Before knowing whether a baby has a fever, it helps to know what's "normal" — and the answer isn't a single number, it depends on the method you use to take the temperature. Here's the full reference.
Normal temperature by method
| Method | Normal range |
|---|---|
| Armpit (axillary) | 97.7°F – 99.0°F (36.5–37.2°C) |
| Rectal | 97.9°F – 100.4°F (36.6–38.0°C) |
| Ear (tympanic) | 97.9°F – 100.4°F (similar to rectal) |
| Forehead (temporal) | 97.7°F – 99.1°F (similar to armpit) |
Rectal readings usually run 0.5°F to 1.4°F higher than an armpit reading taken at the same time — which is why comparing an armpit value against a threshold meant for rectal readings (or the other way around) causes more confusion than it solves.
How to take it correctly, by method
- Armpit — thermometer snug against the skin, baby's arm pressed against the body, 3 to 5 minutes for glass/mercury ones (digital ones beep when done).
- Rectal — the most accurate under 3 months. Lubricated tip, insert 0.5-1 inch, keep baby still, face up or lying on your lap.
- Ear — quick, but can be less accurate under 6 months due to how small the ear canal is. Line it up properly before measuring.
- Forehead — the fastest and least invasive, good for a first check, though slightly less precise than a well-taken rectal or armpit reading.
So when does that temperature actually become a fever?
Generally speaking, fever starts at 100.4°F (38°C) rectal or 99.5°F (37.5°C) armpit — but the baby's age changes quite a bit what to do with that number. The fever post covers the full detail: when to give fever reducer, when it's fine to just watch, and when to call the pediatrician without waiting.
Checkup log — a real screen from the Alfonso app.
Log every reading, with the method used
Create your baby's profile in 30 seconds, no account or password needed, and note every measurement — to compare same-method readings over time, not stray numbers. Syncs automatically with your partner.
Frequently asked questions
What is a baby's normal temperature?
Between 97.7°F and 99.5°F depending on the method — rectal runs a bit higher than armpit.
Which method is most reliable?
Rectal under 3 months. Armpit is more practical after that age.
Does temperature change during the day?
Yes, a bit higher in the afternoon/evening than in the morning — that's normal.
Keep reading
ℹ️ This article is informational and doesn't replace a pediatrician's judgment. If you're unsure about your baby's temperature or health, always check with a healthcare professional.