An axillary temperature, the reading you get by holding a thermometer snugly in the armpit, generally registers lower than temperatures taken by mouth or rectum. Most clinical references place the axillary fever threshold at around 37.2°C to 37.5°C (about 99°F to 99.5°F), rather than the familiar 38°C (100.4°F) used for oral or rectal readings. That lower cutoff exists because the armpit is a surface site, not a body cavity, so it consistently underestimates what is happening deeper inside. Understanding the size of that gap and when it matters can help you decide whether to treat, when to call a doctor, and whether the thermometer you grabbed is giving you a useful number.
Why Armpit Readings Run Lower
Your body’s true “core” temperature lives deep in your trunk and head, regulated by a small region in the brain called the hypothalamus. The armpit, by contrast, is skin surface sandwiched between your arm and torso. It picks up heat radiating outward from blood vessels, but it also loses heat to the surrounding air every time you lift your arm or shift position. The result is a reading that tracks core temperature reasonably well but sits below it by a predictable margin.
Research comparing axillary readings to oral and rectal readings in children found a mean difference of about 0.56°C below oral and about 0.38°C below rectal temperatures under normal conditions.1Annals of Emergency Medicine. Comparison of axillary, oral, and rectal temperatures in children – Section: Results A separate study in non-surgical adult patients found a very similar pattern, with the correlation between axillary and core readings hovering around 0.87 to 0.90, showing that the two track each other closely even though they do not match exactly.2PubMed. The relationship between axillary and core body temperature measurements The armpit is not a random approximation. It is a reliable but systematically low mirror of what is going on inside.
What Number Actually Counts as a Fever
Because axillary temperatures sit below core temperatures, applying the standard oral-fever cutoff of 38°C (100.4°F) to an armpit reading would miss many genuine fevers. Research on children being treated for Kawasaki disease found that using 38°C as the axillary cutoff had poor sensitivity for detecting fever defined by internal temperatures. Lowering the axillary threshold to 37.5°C provided the best balance of sensitivity and specificity for catching oral fevers of 38°C, while a threshold of 37.2°C worked best for matching rectal fevers of 38°C.3PubMed Central. Axillary, Oral, and Rectal Routes of Temperature Measurement During Treatment of Acute Kawasaki Disease – Section: Results
A study in a private pediatric practice confirmed this picture from a different angle: axillary thermometers showed about 95% sensitivity for detecting rectal fevers above 38°C, meaning they caught nearly all true fevers. However, specificity was only about 75%, so roughly one in four children flagged as febrile by armpit did not actually meet the rectal-fever threshold.4PubMed. Performance of axillary and rectal temperature measurement in private pediatric practice In plain terms, the armpit is better at ruling fever in than ruling it out. If you get a high armpit reading, it is worth taking seriously. If the reading looks normal but the person feels warm and unwell, it could be a miss.
For practical purposes, most guidelines suggest treating an axillary reading of 37.2°C to 37.5°C (roughly 99°F to 99.5°F) as a fever-equivalent, depending on the patient’s age and the clinical situation. Some parents and clinicians add a fixed correction factor (typically 0.5°C or about 1°F) to the armpit number to estimate oral temperature, but as we will see, that shortcut works less well as temperatures climb.
The Gap Gets Wider When Fevers Get Higher
One of the most clinically important quirks of armpit measurement is that the offset from core temperature is not constant. At low body temperatures, the gap is small, around 0.4°C. But once rectal or oral temperatures reach 39°C (about 102.2°F) or higher, the gap can exceed 1°C.1Annals of Emergency Medicine. Comparison of axillary, oral, and rectal temperatures in children – Section: Results This means that a child with a true core fever of 40°C might show an armpit reading of only 38.5°C or even lower, potentially leading a caregiver to underestimate the severity.
The reason likely involves blood flow. When the body heats up during a genuine fever, the hypothalamus orchestrates widespread changes: blood vessels in the skin constrict to conserve heat, shivering kicks in, and the body behaves as though its thermostat has been turned up.5PubMed. Central circuitries for body temperature regulation and fever Those same vasoconstriction signals reduce blood flow to the periphery, including the armpit, which means less heat reaches the thermometer at exactly the moment when core temperature is rising fastest. So the higher the fever, the more the armpit lags behind.
This is worth remembering if you are monitoring a sick child overnight. An armpit reading that seems only mildly elevated could correspond to a core temperature that is meaningfully higher. When fevers are clearly climbing, switching to an oral or rectal measurement gives a more accurate picture.
Armpit Temperatures in Newborns
Axillary measurement holds a special place in newborn care because rectal thermometers carry a small but real risk of perforation in tiny infants, and oral measurement is obviously impractical. For this reason, the armpit is often the first-line site in nurseries and NICUs. Research in newborns found that the axillary-to-rectal gap is smaller than in older children: premature infants showed a mean difference of about 0.33°C, while full-term newborns showed about 0.43°C.6PubMed. Using axillary temperature to approximate rectal temperature in newborns The same study suggested that adding 0.3°C for preterm infants or 0.4°C for full-term infants to the armpit reading produces an acceptable approximation of rectal temperature in most cases.
The narrower gap in newborns likely reflects their unique thermal biology. Newborns, especially premature ones, have a high surface-area-to-mass ratio and less subcutaneous fat, which means heat moves from their core to the skin surface more easily. They also possess deposits of brown adipose tissue concentrated between the shoulder blades, which can generate heat rapidly in response to cold stress.7PubMed Central. BMP8 and activated brown adipose tissue in human newborns – Section: Results Because the armpit sits near this heat-generating tissue, neonatal axillary readings may hew closer to core temperature than they do in older children or adults, where the thermal gradient from core to skin is steeper.
Even so, in a newborn under three months with an axillary reading of 37.2°C or above, most pediatric guidelines recommend confirming with a rectal measurement before making clinical decisions, because even a small error matters a great deal in that age group.
How Long You Need to Hold the Thermometer
Getting a reliable armpit reading depends heavily on technique. Digital thermometers beep when they think they have stabilized, typically after 30 to 60 seconds, but armpit readings often need longer to plateau than oral or rectal ones. The arm needs to be held snugly against the body with no air gaps, and the tip of the thermometer should sit in the deepest part of the armpit, pointing toward the center of the chest.
With mercury-glass thermometers (still in use in parts of the world), studies have found it can take several minutes for the reading to stabilize. Age appears to influence stabilization time, with younger children reaching a final temperature faster than older ones.8Scientific Research Publishing. Accuracy of Measuring Axillary Temperature Using Mercury in Glass Thermometers in Children under Five Years Digital thermometers are faster and more standardized, but an early beep does not always mean the reading is done. If you suspect the number is suspiciously low, try again, making sure the arm stays pressed tight for the full measurement period. A common mistake is placing the thermometer loosely or over clothing, which lets ambient air cool the sensor and produces a falsely low result.
Mercury-in-glass thermometers, while historically common for axillary use, are being phased out in many countries because of mercury toxicity concerns. Digital and infrared devices dominate the market now, and a comparison of various technologies found that tympanic (ear) thermometers tended to have the smallest mean difference from a reference standard, while some infrared devices showed wider variation.9PubMed Central. Studying the Accuracy and Function of Different Thermometry Techniques for Measuring Body Temperature – Section: Results For armpit use specifically, a basic digital thermometer remains the cheapest and most widely validated option.
Fever Versus Other Causes of Elevated Temperature
Not every elevated reading on a thermometer means fever. True fever is a regulated process: the brain deliberately raises its temperature set point, usually in response to infection. Pyrogens, signaling molecules released by immune cells or produced from bacterial products, prompt the hypothalamus to shift the balance of certain ion gradients, effectively “resetting the thermostat” to a higher temperature.10PubMed. Fever: reciprocal shift in brain sodium to calcium ratio as the set-point temperature rises Once the set point rises, the body actively works to reach it by shivering, constricting blood vessels, and seeking warmth. That is why you feel cold at the start of a fever even though your temperature is climbing.
Hyperthermia, by contrast, is an unregulated rise in body temperature caused by external heat exposure, drug reactions, or conditions like heat stroke. The critical practical difference is that fever responds to antipyretics like ibuprofen or acetaminophen, which block the prostaglandin signaling that raised the set point. Hyperthermia does not respond to these drugs; the only effective intervention is physical cooling.11PubMed. Fever versus hyperthermia If you take an armpit reading on a child who has been sitting in a hot car, the number may look like a fever, but the mechanism and the treatment are completely different.
This distinction matters for axillary measurement in particular because the armpit is easily influenced by environmental heat. A child who has been bundled in heavy clothing or blankets may show an elevated axillary reading that has nothing to do with infection. Letting the child cool down for a few minutes and re-measuring usually clarifies whether the reading reflects a genuine fever or just trapped heat.
Situations Where the Armpit Falls Short
Axillary measurement works well as a first-pass screening tool, but there are situations where it becomes unreliable enough to warrant a different site. Patients who are shivering intensely, for instance, generate a lot of muscle heat that does not distribute evenly to the skin surface, so the armpit may not capture the true rise in core temperature. Conversely, people with very low body fat or severe dehydration may have less insulation between core and skin, which can shrink or distort the usual offset.
In critical care settings, armpit readings are generally avoided because the stakes of a small error are high. Intensive care units typically rely on esophageal, bladder, or pulmonary artery probes to track core temperature with precision that no surface site can match. Even in ordinary hospital wards, the wide limits of agreement between axillary and tympanic or non-contact infrared thermometers have raised concerns. One study in primary care found that the 95% limits of agreement between a non-contact infrared thermometer and an axillary device spanned a range of nearly 3°C, meaning individual readings could differ by quite a lot even if the average difference was small.12PubMed Central. Non-contact infrared versus axillary and tympanic thermometers in children attending primary care: a mixed-methods study of accuracy and acceptability A narrow average difference can mask wide individual scatter.
That said, for the vast majority of home monitoring, the armpit is perfectly adequate. You are usually not trying to distinguish 38.0°C from 38.2°C; you are trying to decide whether someone is febrile at all, and if so, whether the fever is going up or coming down. For that purpose, repeated armpit readings taken consistently with the same thermometer and the same technique give you a useful trend line, even if the absolute number is slightly off.
Cleaning and Infection Control
The armpit seems like a low-risk measurement site compared to the mouth or rectum, but shared thermometers can still spread pathogens. A 2018 investigation linked reusable axillary temperature probes to an outbreak of Candida auris, a drug-resistant fungal pathogen, in a healthcare setting.13PubMed. Candida auris outbreak was linked to reusable axillary temperature probes, study finds A separate study examining non-critical medical devices, including thermometers, found that they could harbor a range of potentially infectious organisms, including antibiotic-resistant bacteria, reinforcing the need to sanitize between patients.14World Health & Population. The Potential for Transmission of Hospital-Acquired Infections by Non-critical Medical Devices: The Role of Thermometers and Blood Pressure Cuffs – Section: Conclusion
At home, this is less of a concern if only family members share a thermometer, but wiping the tip with rubbing alcohol or a disinfectant wipe before and after each use is still good practice. In hospitals and clinics, disposable probe covers or single-use thermometers eliminate the cross-contamination issue. If you are caring for someone with a known infection, especially in an institutional setting, treat the thermometer as a potential vehicle for transmission rather than assuming the armpit is a “clean” site.
When to Rely on Axillary Temps and When to Switch
Armpit measurement occupies a useful middle ground: it is noninvasive, painless, and accessible to anyone with a basic thermometer. For routine home monitoring of a mildly ill child or adult, it provides enough information to guide decisions about comfort measures, when to give a fever reducer, and when to call a healthcare provider. A reading at or above 37.2°C to 37.5°C under the arm is worth treating as a potential fever, especially if the person has other symptoms of infection.
Consider moving to a different measurement site when any of the following apply:
- Newborns under 3 months: A rectal temperature is the standard for confirming fever in very young infants, where even a low-grade fever can signal serious infection.
- High fevers: When the armpit reading is already above 38°C, the true core temperature could be substantially higher due to the widening gap described earlier. An oral reading in a cooperative older child or adult gives a clearer picture.
- Inconsistent readings: If you are getting numbers that do not match how the person looks or feels, the thermometer may not be making good contact. Try oral or tympanic instead.
- Post-exercise or heat exposure: An elevated armpit reading after physical activity or time in a hot environment may reflect surface heat, not a true fever. Wait 15 to 20 minutes in a comfortable room and remeasure, or take an oral reading for comparison.
There is nothing wrong with starting at the armpit for convenience and escalating to a more precise site if the number raises questions. The goal of any temperature check at home is to detect a meaningful change from baseline, and the armpit does that reliably as long as you account for its systematic offset and its tendency to underestimate at the high end of the fever range.
Why Temporal and Ear Thermometers Have Not Replaced the Armpit
Tympanic (ear) and temporal artery (forehead) thermometers are marketed as faster, more convenient alternatives to the armpit. In controlled settings, tympanic devices can produce readings very close to core temperature, and one comparison found the tympanic thermometer had the smallest mean difference from a reference standard among several devices tested.9PubMed Central. Studying the Accuracy and Function of Different Thermometry Techniques for Measuring Body Temperature – Section: Results But in real-world use, ear wax, improper probe angle, and small ear canals in young children all introduce error. Temporal artery thermometers scan the forehead and are well tolerated even in sleeping infants, but their accuracy has been questioned, particularly in newborns.15Paediatrica Indonesiana. Comparison of the accuracy of body temperature measurements with temporal artery thermometer and axillary mercury thermometer in term newborns
Cost also plays a role. A basic digital stick thermometer suitable for armpit use costs a few dollars, while tympanic and temporal artery devices run considerably more. In low-resource settings, the armpit and a simple thermometer remain the most accessible combination, which is one reason axillary measurement continues to feature prominently in global health guidelines. For families deciding what to buy, there is no single “best” device. What matters more than the gadget is consistent technique and knowing the normal range for whichever site you choose.