To convert 78 degrees Fahrenheit (°F) to Celsius (°C), you can use the following formula:

°C = (°F - 32) x 5/9

Plugging in the value of 78°F:

°C = (78 - 32) x 5/9

°C = (46) x 5/9

°C = 25.56

Therefore, 78 degrees Fahrenheit is equal to approximately 25.56 degrees Celsius.