An oral temperature is the easiest method to obtain an accurate temperature in an adult.

Oral Method
The thermometer must be placed under the tongue with the lips closed. Most digital thermometers will beep when the temperature has been determined and the thermometer can be removed.

A glass (mercury) thermometer should be left inside the mouth for a full three minutes before reading. If performed correctly, oral temperatures can be very reliable. Oral temperatures normally vary anywhere from 97.5 to 99.5 degrees Fahrenheit. This is a little lower than the core body temperature.

Oral temperature should not be taken for at least 20-30 minutes after drinking a hot or cold liquid, smoking, or eating. Wait at least one hour after taking a hot (or cold) shower or bath prior to measuring body temperature.

Rectal Method
This method is the most accurate and should be the only method used to assess body temperature in people who cannot cooperate with an oral method. A well lubricated (e.g. KY jelly, Vaseline) rectal thermometer should be inserted gently into the rectum no more than 1/2 to 1 inch. Rectal temperatures are very safe and are not harmful. Do not use oral thermometers rectally. Rectal temperatures tend to run about 1 degree Fahrenheit above oral temperatures.

Wait at least one hour after taking a hot (or cold) shower or bath prior to measuring body temperature.