Here's how to convert Fahrenheit to Celsius:

Formula:

°C = (°F - 32) × 5/9

Calculation:

* °C = (99.9 - 32) × 5/9

* °C = 67.9 × 5/9

* °C = 37.72

Therefore, 99.9 degrees Fahrenheit is equal to 37.72 degrees Celsius.