What Is an Underarm Temperature?

An underarm temperature is measured by placing the sensing tip of a digital thermometer against the skin in the center of the armpit. The arm is then held tightly against the body, trapping warmth around the thermometer until the device signals that the reading is complete.

The medical term axillary refers to the axilla, or armpit. Unlike a rectal measurement, which more closely reflects core temperature, an axillary thermometer measures heat near the surface of the body. That makes the result more vulnerable to sweat, cool air, loose positioning, clothing, and movement.

Despite this limitation, underarm temperature measurement remains popular because it is:

• Painless and noninvasive
• Safe for people of nearly any age
• Easy to perform at home
• Useful as a quick fever screening method
• Less likely to wake a sleeping child than some other methods

In other words, it is the comfortable middle seat of temperature-taking methods: not anyone’s definition of luxury, but often practical enough to get the job done.

What Is a Typical Underarm Temperature?

There is no single temperature that is normal for every person. Body temperature changes with age, activity, hormones, time of day, sleep, illness, environmental conditions, and the part of the body where it is measured.

A frequently quoted average body temperature is 98.6°F, or 37°C. Healthy temperatures, however, can fall above or below that number. Underarm readings are commonly about 0.5°F to 1°F lower than oral readings, although the difference is not consistent enough to use as an exact conversion.

For adults, published axillary temperature ranges can be surprisingly broad. This is one reason it is more helpful to compare the number with the person’s usual temperature and symptoms than to stare at 98.6°F as though it were a pass-or-fail exam.

Time of Day Matters

Temperature is generally lower in the morning and higher in the late afternoon or evening. A reading that looks slightly elevated at bedtime may be normal for that person, while the same number early in the morning may be more meaningful.

Your Personal Baseline Matters, Too

Consider measuring your underarm temperature a few times when you feel healthy. Take it at approximately the same time each day and use the same thermometer. This can help establish your personal baseline, making future changes easier to recognize.

How to Measure Underarm Temperature Correctly

A digital stick thermometer is usually the simplest choice. Do not use an old glass mercury thermometer. Glass can break, and mercury exposure creates an unnecessary health and environmental hazard.

Step 1: Prepare the Thermometer

Read the manufacturer’s instructions because devices vary. Clean the sensing tip before use according to the product directions. Depending on the model, this may involve cool soapy water or an alcohol wipe.

Step 2: Prepare the Armpit

Move clothing out of the way so the thermometer touches bare skin. Check that the armpit is dry. Sweat can cool the skin and contribute to an inaccurate result, so gently pat the area dry if necessary.

Step 3: Position the Tip

Turn on the thermometer and place its tip high in the center of the armpit. The sensor should rest against skin, not fabric. Aim the thermometer slightly upward so the tip sits securely in the deepest part of the underarm pocket.

Step 4: Hold the Arm Snugly

Lower the arm and press the elbow firmly against the chest. The thermometer tip should remain completely covered by skin. A loose arm allows room air to circulate around the sensor, producing a reading that may be falsely low.

Step 5: Wait for the Signal

Keep the person still until the thermometer beeps or indicates that the measurement is finished. Do not remove it early, even when an impatient toddler begins negotiating like a tiny labor attorney.

Step 6: Record the Reading and Measurement Site

Write down the number, the time, and the fact that it was measured under the arm. When speaking with a healthcare professional, always report the actual result and the location used.

Step 7: Clean and Store the Device

Clean the thermometer again according to its instructions, dry it, and store it in a protective case. If a thermometer is used rectally, label it clearly and never use that same device orally.

Should You Add a Degree to an Underarm Reading?

You may have heard that you should automatically add 1°F to an axillary temperature. Underarm readings are often approximately 0.5°F to 1°F below oral readings, but the difference varies by person, device, technique, and situation.

Adding a fixed amount may offer a rough comparison, but it does not magically turn an underarm measurement into an oral or rectal one. For medical decisions, it is safer to record the original number, identify the measurement site, and confirm a concerning result with a more accurate method.

For example, instead of reporting, “The temperature was 100°F after I added a degree,” say, “The underarm temperature was 99°F.” That gives the healthcare professional information that has not been adjusted by home mathematics.

How Accurate Is an Underarm Temperature?

Axillary temperature measurement is generally considered the least accurate of the commonly recommended digital methods. It can miss a fever because the skin under the arm may remain cooler than the body’s core, particularly when circulation near the skin changes during the early stages of illness.

Underarm measurement is still useful for screening. A normal-looking reading may be reassuring when the person otherwise feels well, but it should not overrule obvious signs of illness. If someone feels hot, shivers, appears unusually sleepy, has trouble breathing, or behaves very differently from normal, confirm the result or seek medical guidance.

Methods That May Provide Better Confirmation

• Rectal temperature: Often considered the most accurate method for young infants.
• Forehead temperature: Appropriate for all ages when a temporal artery thermometer is used correctly.
• Oral temperature: Useful for adults and children old enough to hold the thermometer properly, generally around age 4 or older.
• Ear temperature: Fast and convenient for children older than about 6 months, provided the device is positioned correctly.

Use the same method when tracking changes over time whenever possible. Comparing an underarm reading taken in the morning with an ear reading taken at night can create confusion because both the location and time have changed.

Common Reasons an Underarm Reading May Be Wrong

Many questionable results come from technique rather than a defective thermometer. The most common causes include:

• Placing the sensor over clothing
• Positioning the tip too low in the armpit
• Failing to dry sweaty skin
• Allowing the arm to remain loose
• Removing the thermometer before it signals completion
• Measuring immediately after vigorous exercise or a hot bath
• Taking a reading after the person has been heavily bundled
• Using a device with a weak battery
• Switching repeatedly between different thermometers

After strenuous exercise or a hot bath, wait about an hour before checking temperature when circumstances permit. If the first number seems inconsistent with how the person looks or feels, wait a few minutes, check the positioning, and repeat the measurement.

Underarm Temperature in Babies and Children

Parents often choose the armpit because it is gentle and easy. Pediatric guidance generally considers it acceptable for screening children of any age, but a suspicious result may need to be confirmed.

This is particularly important for infants younger than 3 months. A confirmed temperature of 100.4°F, or 38°C, in a young infant requires prompt medical evaluation, even when the baby appears relatively comfortable. If an infant’s underarm temperature reaches approximately 99°F or higher, contact the child’s healthcare professional and follow instructions for confirming the measurement.

Do not rely solely on the number. Seek medical advice when a child:

• Is difficult to wake, unusually weak, or extremely irritable
• Has trouble breathing
• Develops a stiff neck, seizure, severe headache, or unexplained rash
• Shows signs of dehydration, such as very little urine, a dry mouth, or no tears
• Has repeated vomiting or severe diarrhea
• Has an immune system condition or takes medicines that suppress immunity
• Has a fever that persists or repeatedly returns

How the child looks, drinks, breathes, and responds to you can be as important as the temperature itself.

When Adults Should Seek Medical Attention

Most mild fevers in otherwise healthy adults can be monitored at home with rest, fluids, and attention to symptoms. Contact a healthcare professional when fever is high, lasts more than a few days, repeatedly returns, or occurs with concerning symptoms.

Prompt evaluation is appropriate for difficulty breathing, chest pain, confusion, severe headache, stiff neck, persistent vomiting, seizure, a spreading rash, significant dehydration, or rapidly worsening weakness. People who are pregnant, immunocompromised, receiving cancer treatment, or living with serious chronic illness should seek individualized advice sooner.

A body temperature below 95°F, or 35°C, may indicate hypothermia and is a medical emergency, especially when accompanied by confusion, slurred speech, poor coordination, intense shivering, or unusual drowsiness. Because underarm readings may run low, confirm the number quickly if possible without delaying emergency care.

Frequently Asked Questions

Is 99°F under the arm considered a fever?

In young children, an axillary temperature around 99°F may be considered a fever-screening threshold and should be confirmed using a more accurate age-appropriate method. In adults, interpretation depends on the person’s normal baseline, symptoms, and the quality of the measurement.

Is an underarm temperature lower than an oral temperature?

Usually, yes. An axillary reading is commonly about 0.5°F to 1°F lower than an oral reading, but the difference is not identical for everyone.

Can I take an underarm temperature on either side?

Yes, but use the same side when following a fever over time. Small differences can occur because of positioning, moisture, circulation, and recent activity.

Can deodorant affect an underarm temperature?

Deodorant is not usually a major source of error, but a thick or wet product may interfere with complete skin contact. Wipe away excess product and dry the area before measuring.

Can I use an underarm thermometer on a sleeping child?

Yes. This is one of the method’s practical advantages. Position the sensor against bare, dry skin and keep the arm snug against the body until the reading is complete.

What should I do when two readings are different?

Check the battery, dry the skin, reposition the thermometer, and repeat the measurement after a few minutes. If the result remains questionable or symptoms are concerning, confirm it with an oral, rectal, forehead, or ear measurement appropriate for the person’s age.

Practical Experiences and Lessons From Real-Life Temperature Checks

The most common real-world lesson is that taking an underarm temperature looks simpler than it actually is. Families often place the thermometer somewhere in the general armpit neighborhood, lower the arm loosely, and assume the beep guarantees precision. In reality, the sensing tip needs to sit high in the center of the armpit and remain completely surrounded by skin.

A typical example involves a child who feels warm and tired but produces an underarm reading of only 97.8°F. The first instinct may be to dismiss the possibility of fever. On a second attempt, the parent dries the child’s sweaty armpit, moves the thermometer higher, and holds the elbow firmly against the chest. The result rises to 99.2°F. A forehead or rectal confirmation may then show a clearer fever. The thermometer did not suddenly discover new mathematical skills; the technique improved.

Another useful experience is learning not to compare unrelated readings. Imagine recording 98.1°F under the arm in the morning, 99.5°F by mouth after lunch, and 100.2°F with an ear thermometer before bed. Those numbers may suggest an upward trend, but they were taken at different sites and at different times of day. A more reliable approach is to choose one appropriate method and use it consistently.

Parents also discover that cooperation is part of accuracy. A squirming toddler can turn a straightforward temperature check into a wrestling match with no clear winner. Sitting the child on a caregiver’s lap, giving a simple explanation, and holding the child in a gentle hug can stabilize the arm without making the thermometer feel like an enemy agent. For older children, allowing them to press the start button can create just enough involvement to reduce resistance.

Adults encounter different problems. A person may check immediately after exercising, showering, cooking in a hot kitchen, or wearing several layers of winter clothing. The result reflects more than illness. Waiting in a comfortable indoor environment, removing heavy layers, and allowing the body to settle often produces a more meaningful number.

It is also common to become overly focused on tenths of a degree. A change from 98.3°F to 98.6°F does not automatically mean an illness is advancing. Household thermometers have technical limits, and human temperature naturally fluctuates. Trends of meaningful size, combined with symptoms, matter more than tiny changes on the display.

Keeping a brief temperature log can be remarkably helpful. Record the time, reading, measurement site, symptoms, and any fever-reducing medicine taken. A note such as “7:30 p.m., 99.6°F underarm, tired, drinking normally, no medicine” gives a clinician far more useful information than “The temperature was kind of high yesterday.” Memory becomes creatively unreliable during a sleepless night, so written notes prevent accidental guesswork.

Finally, experienced caregivers learn that the thermometer is one tool, not the entire medical evaluation. A playful child drinking fluids with a mild temperature may be less concerning than a child with a normal underarm reading who is struggling to breathe or cannot stay awake. Measure carefully, confirm doubtful results, and pay attention to the whole person rather than allowing one glowing number to run the household.

Conclusion

Underarm temperature measurement is safe, comfortable, and useful for quick screening, particularly in babies and young children. Its main weakness is reduced accuracy compared with rectal, forehead, oral, or properly performed ear measurements.

For the best result, use a digital thermometer, dry the armpit, place the sensor against bare skin in the center of the underarm, and hold the arm firmly against the chest. Record the original number and identify the measurement site instead of relying on an automatic one-degree adjustment.

When an underarm reading is elevated, unexpectedly low, inconsistent with symptoms, or obtained from a young infant, confirm it using a more accurate method and seek professional guidance when appropriate. The goal is not to achieve the world’s most photogenic thermometer reading. It is to obtain enough reliable information to make a safe decision.