Most thermometers offer readings in both Celsius and Fahrenheit; however, some simple digital models may offer only Fahrenheit temperature information. Converting thermometer readings from Fahrenheit to Celsius requires use of a simple mathematical equation. This equation allows you to exactly calculate the Celsius temperature reading from the Fahrenheit reading without the use of charts or graphs. You may wish to use a calculator to work the equation to ensure you get the answer right or to double-check your work.
Obtain the Fahrenheit temperature reading from the thermometer. This is the number you'll start with in the temperature conversion equation.
Subtract 32 from the Fahrenheit reading. For example, if the Fahrenheit reading is 37.8 degrees C, then you would subtract 32 from it to get 68.
Multiply that number by 5/9 to get the Celsius reading. If the number is 68, for instance, then multiplying it by 5/9 would yield a Celsius temperature reading of 37.8 degrees Celsius.
Check your work, if desired. Multiply the Celsius temperature by 9/5, then add 32 to the result. If you were checking the example problem, you would multiply 37.8 by 9/5 to get 68. Then, add 32 to 68 to get 100, which is the temperature you started with in Fahrenheit.