What temperature is a fever? Check a reading in seconds
This fever temperature checker tells you whether a thermometer reading counts as a fever, taking into account where it was measured and the age of the person. Type the number in °C or °F, choose ear, forehead, mouth, armpit or rectal, pick an age band from newborn to adult, and it shows the reading in both units, the fever line for that method, and plain guidance on when to call a doctor.
It matters because the fever range is not one number. An armpit reads lower than the body’s core and a mouth reading sits in between, so 37.5 °C is a fever under the arm but not in the ear. And for babies, the same reading can mean “call a doctor now”. This page is information only, not a diagnosis; if you are worried about someone, get medical advice whatever the thermometer says.
How to use the Fever Temperature Checker
- Take the temperature as the thermometer’s instructions say, then type the reading.
- Choose °C or °F. If the number looks like the other unit, the checker says so.
- Choose how it was measured — rectal, ear, forehead, mouth or armpit.
- Choose the age band. Babies under 3 months and 3 to 6 months have their own rules.
- Read the result, any urgent warning in red, and the emergency signs below it.
Fever Temperature Checker features
Method-specific fever lines
Rectal, ear and forehead 38.0 °C (100.4 °F); mouth 37.8 °C (100 °F); armpit 37.2 °C (99 °F).
Baby rules built in
Under 3 months with 38 °C or more, and 3 to 6 months with 39 °C or more, are flagged as urgent.
Celsius and Fahrenheit
Every reading is shown in both, so a US thermometer and a UK leaflet can be compared.
Method checks
Warns when an ear reading is used under 6 months or a mouth reading under 4 years.
Emergency signs
The symptoms that need 999 or A&E, whatever the temperature.
Fever temperature range by method
Fever lines from Seattle Children’s Hospital (reviewed May 2025); ages for each method from the American Academy of Pediatrics. The NHS simply treats 38 °C or more as a high temperature in children and adults. This checker also labels 39 °C or more as a high fever and 40 °C (104 °F) or more as very high, and below 35 °C (95 °F) as a low body temperature — hypothermia — which needs urgent help. To convert any other reading, use the temperature converter.
| Method | Fever from | In °F | Best for |
|---|---|---|---|
| Rectal (bottom) | 38.0 °C | 100.4 °F | Babies and toddlers; the most accurate |
| Forehead (temporal artery) | 38.0 °C | 100.4 °F | Any age |
| Ear (tympanic) | 38.0 °C | 100.4 °F | 6 months and older |
| Mouth (oral) | 37.8 °C | 100 °F | 4 years and older |
| Armpit (axillary) | 37.2 °C | 99 °F | Any age, to screen; confirm another way |
Fever temperature for babies and kids: when to get help
Keep a record of readings and doses with the medication schedule maker, and check routine jabs with the vaccination schedule.
- Under 3 months: 38 °C (100.4 °F) or higher — contact a doctor or NHS 111 straight away, even if the baby seems well.
- 3 to 6 months: 39 °C (102.2 °F) or higher — get urgent advice.
- Under 2 years: call the doctor if a fever lasts more than 24 hours. 2 years and older: if it lasts more than 3 days. The NHS advises help for any high temperature lasting 5 days or more.
- Any age: 40 °C (104 °F) or higher, especially if it keeps coming back, or a child who is not feeding, not acting as usual or showing signs of dehydration (fewer wet nappies, no tears, a dry mouth).
Getting an accurate reading
- Wait 20–30 minutes after a hot or cold drink, a bath or exercise before an oral reading.
- For an armpit reading, the armpit must be dry and the tip covered by skin; if it reads 37.2 °C (99 °F) or more, confirm with another method.
- Forehead strips and feeling the forehead are not reliable. Digital thermometers are; glass mercury thermometers should not be used.
- Fever is the body fighting an infection and is not dangerous in itself; how the person looks and behaves tells you more than the exact number.
Fever Temperature Checker FAQ
What temperature is a fever?
In the ear, on the forehead or rectally, 38.0 °C (100.4 °F) or higher. By mouth, 37.8 °C (100 °F). Under the arm, 37.2 °C (99 °F). The NHS uses 38 °C as the general line.
What is the fever temperature for a baby?
The same 38 °C (100.4 °F) rectal or forehead line, but any fever in a baby under 3 months needs a doctor straight away, and 39 °C or more at 3 to 6 months needs urgent advice.
What is the fever temperature range for kids?
38–38.9 °C (100.4–102 °F) is a fever, 39–39.9 °C (102.2–103.8 °F) a high fever and 40 °C (104 °F) or more very high, measured rectally, in the ear or on the forehead.
Is 99 °F a fever?
Under the arm, yes — 99 °F (37.2 °C) is the armpit fever line, so confirm it another way. By mouth, ear or forehead, 99 °F is normal.
Should I add a degree to an armpit reading?
No. Use the armpit line of 37.2 °C (99 °F) instead, as this checker does, rather than adjusting the number.
Can this fever temperature checker diagnose an illness?
No. It is information only. Seek medical advice if you are worried, and call emergency services for any of the emergency signs listed with the result.