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By Mayo Clinic Staff

A thermometer is a device used to measure body temperature. It helps tell whether someone has a fever or is too hot or too cold. Average temperatures can be different for each person.

On average, temperatures vary slightly by age:

• Adults: 97.9 degrees Fahrenheit (36.6 degrees Celsius), not 98.6 degrees Fahrenheit (37 degrees Celsius) which is commonly cited.
• Children: 98.1 degrees Fahrenheit (36.7 degrees Celsius).
• Infants: 98.9 degrees Fahrenheit (37.2 degrees Celsius).

Body temperature may be higher for people assigned female at birth and temperatures tend to be higher for all age groups in the later afternoon.

But no matter what your temperature is daily, if you think you have a fever, you need a thermometer. Choosing one can be confusing, so here's what you need to know about the most common thermometers.