66 degrees Fahrenheit is equal to 18.89 degrees Celsius. To convert Fahrenheit to Celsius, you can use the formula: Celsius = (Fahrenheit - 32) x 5/9. Applying this formula to 66 degrees Fahrenheit, we get:

Celsius = (66 - 32) x 5/9 Celsius = 34 x 5/9 Celsius = 170/9 Celsius = 18.89

Therefore, 66 degrees Fahrenheit is equal to 18.89 degrees Celsius.