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Measuring temperature: which thermometer at which age
Rectal is the reference, the armpit is the compromise, and the ear is only as good as its aim.
This is a reference, not a prescriber: the package insert of the bottle in your hand outranks every number on this page, aspirin is never for children, and in a baby under three months any measured fever is urgent — call now, do not dose. If the child is on other medicines, or has a known liver, kidney or stomach condition, dosing belongs to a clinician.
The sites, ranked
Rectal measurement is the reference standard for children and the one to use under three months, where half a degree changes the decision. Axillary (armpit) measurement is the standard compromise: comfortable and safe, reading about 0.5 °C below the core. Ear thermometers read the eardrum's infrared and are fast, but the reading depends on a straight aim at the drum and is less reliable in babies under six months, whose canals are small and curved. Forehead strip and skin readings are the least reliable and are screening at best.
Method notes that change the number
For a rectal reading, lubricate the tip, insert about 1–2 cm in a baby and no further than the tip's metal portion, and hold the child still. For axillary, the armpit must be dry and the arm held against the body for the full measuring time. Wait 15–20 minutes after a hot drink, a bath or exercise — all three raise skin temperature without raising the core.
Consistency beats precision
The practical rule is one site, always the same site, during an illness — because the trend across hours is the information, and switching sites mixes two different scales. Note the site when you note the number.
Cleaning and the sibling rule
Thermometers are shared instruments and should be cleaned after each use per the maker's instructions — alcohol wipe for the tip, not a rinse under the tap for an electronic one. If a thermometer has been used rectally, it stays a rectal thermometer: label it, and do not put it back in the family's mouth.
Fever thresholds and red flags: the national paediatric guidance in the region. Dosing: the standard weight-based rules — paracetamol 15 mg/kg per dose every 4–6 hours, at most four doses a day (60 mg/kg/day ceiling); ibuprofen 10 mg/kg per dose every 6–8 hours, at most three doses a day (30 mg/kg/day ceiling), from 3 months and about 5 kg. Febrile seizures: the standard paediatric emergency guidance (simple vs complex, the five-minute line). Checked 2026-09-06.
Frequently asked questions
Which thermometer is most accurate for a baby?
A rectal digital reading is the reference for babies, and it is the only site reliable enough for the under-three-month rule. Ear and forehead devices are convenient but are screening tools in that age group.
Do I add or subtract degrees depending on the site?
The usual correction is that axillary reads about 0.5 °C below the core, so 37.5 °C in the armpit is a core fever. Better than correcting is recording the site with the number, and using one site throughout an illness.
The rest of the fever pages
Fever at every age
All seven fever pages, with the thresholds, the ladders and the red flags.
What counts as a fever at each age
The measurement thresholds, why the site matters, and the one age where any fever is urgent.
Paracetamol dosing by weight
15 mg per kilogram per dose, every 4–6 hours, four doses a day at most — with the arithmetic worked.
Ibuprofen dosing by weight
10 mg per kilogram per dose, every 6–8 hours, from 3 months and about 5 kg — and the situations where it is the wrong choice.
When to call instead of waiting
The red flags that override the thermometer — by age, and the combinations that are always urgent.
A medicine log that answers the call
TempoLife timestamps every dose so the daily ceilings stay visible and the doctor gets exact numbers instead of a guess.