Last Updated on April 25, 2025
Body temperature tells us a lot about how our body is functioning. When you’re sick, feeling cold, or running hot, the first thing many people do is grab a thermometer. But how we measure body temperature — and where — makes a difference.
What is Normal Body Temperature?
The average normal body temperature is about 98.6°F (37°C), but that’s not a fixed number for everyone. Your temperature can naturally vary based on:
- The time of day (lower in the morning, higher in the evening)
- Your age
- Physical activity
- Hormonal cycles (especially in women)
Recent studies suggest that “normal” body temperature might actually range between 97°F (36.1°C) and 99°F (37.2°C) for healthy adults [1].
Methods of Measuring Body Temperature
There are different ways — and places — to measure body temperature. Some are more accurate than others, depending on the situation.
Oral (Mouth) Measurement
This is the most common method of taking body temperature.
The person should be mature enough to cooperate and follow instructions. He should not have taken any hot or cold stuff for some time.
For accurate measurement, it is important that the person whose temperature is to be taken should be able to breathe through the nose.
- The person is asked to place the thermometer under the tongue, on one side of the center. The lips should be tightly closed around it.
- Leave the thermometer in place for the required amount of time. Digital thermometers give beeps on completion of reading.
- Check the reading
- Clean the thermometer with clean water.
- Common for adults and older children.
- Place the thermometer under the tongue and close the mouth.
- Drinking hot or cold beverages just before can give false readings.
Rectal (Bottom) Measurement
This method is recommended for infants, small children, and people who are unable to hold a thermometer safely in their mouths.
- Apply a lubricant on the bulb of the thermometer. A jelly-like Vaseline would do. This makes the insertion of the thermometer easier.
- In the case of children, make the child lie prone on a lap or soft padded surface.
- Spread the buttocks and insert the bulb end of the thermometer about 1/2 to 1 inch into the anal canal. Remove the thermometer after 3 minutes or when the device beeps. Pressing the child’s buttocks together will help keep the thermometer in place.
- Remove the thermometer and read
- Clean the thermometer with cool, soapy water and rinse it.
A thermometer used for measuring rectal temperature should not be used to measure oral temperature.
Rectal measurement of body temperature is the Gold standard for babies and young children as it gives the most accurate core body temperature. It gives about 0.5°F (0.3°C) higher than oral readings.
Axillary (Underarm) Measurement
It is easy but less reliable. It often reads lower than oral or rectal measurements. Good for quick checks but not diagnostic decisions.
Though not as accurate as oral or rectal methods, the armpit is commonly used as a place for measuring the temperature. It is quite a simple method to execute.
- The thermometer is placed underarm with the bulb in the center of the armpit.
- Press the arm against the body. The usual time for which the thermometer is kept is 5 minutes or beep.
- An armpit temperature reading may be as much as 1°F (0.6°C) lower than an oral temperature reading.
- Clean the thermometer with cool, soapy water and rinse it off before putting it away.
Tympanic (Ear) Measurement
It uses infrared sensors to measure heat from the eardrum. The method is fast and convenient, but placement errors can lead to wrong readings.
- Check that the probe is clean and free of debris. If dirty, wipe it gently with a clean cloth. Do not immerse the thermometer in water.
- Attach the disposable cover to the probe. Use a disposable probe cover to keep the probe clean. Use a new probe cover each time you take an ear temperature.
- Turn the thermometer on.
- For children < 12 months, pull the earlobe down and back to place the probe in the ear canal.
- For children >12 months and adults, pull the earlobe up and back.
- Press the “on” button to display the temperature reading.
- Remove the thermometer and throw away the used probe.
Remote [non-contact] thermometers can also be used for measuring temperature from the ear.
Temporal (Forehead) Measurement
Infrared scanners can detect heat from the temporal artery. It is a quick method, but it can vary with the environment
You must have come across the forehead temperature scanners during COVID at the airport theaters etc. The following site was common in those times.
This type of thermometer is a non-contact or remote thermometer. Another type is a contact thermometer that uses a cup to be placed against the forehead.
Using a Contact Thermometer on the Forehead
- Press the soft cup of the thermometer or the entire plastic strip firmly against a dry forehead.
- Hold the thermometer in place for the required time.
- Read the temperature before removing the thermometer. [Note the difference from other methods]
- Clean the thermometer.
Remote thermometers are aimed at the forehead from a distance. They use infrared scanners to know the temporal artery temperature.
Using a no-touch forehead thermometer
Power on and let the device be ready- Position the thermometer about 5 cm from the center of the forehead. A light on some devices might help you to position
- The device beeps in 2-3 seconds when it is ready for the reading
Forehead thermometers are not as accurate as other thermometers. In case of high fever, the temperature should be confirmed by other methods.
Types of Thermometers
Digital Thermometers
These are quick, accurate, and easy to read. But they require a battery and are not sturdy.
Mercury Thermometers
These are classically used and quite reliable. But the glass can break, and mercury toxicity can occur if ingested.
Infrared Scanners
These are contactless and fast but sensitive to technique and environment
Smart App-Based Thermometers
These thermometers sync with apps and can store trends over time. However, these are expensive and require connectivity.
Most medical bodies now recommend digital thermometers for home use. Mercury thermometers have been largely phased out due to health and environmental risks [2].
When Should the Temperature Be a Concern
- Above 100.4°F (38°C) and below 95°F (35°C) — seek urgent help.
- Persistent low-grade fever can be due to signal infections, autoimmune diseases, or post-viral syndromes [3].
Quick Tips for Accurate Body Temperature Measurement
- Wait at least 15 minutes after eating, drinking, or exercising.
- Always use the thermometer correctly for the method.
- For kids under 3, prefer rectal or tympanic measurements. [4]
- Clean thermometers after every use.
References
- Protsiv M, Ley C, Lankester J, Hastie T, Parsonnet J. Decreasing human body temperature in the United States since the industrial revolution. eLife. 2020;9:e49555. [Link]
- U.S. Environmental Protection Agency (EPA). Mercury Thermometers. [Link]
- MedlinePlus. Fever. U.S. National Library of Medicine. [Link]
- Batra P, Goyal S. Comparison of rectal, axillary, tympanic, and temporal artery thermometry in the pediatric emergency room. Pediatr Emerg Care. 2013;29(1):63–66. doi: 10.1097/PEC.0b013e31827b5427. [Link]