How to Measure Fever Correctly: Thermometers, Sites and Common Mistakes

Taking someone's temperature seems simple – but many readings are wrong because the thermometer is used incorrectly or the numbers are compared across different body sites. This guide explains how to measure fever accurately in babies, children and adults, which thermometer to choose and how to avoid common pitfalls.

Why Accurate Temperature Measurement Matters

Body temperature is a key vital sign. It helps you and your healthcare provider judge how serious an illness may be, whether an infection is getting better or worse, and how well treatment is working.

• Fever can be a sign of infections such as flu, COVID-19 or other illnesses
• Trends over time are more important than a single number
• Documented readings (time, site, value) are very helpful during medical consultations

Normal Body Temperature and Fever Thresholds

For most healthy adults, normal body temperature ranges around 36.5–37.5 °C (97.7–99.5 °F). Temperature is usually lower in the morning and higher in the late afternoon or evening.

Remember: Numbers from different sites cannot be compared directly. Use the same thermometer and the same site whenever you monitor fever over time.

Thermometer Types

Digital Contact Thermometers

Standard digital thermometers are placed in the mouth, underarm or rectum. They are affordable, robust and generally reliable when used correctly.

Ear (Tympanic) Thermometers

Ear thermometers measure infrared heat from the ear canal. They are quick and convenient for older babies, children and adults but require correct positioning and a clean ear canal.

Forehead and Non-Contact Infrared Thermometers

Temporal or forehead thermometers and non-contact infrared devices are popular for rapid screening. They work best when the skin is clean, dry and at room temperature.

Glass / Mercury Thermometers

Mercury thermometers are no longer recommended because mercury is toxic if the device breaks. Choose a digital thermometer instead.

Best Sites to Take a Temperature

Rectal

Rectal readings are considered the most accurate way to measure core temperature in infants and young children. Use a digital thermometer, insert only a short distance and never leave a child unattended.

Oral

Oral measurement is suitable for cooperative older children and adults. Place the tip under the tongue, close the lips and wait for the signal.

Axillary (Underarm)

The underarm method is easy but less accurate. Use it mainly for trend monitoring, making sure the skin is dry and the arm is held tightly against the body.

Ear

Ear thermometers are handy for older babies, children and adults but are generally not recommended for infants under 3 months of age.

Forehead / Temporal

Forehead and temporal thermometers are convenient for quick checks, for example in sleeping children. Allow time to acclimatise after coming indoors from heat or cold.

Step-by-Step Checklist

1. Wash your hands and clean the thermometer
2. Make sure the device is set to body mode, not surface mode
3. Let the person rest for at least 15 minutes before measurement
4. Wait 30 minutes after eating, drinking or exercising
5. Position the thermometer correctly for the chosen site
6. Wait for the beep and record the reading with time and site

Common Mistakes to Avoid

• Comparing values from different measurement sites
• Measuring too soon after eating or drinking something hot or cold
• Using a dirty thermometer or one with low battery
• Confusing surface mode with body mode on infrared devices