1. Temperature is usually measured using either the Fahrenheit (°F) or Celsius (°C) scale, both of which are fixed by the freezing and boiling points of water.
  2. The Celsius scale is part of the metric system. It has 100 units (degrees) between the freezing and boiling points of water. This scale is used by people in most countries and by most U.S. meteorological services.


  • mathematical worksheet (optional)
  • examples of use of the Celsius scale
  • student journals




  1. Display the following conversion formulas for student reference:
    CELSIUS °C (9/5 °C)+ 32 = °F
    FAHRENHEIT °F 5/9 (°F - 32) = °C
  2. Identify the freezing points and boiling points for both the Celsius and Fahrenheit scales.
  3. As a group, use the conversion formula to find the Fahrenheit equivalent of 21°C.
  4. Enter the conversion formula in the student journals and challenge students to calculate the Celsius equivalents of several Fahrenheit temperatures and Fahrenheit equivalents of several Celsius temperatures.
  5. Discuss the advantages and disadvantages of both systems.
  6. This "standard" formula is based on converting at the freezing point of water. This requires the addition (sometimes) and the subtraction (sometimes) of "32" -- and this can be somewhat confusing. On the other hand, if the coversion is done at the temperature that is the same on both scales -- which is "-40" degrees -- there is little room for confusion. Think of it this way:
    • 9/5 is greater than one, and will increase a value. The Fahrenheit scale ALWAYS has a larger value than the corresponding Celsius temperature.
    • 5/9 is less than one, and will decrease a value. The Celsius scale ALWAYS has a smaller value than the corresponding Fahrenheit temperature.
      1. When given a temperature in either Fahrenheit or Celsius, add 40 to it.
      2. Then multiply by the appropriate factor:
        • by 9/5 if your original temperature is given in degrees-C
        • by 5/9 if your original temperature is given in degrees-F
      3. Then subtract 40.
        • Try it! (Is this easier to remember then the conversion given in the table above?)

Assessment Ideas

  • Quickness and accuracy of student conversions using the "standard" method and the method described in Step 6.


  • History (research topics): Who were Fahrenheit and Celsius? Look how these scientist developed these methods for measuring temperature.


  • temperature: the degree of heat or cold, as measured on a scale.


  • Adapted from "Winds of Change" educational CD-ROM, Copyright Caltech and NASA/Jet Propulsion Laboratory
  • Step 6 conversion approach suggested by Tom Ford, The Science Source