Can an Armpit Thermometer be Wrong? Factors That Affect Axillary Accuracy

September 30, 2025 •

4 min read

According to the Cleveland Clinic, research shows that armpit temperature measurements consistently run lower than oral or rectal temperatures. Given this, the question of "Can an armpit thermometer be wrong?" is a valid one, and understanding its limitations is key to effective fever screening. While convenient and non-invasive, axillary measurements should be interpreted with caution, especially when dealing with young children or when precision is critical.

Quick Summary

Armpit (axillary) thermometers are less accurate than other methods due to measuring skin temperature rather than core body heat. Numerous factors, from placement and movement to environmental conditions and sweat, can impact the final reading. While useful for initial fever screening, confirmation with a more reliable method like rectal or oral is often necessary for accuracy.

Key Points

Less Accurate than Core Methods: Axillary readings are typically lower and less accurate than core measurements taken rectally or orally, as they measure skin surface temperature.
Factors Impacting Accuracy: Sweat, ambient room temperature, and peripheral vasoconstriction during fever can all cause an armpit reading to be falsely low or inconsistent.
Technique is Crucial: For the most reliable result, the thermometer must be placed high in a dry armpit, held snugly against the skin for the recommended duration, often longer than the initial beep.
Consider the 'Add a Degree' Rule: Some sources recommend adding 0.5 to 1 degree Fahrenheit to an axillary reading, but this is an estimate and not a substitute for a precise core temperature.
Poor for Infants and Screening: For babies under 3 months, an armpit reading is not reliable enough for fever diagnosis, and a rectal temperature is the standard.
Use as a Screening Tool: An armpit thermometer is best used for an initial, quick fever check. If a fever is suspected, confirm it with a more reliable method.

Why an Armpit Thermometer Can Be Wrong

When taking a temperature, the goal is to measure the body's internal, or core, temperature. The armpit, however, is a surface location, which inherently makes it less reliable than sites like the rectum or mouth. Unlike core temperature, the skin's temperature fluctuates more easily in response to both internal and external factors. This means that the reading on an armpit thermometer is a reflection of the skin's temperature, not a precise measure of the body's core. For this reason, medical professionals generally consider axillary measurements a screening tool rather than a diagnostic standard.

Factors Affecting Axillary Readings

Several variables can influence the outcome of an armpit temperature reading, leading to potential inaccuracies:

Peripheral Vasoconstriction: At the onset of a fever, the body often constricts blood vessels in the extremities to conserve heat, a process known as peripheral vasoconstriction. This can cause the skin in the armpit to feel cooler even as the core body temperature is rising, resulting in a misleadingly low reading.
Sweating and Evaporation: Sweat in the armpit can cause evaporative cooling, which lowers the temperature of the skin and can significantly skew the reading. A moist armpit will not give an accurate result.
Incorrect Placement: The thermometer must be placed high in the center of the armpit and held securely against the skin. If the probe is not in direct contact with the skin or is touching clothing, the reading will be unreliable.
Environmental Factors: The temperature of the room or exposure to cold air can impact the skin's surface temperature. This makes axillary readings particularly sensitive to external environmental conditions.
Duration of Measurement: Failing to hold the thermometer in place for the full recommended time, typically 4 to 5 minutes for a standard digital thermometer, will produce an inaccurate, low reading. Some digital devices may beep sooner, but for axillary use, a longer hold time is often advised for a more stable temperature.
Movement: A child's movement can break the secure contact between the thermometer and the skin, interrupting the measurement and causing an error.

How to Get the Most Accurate Armpit Reading

If using an armpit thermometer is the only option, following a proper technique can minimize errors. These steps can help improve the reliability of the reading:

Use a Digital Thermometer: Avoid using old-style mercury thermometers, which are now considered a safety hazard. A reliable digital thermometer is the safest and most accurate option for axillary use.
Prepare the Area: Ensure the armpit is dry before taking the temperature. If the person has been sweating or has recently bathed, wait for the skin to return to a normal state.
Position Correctly: Place the tip of the thermometer high into the center of the armpit, ensuring it is in direct contact with the skin. Clothing should not be in the way.
Hold Securely: Lower the arm and hold it snugly against the body. For a small child, holding them close can help keep their arm in place. The thermometer should be fully concealed by the armpit.
Wait Long Enough: Leave the thermometer in place for the manufacturer's recommended time, or for a full 4 to 5 minutes, even after it beeps. This allows the reading to stabilize.
Adjust the Reading: As axillary temperatures are typically lower, some pediatricians recommend adding a half to one degree Fahrenheit to the reading to estimate the core temperature. However, this is an estimation, not a precise measurement.
Confirm the Reading: If a fever is suspected, or if the reading is concerning, confirm it with a more accurate method, such as a rectal, oral, or temporal artery thermometer.

Comparing Axillary to Other Thermometer Types

For an accurate temperature reading, especially in critical situations, other methods are often preferred. Here's a comparison of common thermometer types:


Measurement Method	Accuracy	Recommended For	Pros	Cons
Rectal	Most accurate, gold standard for infants	Infants (under 3 months for definitive results), children	Most reliable measure of core body temperature	Invasive, uncomfortable, requires proper technique
Oral	Generally accurate for cooperative users	Children over 4-5 years old, adults	Non-invasive, quick results	Inaccurate if user has recently eaten, drank, or smoked; requires user cooperation
Tympanic (Ear)	Good accuracy if used correctly	Older infants (>6 months), children, adults	Quick, easy to use	Accuracy can be affected by earwax, ear canal shape, or infection
Temporal Artery (Forehead)	Generally accurate, can be affected by external factors	All ages, especially newborns	Quick, non-invasive, less disturbing to newborns	Can be affected by sweat, sunlight, or cold environment
Axillary (Armpit)	Least accurate; best for initial screening	All ages for screening purposes	Non-invasive, safe, simple	Less accurate, readings often lower than core, susceptible to external factors

A Note on Fever in Infants

For infants under 3 months of age, a rectal temperature is considered the gold standard for accuracy. An underarm reading is only a preliminary check. If a baby shows signs of illness, or if an axillary reading is elevated, it should be confirmed with a rectal temperature. Relying solely on an inaccurate armpit reading could lead to a delay in detecting a serious bacterial infection, which is a significant risk in this age group.

Conclusion

Yes, an armpit thermometer can be wrong, and its readings should always be viewed as an approximation rather than a definitive measurement. While convenient for an initial, non-invasive fever check, especially for older children and adults, it is the least accurate method. Factors such as sweat, environmental temperature, and improper technique can easily throw off the result. To ensure health decisions are based on the most reliable information, a more accurate method like rectal or oral thermometry should be used for confirmation, particularly for infants and young children. Understanding the limitations and proper usage of your thermometer is the first step toward accurate health monitoring. More detailed information can be found on authoritative health websites, such as the Centers for Disease Control and Prevention.

Frequently Asked Questions

No, armpit thermometers are not considered reliable for babies, especially those under 3 months. For this age group, a rectal temperature is the gold standard and the most accurate way to check for a fever. Axillary readings can be used for initial screening in older infants but should be confirmed with a rectal temperature if a fever is detected.

An armpit temperature is lower because it measures the temperature of the skin surface, not the body's internal, or core, temperature. It is less insulated and more susceptible to environmental factors, unlike the internal temperature measured by rectal or oral methods.

To get the most accurate armpit reading, use a digital thermometer. Place the tip high in a dry armpit, ensuring it makes direct skin-to-skin contact. Hold the arm snugly against the body for 4 to 5 minutes, or until the manufacturer's directions are fully followed.

The "add a degree" rule is based on the observation that armpit temperatures are often lower than core body temperatures. Some pediatricians suggest adding 0.5 to 1 degree Fahrenheit to an axillary reading to estimate a more accurate core temperature. This is an estimation, and any suspected fever should be confirmed with a more accurate method.

Yes, sweat can significantly lower your armpit temperature reading. Evaporative cooling from sweat can cause a falsely low reading, so it is important to ensure the armpit is clean and dry before taking the measurement.

You should use a more accurate thermometer, such as a rectal, oral, or temporal artery type, whenever a precise temperature is needed. This is especially important for infants, young children, or when a fever is suspected to properly inform treatment decisions.

For hygienic purposes, it is recommended to have two separate digital thermometers: one for oral use and one for rectal/axillary use. Labeling them clearly can prevent cross-contamination.