Measuring Fever Right: How to Use a Thermometer Correctly
A thermometer is only useful if you read it right. Learn how to measure temperature accurately at home, what counts as a fever, the different types, and when a temperature needs a doctor.
A thermometer is one of the most basic home health devices — yet a surprising number of "fevers" and "normal" readings are wrong simply because of how the temperature was taken. Getting an accurate reading matters, because it guides whether you rest at home or seek help. This is a clear, plain-language guide to measuring temperature correctly, understanding the numbers, and knowing when a fever needs a doctor.
What Counts as a Fever
Normal body temperature is around 37°C (98.6°F), though it varies a little through the day and between people. A fever is generally a temperature of 100.4°F (38°C) or above. Remember that a fever is usually the body's defence working, not the illness itself — so how the person looks and feels overall matters as much as the exact number.
Types of Thermometer
| Type | How it is used | Note |
|---|---|---|
| Digital (oral/underarm) | Under the tongue or in the armpit | Common, reliable; underarm reads slightly lower |
| Digital forehead/temporal | Swiped/held on the forehead | Quick, easy for children |
| Ear (tympanic) | In the ear canal | Fast; technique matters for accuracy |
| Mercury (old glass) | Being phased out | Avoid — breakage releases toxic mercury |
How to Measure Accurately
- Wait a while after hot/cold drinks, food, a bath, or exercise (they skew a reading)
- For oral: place under the tongue, mouth closed, and wait for the beep
- For underarm: place in a dry armpit, arm held down snugly; note it reads a touch lower than oral
- Follow the device instructions for ear/forehead types (position matters a lot)
- Read it in a consistent way each time if tracking a fever, and note the time
- Clean the thermometer as directed between uses
When a Fever Needs a Doctor
When to See a Doctor
A thermometer helps you monitor, but it does not diagnose. Most mild fevers settle with rest and fluids, but see a general physician for a high or prolonged fever, or any of the warning signs above — and any fever in a baby under 3 months needs same-day care. For guidance on a fever, an online consultation is a convenient first step, and the doctor will tell you if you need to be seen or tested in person. (For safe fever home care, see our fever guide.)