Baby Fever Calculator (Is It a Fever and When to Call the Doctor)
Check whether your baby or toddler has a fever from the temperature, the measuring method and age, and see when to manage at home, call the doctor or seek care now (AAP and NICE).
How it works
Fever if: rectal, ear, forehead or mouth ≥ 38.0 °C (100.4 °F); armpit ≥ 37.2 °C (99 °F) · Under 3 months with fever → care now · 3–6 months ≥ 38.3 °C (101 °F), over 6 months ≥ 39.4 °C (103 °F), fever > 24 h under 2 years, > 3 days at 2+ years or ≥ 5 days → call the doctor
The American Academy of Pediatrics treats 38.0 °C (100.4 °F) or higher as a fever for rectal, ear, forehead and mouth readings, with a lower cut-off of about 37.2 °C (99 °F) for armpit readings. Age changes how serious it is: any fever in a baby 3 months or younger needs prompt medical assessment, the AAP advises calling the doctor at 38.3 °C (101 °F) or more at 3–6 months and 39.4 °C (103 °F) or more after 6 months, and NICE guideline NG143 treats fever lasting 5 days or more as a warning sign. How your child looks and behaves matters more than the number — a playful child with 39 °C may be less worrying than a floppy child with 38 °C.
Worked example
An 8-month-old has a rectal temperature of 38.5 °C (101.3 °F) for 1 day. The rectal threshold is 38.0 °C, so the result is Fever; she is over 6 months, below 39.4 °C and the fever is under 24 hours, so you can manage at home and watch for warning signs.
Sources: American Academy of Pediatrics (HealthyChildren.org) — Fever and your baby · NICE guideline NG143 (2019, updated 2021) Fever in under 5s: assessment and initial management
Frequently asked questions
What temperature is a fever in a baby?
A rectal, ear, forehead or mouth temperature of 38.0 °C (100.4 °F) or higher. Armpit readings run lower, so about 37.2 °C (99 °F) or more counts as a fever — confirm with a rectal reading in a baby.
When is a fever an emergency in a baby?
Any fever of 38.0 °C (100.4 °F) or more in a baby under 3 months needs medical care right away, even if the baby seems well. At any age, get urgent help for breathing difficulty, a non-blanching rash, stiff neck, seizure, unusual drowsiness or dehydration.
Which thermometer is best for babies?
A digital rectal thermometer is the most accurate under 3 months. From about 3–6 months ear and forehead thermometers are acceptable, and from age 4 an oral reading is fine. Armpit readings are easy but the least accurate.
Should I give medicine for every fever?
No. Fever helps fight infection. Treat it with paracetamol (acetaminophen) or, from 6 months, ibuprofen only if your child is uncomfortable, using the weight-based dose on the package or from your pharmacist.
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This calculator gives general guidance for healthy children and is not medical advice. If you are worried about your child, or they seem unwell, contact your pediatrician, a nurse line or emergency services.