Step 1: Bring a pot of water to boil on the stove.
Step 2: Fill a glass or small bowl with ice and cover with water
Step 3: Test thermometer in the boiling water; If it doesn't read 212 degrees Fahrenheit or 100 degrees Celsius, adjust the small nut on the backside of the thermometer, located where the face meets the probe. It won't be a large turn, so easy does it!
Step 4: Test thermometer in bowl of ice water. If it doesn't read 32 degrees Fahrenheit or 0 degrees Celsius, make another slight adjustment. It should now be calibrated to read correctly in either temperature of water.
One great thing to note, from food writer Dana McCauley, is that many thermometers have a mini wrench built right into the shield or sleeve that protects the probe and keeps it clean. Check yours before you go digging through your tool box!
Digital thermometers aren't as easily adjusted and either have to be done by a pro, or at least tested to see how far off in either direction they are. If you know your thermometer reads 15 degrees hotter than it really is, just make sure to do the math without worrying about the expensive fee to have it repaired (which is why we still love our inexpensive, no bells and whistles, instant read thermometer!)