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 Celsius scale is equal to 180/100 or 9/5 degrees on the Fahrenheit scale.

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

There is a mental math method to convert from Celsius to Fahrenheit. The ratio of 9/5 is equal to 1.8 and 1.8 is equivalent to 2 - 0.2

How to convert Celsius temperatures to Fahrenheit with mental math.
• Double the Celsius temperature (multiply by 2).
• Take 1/10 of this number (2 * 1/10 = 0.2) and subtract it from the number above.
• Add 32^o to adjust for the offset in the Fahrenheit scale.
• Example: convert 37^o C to Fahrenheit.
  37 * 2 = 74
  74 * 1/10 = 7.4
  74 - 7.4 = 66.6
  66.6 + 32 = 98.6^o F

Note: This lesson includes temperatures that use negative numbers. Negative numbers follow a pattern that is opposite of positive numbers. The number -20 is smaller than the number -10. Example of counting with negative and positive numbers: -5, -4, -3, -2, -1, 0, 1, 2, 3, 4, 5.

Example of converting negative Celsius temperatures to Fahrenheit
• Convert -15^o C to Fahrenheit.
• -15 * 9/5 = -135/5 = -27
• -27 + 32 = 5^o F