How to Check for a Fever: Thermometer Tips and When to Worry
How to take an accurate temperature with each kind of thermometer, what counts as a fever by age, and the signs that mean it is time to call a doctor.
Tom Gallagher
30 de junio de 2026 · 2 min de lectura
Someone at home feels warm, you grab the thermometer, and three tries give you three different numbers. Most of that comes from how the reading was taken, not from the thermometer.
Below: how to take each reading properly, what counts as a fever and when to call a doctor.
Which method fits which age
Forehead scanners have taken over medicine cabinets, and I'd bet they stay, because they are fast. Mercury thermometers should go: they can break and release toxic vapor.
| Method | Age | Note |
|---|---|---|
| Rectal | Babies, especially under 3 months | Most accurate |
| Oral | 4 years and up | Good for adults |
| Ear | 6 months and up | Fast |
| Forehead | Any age | Easy to throw off |
| Armpit | Any age | Least accurate |
How to get a reading you can trust
- Oral: wait 30 minutes after a hot or cold drink. Tip under the tongue toward the back, lips closed.
- Rectal: a little petroleum jelly, then in about half an inch for babies under 6 months, up to an inch for older ones. Stop at any resistance.
- Ear: for children over 1, pull the ear gently back and up.
- Forehead: follow the maker's directions. Direct sun or cold outdoor air can skew it.
Hidden tip: the method changes the number. Rectal and ear readings usually run about 1°F higher than oral, forehead about 1°F lower. Track a fever with one method and note which.
Hidden tip: after coming in from the cold, wait 15 minutes before an ear reading. And label the rectal thermometer so it never ends up in a mouth.
What counts as a fever
Generally 100.4°F (38°C) or higher. Normal temperature runs lower in the morning and higher in the evening, so one warm reading is not a crisis.
When to call a doctor
- Under 3 months: 100.4°F (38°C) rectal or higher. Call right away.
- 3 to 6 months: 101°F (38.3°C) or higher.
- Older than 6 months: 103°F (39.4°C) or higher, or a child who seems very ill.
- Adults: above 103°F, or a fever lasting beyond three or four days.
- Any age, get help now: fever with trouble breathing, chest pain, stiff neck, severe headache, a rash that doesn't fade when pressed, or a seizure.
Behind the counter, the question I heard most was "is this number bad?" I'd tell anyone: watch the person, not just the number.
Save this: fever cheat sheet
| Point | Remember |
|---|---|
| Fever | 100.4°F (38°C) or higher |
| Babies | Rectal |
| Oral / ear | From 4 years / 6 months |
| Before oral | 30 min after drinks |
| Offsets | Ear, rectal ~1°F up; forehead ~1°F down |
| Call now | Under 3 months at 100.4°F |
| Call soon | 3 to 6 months 101°F; older 103°F |
| Any age | Breathing trouble, stiff neck, rash, seizure |
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