PRINCIPLE OF TEMPERATURE CONVERSION
Understanding the principle of conversion between Farenheit and Celsius allows us to rebuild the formula anytime we forget it. The secret is in comparing the degree spreads in the two systems between FREEZING and BOILING of water in a standard atmosphere of pressure.
SPREAD from Freezing to Boiling of Water
The Celsius scale spread is approximately 100 degrees between freezing and boiling of water. (The modern and more precise definition of the unit scale of Celsius/Centigrade references absolute zero and a special "triple point" value of water. This puts the boiling of water at 99.9839 °C. But 100 °C is close enough for our application.)
So: 100 °C - 0 °C = 100 degree difference
The Farenheit scale spread is 180 degrees between freezing and boiling of water.
So: 212 °F - 32 °F = 180 degree difference
The ratios of this difference can be used to convert between the two unit systems.
100 / 180 ----> 5/9 when converting from °F to °C
CONVERSION °F to °C
°F - 32 = value
value * 5/9 = °C
CONVERSION °C to °F
This formula is the °F to °C formula in reverse:
°C * 9/5 = value
value + 32 = °F