There are a few different ways to calibrate digital and bi-metal thermometers.
The most common are comparison boiling and freezing. There are also thermometer calibration services who will do this for a price.
Liquid in glass thermometers should be calibrated by a professional.
Note that, if your thermometer does not have an adjusting nut (or calibration screw, etc., usually found on the back of the thermometer), you may have to send it to one of these services for recalibration.
Although you can mark specific temperatures (boiling and freezing points can be marked on thermometer casing with tape) with the following methods, a full range of temperatures may not be achieved.
The first and simplest method is by using comparison. That is, compare the thermometer you believe might need recalibration to a thermometer which has already been calibrated or that has been National Institute of Standards and Technology (NIST) certified. This method can be effective, so long as you are confident that the comparison thermometer is reading correctly, which is why an NIST-Certified thermometer, tested annually, is preferable.
Just compare the two and adjust the thermometer you are calibrating to match the “correct” thermometer’s temperature.
This should be done with distilled water, as tap water has compounds in it which could affect the freezing or melting points of water and skew the process.
Calibration in ice water or “freezing” the thermometer is best for thermometers used to measure low temperatures. Add crushed ice to a bowl or cup of distilled water to form a slushy mix at least 2” deep. Insert the thermometer stem into the slushy mix, for at least one minute. Make sure the stem does not come into contact with the sides or bottom of the container. The thermometer should read between 30° and 34° F. If it doesn’t, adjust the dial 32° F.
To calibrate a thermometer used to measure higher temperatures, you might prefer the boiling method. Again, use distilled water and bring it to a rolling boil in a pot or beaker if you have one handy.
Insert the thermometer’s stem at least 2” into the water for about a minute, taking care not to burn yourself while doing this. Do not let the stem touch the sides or bottom of the container. The thermometer should read between 210° and 214°. If it does not, adjust the dial to 212° F.