How to Take a Baby's Temperature Correctly by Method

Taking a baby's temperature the right way depends mostly on age and method. A quick orientation before the details:

• Under 3 months: a rectal reading with a digital thermometer is the gold standard for accuracy (AAP).
• Fever threshold: 38.0 °C (100.4 °F) rectal is the medically meaningful cutoff for an infant.
• Armpit (axillary): easy and safe, but runs about 0.5–1 °C lower than rectal and is a screening method, not a confirmation.
• Ear and forehead: fast and convenient from roughly 6 months on, but more sensitive to technique.

Knowing how to measure a baby's temperature correctly matters as much as the number itself. A rectal 38.1 °C and a forehead 37.4 °C can describe the very same baby. This guide walks through each method, the correct technique, and which one to reach for at which age.

Quick Reference: methods by age

Always use a digital thermometer. Glass mercury thermometers are no longer recommended and most countries have phased them out (NHS).

Which method should I use at my baby's age?

Age drives the choice more than anything else.

For a newborn or any baby under 3 months, take the temperature rectally. At this age a fever can be the only early sign of a serious infection, so accuracy is not optional. The CDC and AAP both treat 38.0 °C (100.4 °F) rectal in an infant under 3 months as a reason to call the doctor the same day, not to wait and watch.

From 3 to 6 months, rectal is still the most reliable, but a temporal (forehead) scanner is a reasonable screen if you confirm anything borderline rectally.

From 6 months onward, you have room. Tympanic (ear) and temporal readings are practical for a wiggly baby. Armpit remains the gentlest screen. If a quick method reads high or your baby seems genuinely unwell, confirm rectally before you panic or relax.

How to take a rectal temperature, step by step

It feels intimidating the first time. The motion is small and your baby will not be harmed if you go slowly.

1. Clean the digital thermometer tip with soap and water or alcohol, then rinse.
2. Coat the tip with a little petroleum jelly.
3. Lay the baby on their back, knees toward the chest, or face-down across your lap.
4. Insert the tip gently, about 1.3–2.5 cm (½–1 inch), no further. Stop the instant you feel resistance.
5. Hold it steady until it beeps. Read, then wash the thermometer.

Label that thermometer "rectal" and keep a separate one for the mouth. Never share between the two.

Related: many parents take their first rectal reading during those raw early weeks. If you are still finding your feet, the first night home with a newborn covers the wider survival kit.

Armpit, ear and forehead: getting them right

Axillary (armpit). Tuck the digital thermometer tip into a dry armpit, lower the arm to hold it against the skin, and wait for the beep. A wet armpit or a gap reads falsely low. Add roughly 0.5–1 °C in your head to estimate the core reading, but treat it as a screen.

Tympanic (ear). Pull the outer ear gently up and back to straighten the canal, then seat the probe snugly. Earwax, a too-shallow angle, or a recent move from cold to warm air all skew the result. Ear thermometers are unreliable under 6 months because the canal is too small (NHS).

Temporal (forehead). Follow the device instructions exactly, because some scan across the forehead and others touch a single point. Sweat, a hat, sun, or a heater nearby all push the number around. Wipe the forehead dry and take two readings.

What counts as a fever by method?

Fever is defined as elevated core body temperature, and each method has its own threshold because each measures a slightly different place. As a rule of thumb (AAP):

• Rectal, ear, temporal: ≥ 38.0 °C (100.4 °F)
• Oral: ≥ 37.8 °C (100.0 °F)
• Armpit: ≥ 37.2 °C (99.0 °F)

Note the method next to the number whenever you tell a clinician. It also helps to walk into the next check-up prepared with your child's own data, so the pediatrician sees the whole picture at a glance. "38.3 rectal" and "38.3 forehead" are not the same clinical picture.

Decision logic: wait vs. confirm vs. call

• Baby under 3 months and any reading at or above 38.0 °C rectal → call the doctor today, do not wait.
• A forehead or ear reading looks high but the baby seems fine → confirm rectally (under 6 mo) or recheck after wiping and waiting 30 minutes.
• Armpit reads borderline (37.2–37.5 °C) and baby is playing normally → recheck in an hour rather than medicate immediately.
• Baby over 3 months, fever confirmed, but alert and drinking → comfort, fluids, and monitoring usually come before anything else.

This article stays on the how of measuring. For the when to worry side of fever, read our companion guide on fever and cough: when to monitor and when to call the doctor.

Common mistakes to avoid

• Measuring right after a warm bath, a feed, or crying. Wait 20–30 minutes.
• Using one thermometer for both mouth and bottom.
• Trusting a single forehead reading on a sweaty baby.
• Adding or subtracting degrees to "convert" between methods and then treating the converted number as gospel. Report the real reading and the method.
• Reaching for fever medicine based on the number alone rather than how the baby looks and acts.

A feverish baby who won't drink is a separate worry from the temperature itself. If feeding falls off a cliff, our note on a baby not eating much is a useful read.

When to seek professional help

Call your pediatrician or seek urgent care if your baby is under 3 months with any fever, if a fever in an older baby passes 40 °C (104 °F), lasts more than three days, or comes with a stiff neck, a rash that does not fade under pressure, a febrile seizure, breathing trouble, or unusual drowsiness. Trust the overall picture over the digit on the screen.

A fever that drags on also raises the risk of dehydration, especially in hot weather. Our guide on heat and dehydration in babies covers the warning signs to track alongside the temperature.

Frequently asked questions

Is forehead or rectal more accurate for a baby?
Rectal is more accurate and is the reference standard, especially under 3 months. Forehead scanners are convenient and good enough for screening from about 3 months, but confirm anything borderline rectally in a young infant.

Do I really need to take a rectal temperature?
Under 3 months, yes, when accuracy matters. A fever can be the first and only sign of serious infection at that age, and other methods are too imprecise to rule it in or out (CDC).

How much should I add to an armpit reading?
Armpit runs roughly 0.5–1 °C below rectal, but the gap varies by baby and conditions. Use it as a screen and confirm with a more accurate method if it reads near the fever line.

Can I use an ear thermometer on a newborn?
Not reliably. The ear canal is too small and curved under 6 months, so readings are inconsistent (NHS). Use rectal for newborns and young infants.

Why does my baby's temperature change through the day?
Body temperature naturally rises in the late afternoon and evening and dips overnight. A reading that is slightly higher at 6 p.m. than at 9 a.m. is usually normal rhythm, not a developing fever.

What temperature means my baby has a fever?
38.0 °C (100.4 °F) measured rectally, or the equivalent threshold for your method from the table above. Below that, an elevated reading may just be a warm room, recent crying, or daily rhythm.

How KidyGrow helps you

A single temperature reading at 2 a.m. tells you almost nothing on its own. The story lives in the pattern, and the pattern is exactly what a tired parent loses track of.

KidyGrow holds that thread for you. Note a reading and the method alongside what the day looked like, and the app remembers the shape of this illness so you don't have to. By the third feverish night, your Daily Brief might surface something concrete: the last two evenings peaked around the same hour, and the fever has been climbing 24 hours then easing. That kind of "is this getting better or worse" answer is hard to hold in your head at 4 a.m.

It adapts to your child, not a textbook average. Some weeks, honestly, the app won't find a clean pattern, because some illnesses are just three days of chaos. But when there is a pattern, you stop guessing. The morning question shifts from "was last night worse, or did it just feel worse" to "this is what the last three days were. Now I can decide whether to call."

Sources

1. American Academy of Pediatrics, HealthyChildren.org — "How to Take a Child's Temperature."
2. Centers for Disease Control and Prevention (CDC) — Infant and child health.
3. NHS — "How to take your baby's temperature."
4. American Academy of Pediatrics, HealthyChildren.org — "Fever Without Fear."

_This content is educational and does not replace medical advice. Consult your pediatrician for health concerns._