The metric system uses the Celsius scale to measure temperature. However, temperatures are still measured on the Fahrenheit scale in the U.S.

Water freezes at 0^o Celsius and boils at 100^o Celsius which is a difference of 100^o. Water freezes at 32^o Fahrenheit and boils at 212^o Fahrenheit which is a difference of 180^o. Therefore each degree on the Fahrenheit scale is equal to 100/180 or 5/9 degrees on the Celsius scale.

How to convert Fahrenheit temperatures to Celsius
• Subtract 32^o to adjust for the offset in the Fahrenheit scale.
• Multiply the result by 5/9.
• Example: convert 98.6^o Fahrenheit to Celsius.
  98.6 - 32 = 66.6
  66.6 * 5/9 = 333/9 = 37^o C.

There is a mental math method to approximate the Fahrenheit to Celsius conversion. The ratio of 5/9 is approximately equal to 0.55555....

How to approximate the conversion of Fahrenheit temperatures to Celsius with mental math.
• Subtract 32^o to adjust for the offset in the Fahrenheit scale.
• Divide the Celsius temperature by 2 (multiply by 0.5).
• Take 1/10 of this number (0.5 * 1/10 = 0.05) and add it from the number above.
• Example: convert 98.6^o F to Celsius.
  98.6 - 32 = 66.6
  66.6 * 1/2 = 33.3
  33.3 * 1/10 = 3.3
  33.3 + 3.3 = 36.6 which is an approximation of the Celsius temperature