Please read first

General information only — not medical advice. This page explains temperature readings; it cannot diagnose you or anyone in your care. For your situation, talk to a doctor or pharmacist — and if someone seems seriously ill, get medical help right away rather than reading a web page.

Fever: What Temperature Counts, and When to Call

One number causes a lot of worry: is this a fever, or not? This page explains what the numbers mean, why the spot you measure changes the number, and the signs that mean it is time to call a doctor.

What is a normal temperature?

The average normal body temperature is about 98.6°F (37°C). But “normal” is not one exact number — it varies from person to person, and it moves across the day.

Your temperature is usually lowest in the early morning.
It is usually highest in the late afternoon or evening.
A healthy person’s temperature can sit a degree or so either side of 98.6°F and still be normal for them.

So a reading a little above or below 98.6°F does not, by itself, mean anything is wrong.

The fever line: 100.4°F

A temperature of

100.4°F (38°C)

or higher is generally considered a fever.

The scale above runs from 97°F to 104°F. Most healthy readings sit near 98.6°F. The fever line used by doctors and nurses is 100.4°F.

Where you measure changes the number

The same body can honestly give different numbers in different spots. That is normal — the spots are not equally close to the body’s core temperature. Here is how each spot usually compares with a mouth reading:

Mouth (oral)

The comparison point for the others

The reading itself

Rectal

Compared with a mouth reading, it usually runs

0.5–1°F higher

Ear

Compared with a mouth reading, it usually runs

0.5–1°F higher

Armpit

Compared with a mouth reading, it usually runs

0.5–1°F lower

Forehead scanner

Compared with a mouth reading, it usually runs

0.5–1°F lower

That is why the spot matters. An armpit reading of 99.6°F and a mouth reading of 100.4°F can be describing the same body at the same moment.

Spot checker: what would this be as a mouth reading?

Pick the spot you used, type in your reading, and this tool works out the rough mouth-equivalent range — so you can compare it fairly with the 100.4°F fever line.

Check a reading

1. Where did you measure?

2. What was the reading?

Type the number in degrees Fahrenheit (°F), just as your thermometer showed it.

Rough mouth-equivalent

This is a rough estimate — different thermometers and technique change the number.

This checker runs on this page only. Nothing you type is saved or sent anywhere.

Take it right: getting a reading you can trust

Before a mouth reading, wait 20–30 minutes after eating, drinking something hot or cold, or smoking. Food and drink in the mouth change the number.
After heavy exercise or a hot bath, wait about an hour before taking a temperature.
Read and follow your thermometer’s own instructions. Each type is used a little differently, and the instructions that came with yours are the ones to follow.
Use a digital thermometer. The American Academy of Pediatrics advises against glass mercury thermometers — the glass can break and mercury is poisonous. A digital thermometer is the standard choice.

When to call a doctor

Babies under 3 months — call right away

A baby younger than 3 months with a rectal temperature of 100.4°F (38°C) or higher — call a doctor right away, the same day. Do not wait to see if it comes down on its own.

Adults and older children

Call a doctor if:

A fever is above 103°F (39.4°C).
A fever lasts more than about 3 days.
There are worrying signs like the ones below — these mean get medical help promptly:

Trouble breathing
Confusion
A stiff neck
A severe headache
A seizure

Trust how the person looks and acts

The number is only part of the picture. Trust how the person looks and acts, not just the number. A person who seems very unwell needs help even if the number is not high — and when in doubt, call. A nurse line or a pharmacist can advise you on what to do next.

One more time, plainly

Sources

MedlinePlus — “Fever” and “Temperature measurement” (U.S. National Library of Medicine encyclopedia and health information).
Mayo Clinic — “Fever” (when to seek care).

Facts checked November 2026.