The most common thermometers fall into three broad categories: digital probe thermometers, infrared thermometers, and liquid-in-glass thermometers. Each uses a different method to detect temperature, and choosing the right one depends on whether you’re checking a child’s fever, monitoring ovulation, or measuring the surface of a grill. Here’s how each type works and where it fits best.
Digital Probe Thermometers
Digital thermometers are the standard recommendation for home medical use. They contain a small sensor called a thermistor, a material whose electrical resistance shifts predictably as it heats up. A microchip converts that resistance change into a number on the screen. Most digital thermometers are accurate to within about ±0.1°C (±0.2°F), which is more than precise enough for detecting a fever.
You can use a digital probe thermometer in three places on the body, and the readings differ slightly depending on the site. Rectal readings run about 0.3 to 0.6°C (0.5 to 1°F) higher than oral readings, while armpit (axillary) readings run the same amount lower. A rectal or oral measurement is more accurate than an armpit one. For children under four or five, rectal is the preferred method. Once kids reach four or five, oral readings become practical. If you use the same style of thermometer for both, label one “oral” and the other “rectal” and never swap them.
The American Academy of Pediatrics specifically recommends digital electronic thermometers over all other types for children and no longer recommends mercury thermometers at all.
Infrared Thermometers
Infrared thermometers detect the heat energy radiating off a surface and convert it into a temperature reading. Two versions are common for medical use: ear (tympanic) and forehead (temporal artery).
Ear (Tympanic) Thermometers
These read infrared radiation from the eardrum, which shares its blood supply with the brain’s temperature-regulating center, making it a useful proxy for core body temperature. A reading takes about one second. The catch is that earwax buildup or a small, curved ear canal can block the sensor’s path and throw off the result. That’s why most pediatricians still prefer a standard digital probe for young children. Each use requires either a disposable cover over the tip or cleaning between patients.
Forehead (Temporal Artery) Thermometers
Temporal artery thermometers scan the skin over the blood vessel that runs across the forehead. Some models require a slow sweep from the center of the forehead to the hairline; others are point-and-click from a short distance. During a sweep, the device takes up to 1,000 readings per second and reports the highest one. A measurement takes about three seconds, and the thermometer works best at a distance of 2 to 5 cm from the skin. Because they’re completely contactless, these became especially popular during the COVID-19 pandemic for screening in public settings.
Industrial Infrared Thermometers
The infrared guns used in kitchens, HVAC work, and manufacturing look similar to medical forehead models but are calibrated very differently. Medical infrared thermometers cover a narrow range, roughly 30 to 42.5°C, with high display precision. Industrial models span a far wider range, sometimes from -50°C all the way to 2,200°C, but sacrifice fine resolution at body-temperature scales. They’re also designed to be used at a distance greater than one meter, whereas medical models need to be within a few centimeters. You cannot reliably use an industrial infrared thermometer to check for a fever.
Liquid-in-Glass Thermometers
These are the classic thermometers: a glass tube with a narrow channel connected to a bulb filled with liquid. When the liquid heats up, it expands faster than the glass around it, and the only place it can go is up the thin tube. The narrowness of the tube magnifies even small expansions, making the temperature easy to read against a printed scale.
Older versions used mercury, but mercury is a well-documented toxic substance. Spills from broken thermometers release vapor that can be inhaled, and disposal sends mercury into the environment. The EPA called for a phase-out of mercury thermometers in its own laboratories in 2008, and the National Institute of Standards and Technology stopped calibrating mercury-in-glass thermometers entirely in 2011. If you still have one at home, contact your local hazardous waste authority for safe disposal options rather than tossing it in the trash.
Modern liquid-in-glass thermometers use safer alternatives. Red-dyed alcohol is the most familiar, and a gallium-indium-tin alloy called galinstan mimics mercury’s appearance and behavior without the toxicity. These remain useful as backup thermometers since they need no batteries, but they’re slower to read and easier to break than digital models.
Basal Body Thermometers
Basal thermometers are a specialized subset of digital thermometers designed for fertility tracking. After ovulation, your resting body temperature rises by a tiny amount, often just a few tenths of a degree. A standard digital thermometer that reads to one decimal place (like 98.6°F) can miss this shift. Basal thermometers measure to two decimal places (like 97.67°F), giving you the resolution needed to spot the small, sustained temperature rise that confirms ovulation has occurred. To get a reliable reading, you take your temperature at the same time each morning before getting out of bed.
Wearable Temperature Monitors
A newer option is the wearable temperature patch, a thin adhesive sticker placed under the arm. Disposable versions are battery-free and provide continuous temperature readings for up to 48 hours, making them practical for monitoring a sick child overnight without repeated wake-ups. Some reusable wearable monitors sync to a smartphone app and log temperature trends over time, which can be useful for tracking a fever’s trajectory or for fertility charting without the hassle of a daily manual reading.
Choosing the Right Thermometer
- Infants under 3 months: A digital probe used rectally gives the most reliable reading, and accuracy matters most in this age group.
- Toddlers and young children: Rectal remains most accurate. Ear thermometers are acceptable but can be thrown off by earwax or small ear canals. Forehead thermometers offer convenience with slightly less precision.
- Children 4 to 5 and older: Oral digital readings become practical once a child can keep the thermometer under their tongue with their mouth closed.
- Adults: Any type works well. Oral digital thermometers are inexpensive and accurate. Forehead and ear thermometers trade a small amount of precision for speed and convenience.
- Fertility tracking: A two-decimal-place basal thermometer, or a continuous wearable monitor, gives the sensitivity this application requires.
Regardless of type, keep in mind that “normal” body temperature varies from person to person and throughout the day. The commonly cited 37°C (98.6°F) is an average oral temperature, not a fixed standard. What matters more than any single reading is the pattern: a clear rise above your personal baseline, especially when accompanied by symptoms.