The normal body temperature range for adult dogs is 37.5-39.2 °C (99.5-102.5 F°), taken in normal room temperatures for a resting dog.
There are many different types of thermometers; each model has an optimum time needed to complete the job.
The most common inexpensive thermometer choice is a rectal "mercury" thermometer. These days, manufacturers have replaced the mercury inside with a non-toxic red liquid. This non-mercury type is the safe model to use. For most accurate use, slide it into the rectum so that almost all of it is inside. Keep a hold of it, or use one with a string or tape attached to prevent loss of the instrument! If it feels like it has entered some stool (high resistance), remove it and replace it. Accurate readings require the thermometer to be along the rectum wall. Make sure that the dog is restrained so that sudden movement cannot occur. Thermometer breakage can occur if an active dog moves vigorously. Apply a thin layer of lubricant before inserting. Vaseline® or water-soluble jelly will do. Make sure the thermometer is properly cleaned between uses by a soapy wash, then a wipe with alcohol. Many people prefer to wear disposable gloves while taking a rectal temperature. This is a good way to prevent soiling of hands. If gloves are not worn, it is important to remember to wash hands carefully with soap and water afterwards. This type of thermometer usually requires between one and three minutes to stabilize. If the temperature is not going up within a 15 second interval, you can generally assume it has reached the correct temperature reading. Remember to shake down the thermometer before each use. Not doing this is a common source of error. It should be shaken down adjacent to the lower end of the scale, not just back into the normal temperature range if the last registry was in a fever range. Many find reading a traditional thermometer a bit of a challenge since it often needs to be rotated to view the liquid core, and interpreting temperature readings between the marks can lead to inaccuracy.
If there is soiling on the outside of the thermometer, remove this material before taking the reading to ensure an unobstructed view of the core.
Newer ear thermometers can take a temperature in a matter of seconds. An audible beep usually occurs to help the user know the temperature is registered. There are those that suggest that this type of thermometer is less accurate. Human ear thermometers are made to assess temperature at the tympanic membrane (ear drum), but since the dog's ear is built with a 90 degree turn between the tympanic membrane and the canal exit, and not straight like our ear canal, the path of the registry is interrupted due to this anatomy variation. Gently pulling the ear up and out to straighten the external ear canal can overcome this difference in anatomy though.
Digital thermometers can be used as rectal thermometers. When the temperature is taken, an audible beep occurs, and the temperature readout appears in the window at the base of the instrument. It is not quicker than the traditional thermometer, but provides an easy-view readout with an exact temperature.
Do not select the type of human thermometers that have strips that are applied to the forehead. These are not accurate in dogs because the haircoat prevents contact with the skin.
Never try to take a temperature orally in dogs. The sharp teeth invariably will damage or break the thermometer, and those made of glass will shatter, resulting in lacerations to the tongue.