Whether you’re checking your child for a fever or monitoring your own health, thermometers are a household tool we often take for granted. With so many kinds to choose from, it can be hard to know which ones to trust or how to use them correctly.
To help clear things up, Kirsten Rupp, MD, internal medicine and pediatrics doctor, explains how thermometers work and which type might be best for you.
How does a thermometer work?
“All thermometers measure the heat your body gives off,” says Dr. Rupp. “The difference lies in the technology used.”
Here’s how the most common types of thermometers work:
- Digital thermometers: Use sensors to check your body heat and display the number on a screen.
- Ear and forehead thermometers: Use infrared light to detect heat from your skin or ear.
- Glass thermometers: Filled with mercury or colored liquid, they measure temperature by how much the liquid expands and rises.
Which type is most accurate?
According to Dr. Rupp, rectal thermometers are the most accurate, but they’re typically only needed for babies and very young children.
Accuracy by type:
- Rectal (digital): Most accurate; best for infants.
- Oral (digital): Still accurate, but eating or drinking can change results.
- Ear: Accurate if used correctly, though earwax can affect readings.
- Forehead: Easy to use, but slightly less precise.
- Armpit: Least accurate.
Why do thermometer readings differ depending on where they’re taken?
“The further you get from your body’s core, the cooler the temperature tends to be,” Dr. Rupp explains.
Here’s how typical readings compare depending on where the thermometer is used:
| Site | Typical Reading | Difference from Core |
|---|---|---|
| Rectal | 99.5°F (37.5°C) | Closest to core temperature |
| Oral | 98.6°F (37°C) | 0.5°F–1°F lower |
| Ear | 98.6°F–99.5°F | Close to core if positioned correctly |
| Forehead | 97.5°F–99.3°F | 0.5°F–1°F lower |
| Armpit | 97°F–98°F | Often 1°F lower than oral |
Common thermometer mistakes
Avoid these errors for the most accurate reading:
- Taking an oral temperature right after eating or drinking.
- Placing the thermometer incorrectly.
- Using a dirty or damaged thermometer.
- Comparing readings from different body sites.
- Measuring right after coming in from outdoors.
When should you recheck or call your doctor?
Dr. Rupp says you should recheck or contact your doctor if:
- A fever over 101°F lasts more than three days.
- An infant under three months has a rectal temperature of 100.4°F or higher.
- Other symptoms appear like confusion, trouble breathing, rash, stiff neck or seizures.
- You think the reading might be wrong.
Bottom line
Thermometers seem simple, but a few small details, like where and how you use them, can make all the difference.
“If you’re not sure your reading is correct, take a second reading after a few minutes or try a different site,” says Dr. Rupp. “The most important thing is to look at how you or your child feels, not just the number.”