Minus 40 degrees Celsius is exactly minus 40 degrees Fahrenheit. This is the only temperature for which the readings on both scales are equal.

To convert from Celsius to Fahrenheit

C to F -> formula is °C x 9/5 + 32 = °F

-40 * (9/5) + 32 = (-72) + 32 = -40

To convert from Fahrenheit to Celsius

F to C -> formula is (°F - 32) x 5/9 = °C