Temperature Converter
A fever is the body's normal response to infection, but the threshold for concern — and for calling a doctor — depends on a child's age. Convert a temperature reading between Celsius and Fahrenheit below, then see our full fever management guide for when to seek care and how to dose fever-reducing medication.
Fever thresholds by age
- Under 3 months: A rectal temperature of 100.4°F (38.0°C) or higher is a medical emergency — call your pediatrician or go to the ER immediately, even if your baby otherwise seems fine.
- 3–36 months: Call your doctor for a fever of 102.2°F (39°C) or higher, or for any fever lasting more than 24 hours without an obvious cause.
- Over 36 months: Call your doctor for a fever of 104°F (40°C) or higher, or one lasting more than 3 days, or if your child is difficult to wake, refuses fluids, or has a stiff neck, rash, or trouble breathing.
These thresholds are general guidance, not a substitute for medical advice — always contact your child's healthcare provider if you're concerned.
Celsius and Fahrenheit
Celsius and Fahrenheit are simply two different scales for the same measurement, related by a fixed formula, which is what this tool applies. Pediatric offices and thermometers in the United States commonly display Fahrenheit, while most of the rest of the world — and much of the medical literature — uses Celsius, so it's useful to be comfortable converting between the two.
Taking an accurate temperature
How a temperature is taken affects the reading as much as the number itself. Rectal readings are considered the most accurate for infants and young children, oral readings work well once a child can reliably hold a thermometer under the tongue, and temporal (forehead) or tympanic (ear) readings are convenient but slightly less precise. Axillary (armpit) readings tend to run lower than the child's true core temperature and are generally the least precise option. Your pediatrician can advise which method is most appropriate for your child's age.
When a number is concerning
There isn't one universal temperature that means "call the doctor" for every child — the threshold that matters depends heavily on age, with much stricter thresholds for young infants than for older children. Rather than repeat those age-specific numbers here, see our full fever management guide, which walks through when a given reading warrants a call to your pediatrician or a trip to urgent care or the emergency room.
Sources & references
These references informed the guidance on this page. They are provided for transparency and further reading; they are not endorsements, and this page is not a substitute for the advice of your child's own clinician.