. Adults should call their provider if a fever is not improving with home care or is getting worse, and anyone with a very high temperature that will not come down should be seen.

Questions for your provider

• Which temperature should worry me, given my health conditions?
• How and how often should I check it?
• Which fever medicine suits me, and how much can I take?
• How might my regular medicines affect a fever or my symptoms?
• How many days is it reasonable to wait at home?

Questions and answers

Sources

1. MedlinePlus — Body temperature norms
2. MedlinePlus — Fever
3. NHS — High temperature (fever) in adults
4. NHS — High temperature (fever) in children
5. WHO — Mercury and health
6. EPA — What to do if a mercury thermometer breaks

Sources checked on 29 October 2026.