When a thermometer shows 37.5°C or 38°C, the natural question is whether this is a normal temperature for a child or a fever. The number alone, however, does not tell the full story.
A child's age, the site where the temperature was measured, the type of thermometer, the measurement technique, activity level and even a recent bath can affect the reading.
For that reason, a temperature of 37.5°C measured under the arm should not automatically be interpreted in the same way as 37.5°C measured in the ear or mouth. There is also no reliable formula for adding or subtracting a fixed number of degrees to convert a reading from one measurement site to another. [1] [4]
There is no single temperature that is normal for every child at every moment. The American Academy of Pediatrics notes that normal body temperature varies with a child's age, activity level and time of day. The NHS describes the average normal temperature in babies and children as approximately 36.4°C, while noting that normal variation occurs. [1] [3]
For parents, the measurement site is particularly important because different parts of the body can produce slightly different readings.
The Canadian Paediatric Society provides the following reference ranges according to measurement site: [5]
| Measurement site | Approximate reference range | What to know |
|---|---|---|
| Armpit | 36.5–37.5°C | Easy and commonly used but less precise than some other methods. |
| Mouth | 35.5–37.5°C | Suitable for older children who can hold the thermometer correctly. |
| Ear | 35.8–37.9°C | Accuracy depends heavily on correct positioning in the ear canal. |
| Rectum | 36.6–37.9°C | Highly accurate but invasive and not routinely recommended by every guideline. |
These are reference ranges rather than rigid conversion or diagnostic rules. A temperature should always be interpreted alongside the measurement site, the child's age and the child's overall condition.
Different thermometers estimate body temperature from different parts of the body. The American Academy of Pediatrics considers rectal temperature the most accurate, followed by forehead measurements. Oral and ear measurements can also be accurate when performed correctly, while armpit temperatures are generally less accurate and are often used for screening. [1]
NICE takes a less invasive approach for routine temperature measurement in children younger than 5. It recommends an electronic thermometer in the armpit for infants younger than 4 weeks and either an axillary thermometer or infrared ear thermometer for children aged 4 weeks to 5 years. [2]
These recommendations are not necessarily contradictory. They reflect the balance between measurement accuracy, practicality and safe routine use.
Armpit, or axillary, measurement is widely used in babies and young children because it is simple and non-invasive. The Canadian Paediatric Society lists an approximate normal axillary range of 36.5–37.5°C. [5]
Technique matters. The thermometer tip should sit against the skin in the centre of the armpit, with the child's arm held firmly against the body until the thermometer completes the measurement. The NHS recommends digital armpit measurement for children younger than 5 years. [3]
| Age | Practical option | Important consideration |
|---|---|---|
| Newborns and young infants | Digital armpit measurement is a practical routine option supported by NICE and the NHS. | A temperature of 38°C or higher in a baby younger than 3 months needs urgent assessment. |
| From around 6 months | Armpit or a properly used ear thermometer. | The AAP does not recommend ear thermometers in younger infants because their ear canals are narrow. |
| Older children | Ear, oral or forehead measurement may be appropriate depending on the device and the child's ability to cooperate. | The AAP generally considers oral measurement suitable from about age 4 if the child can use the thermometer correctly. |
Although rectal measurements are considered highly accurate by some organisations, they are more invasive. NICE does not recommend routine oral or rectal temperature measurement in children aged 0 to 5 years. If a rectal measurement is considered necessary, parents should use an appropriate technique and follow medical advice. [1] [2]
No fixed amount should automatically be added to an armpit temperature.
A common rule of thumb is to add 0.5°C or even 1°C to an underarm reading to estimate the child's “real” temperature. This can create false precision.
Mayo Clinic explains that readings can differ depending on where temperature is measured, but there is no exact way to add or subtract a fixed degree value to make readings from different parts of the body match. When following a child's temperature over time, it is more useful to compare measurements taken in the same way. [4]
Therefore, if a child's temperature is 37.7°C under the arm, do not automatically convert it to 38.2°C or 38.7°C. Record it as 37.7°C axillary and interpret it with the child's age, symptoms and overall condition.
Small changes do not always mean the illness itself has suddenly changed. Temperature readings can be affected by:
The NHS recommends allowing a child to cool down for a few minutes before repeating the temperature if they have just had a bath, been very active, been heavily wrapped or stayed in a very warm room. [3]
If fever is confirmed, read our detailed guide to child fever treatment by age, temperature and symptoms.
| Reading | How to interpret it |
|---|---|
| 35.5°C | This may fall near the lower end of some measurement ranges, particularly oral readings, but would be relatively low for others. Repeat an unexpected reading, especially if the child feels cold or appears unwell. |
| 36°C | This can be normal depending on the child, measurement site and time of day. |
| 37.5°C | The site matters. This may sit near the upper end of normal reference ranges for some methods and should not be interpreted without knowing how it was measured. |
| 38°C | This is commonly used as a fever threshold. In a baby younger than 3 months, it requires urgent medical assessment. |
| 39°C | This is not a normal temperature. NICE considers 39°C or higher in children aged 3 to 6 months at least an intermediate-risk feature for serious illness. In children older than 6 months, the number alone does not determine illness severity. |
NICE specifically advises that in children older than 6 months, the height of the temperature alone should not be used to identify serious illness. The child's appearance and accompanying symptoms remain crucial. [2]
Normal temperature can vary somewhat with age, activity and time of day. In everyday home monitoring, however, parents do not need to memorise a different “normal number” for every age.
Age is more important because it affects which measurement method is appropriate and how urgently fever should be assessed.
For example, 38°C carries a very different level of concern in a 2-month-old baby than it does in an otherwise well 5-year-old child. [2]
Seek prompt medical assessment if:
If a seizure occurs with fever, see our guide to febrile seizure first aid and what to do in the first few minutes.
The goal of fever care is not to force the thermometer back to 37°C at all costs. The child's comfort, hydration, overall condition and the cause of the fever matter more than chasing a particular number.
A temperature falling after fever medicine does not prove that the underlying illness is minor, and a temperature that does not fall quickly does not by itself diagnose a serious illness.
For safe home care and common cooling mistakes to avoid, see: how to reduce a child's fever safely.
A child's normal temperature cannot be interpreted separately from the way it was measured. Armpit, ear, mouth, forehead and rectal measurements can produce different readings, while age, technique, activity and environmental conditions can also influence the result.
Do not automatically add 0.5°C or 1°C to an armpit temperature. Record the measurement site, make sure the thermometer is being used correctly and repeat an unexpected reading before interpreting it alongside your child's age and overall condition.
For specialist assessment of fever or other concerning symptoms, visit Pediatrics at Mouwasat Hospital.
