Check the thermometer type and manufacturer instructions
Prepare a reference standard, such as ice water or boiling water
For ice-point calibration, fill a container with crushed ice and add just enough water to make a slush
Insert the thermometer probe into the ice slush without touching the container sides or bottom
Wait until the reading stabilizes
Adjust the thermometer to 32°F or 0°C if it has a calibration feature
For boiling-point calibration, bring clean water to a full boil
Insert the thermometer probe into the boiling water without touching the pot
Wait until the reading stabilizes
Adjust the thermometer to 212°F or 100°C at sea level if it has a calibration feature
Account for altitude when using boiling-point calibration
Recheck the thermometer after adjustment
Repeat calibration periodically and after drops or shocks
Replace the thermometer if it cannot be calibrated properly