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 – 100.2°F (36.6–37.9°C); 100.4°F (38°C) or higher is a fever |
| Ear (tympanic) | 97.9°F – 100.2°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, use a digital thermometer and wait for the beep (mercury ones are not recommended).
- 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.
For quick reference, these are the values from which a fever is generally considered, depending on how you measure:
| Method | Considered a fever from |
|---|---|
| Rectal | 100.4°F (38°C) |
| Ear (from 6 months) | 100.4°F (38°C) |
| Forehead | 100.4°F (38°C) |
| Armpit | 99.5°F (37.5°C) (some guidelines use a slightly lower value) |
These are general references: if your pediatrician gave you different numbers for your baby, follow theirs.
If your baby is under 3 months
At this age the rule is simpler and stricter: any temperature of 100.4°F (38°C) or higher means calling your pediatrician right away or going to the ER, even if your baby looks fine and is feeding. Don't wait to see how it goes, and don't give any medicine without medical advice.
At any age, get help without waiting if, besides the temperature, your baby seems listless, is hard to wake, refuses feeds, has fewer wet diapers or is struggling to breathe.
Why it changes during the day
A baby's temperature isn't fixed: it tends to be a little lower overnight and in the morning, and slightly higher in the afternoon. It can also go up a bit after a lot of crying, a feed, a warm bath, heavy clothing or a hot room — which is why it's worth checking the ideal nursery temperature.
If a reading surprises you, take off a layer, wait a while with your baby calm and measure again with the same method. Logging every reading with the method and time lets you compare like with like — and it's exactly what your pediatrician will ask. With Alfonso you can bring it all together in the PDF report for your pediatrician.
Which thermometer to choose
- Digital: the most recommended option, works for armpit and rectal. If you use one rectally, keep it for that only.
- Mercury: better not. If it breaks, mercury is toxic; if you still have one, it's worth replacing.
- Ear: handy, but less reliable under 6 months.
- Forehead (infrared): quick for a first check; if it reads high, confirm with another method.
- Forehead strips, pacifier thermometers or phone apps: not reliable for deciding whether there's a fever.
Low temperature: when to pay attention
It gets less attention, but a temperature below the normal range matters too, especially in newborns and premature babies, who are still worse at regulating their temperature. If a reading comes out low, first check that it was taken correctly, warm your baby up (skin-to-skin contact helps) and measure again. If it stays low, or your baby seems listless, feeds less or is hard to wake, call your pediatrician without waiting. And if there's a fever, the post on fever in babies walks you through the next steps.
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.
Do I need to wake my baby to take their temperature?
If they're sleeping peacefully and don't seem sick, there's no need to wake them just to measure. If your baby is under 3 months and feels hot, seems listless or not like themselves, it's worth measuring and calling your pediatrician.
How long should I wait after a bath to take it?
It's best to wait a while, with your baby calm and not overdressed, because a bath, heavy clothing or a long cry can throw off the reading for some time. If in doubt, measure again with the same method.
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.