The formula to convert degrees Fahrenheit (°F) to degrees Celsius (°C) is:

°C = (°F - 32) x 5/9

Substituting the value of -40°F into the formula, we get:

°C = (-40°F - 32) x 5/9

°C = (-72) x 5/9

°C = -40°C

Therefore, -40°F is equivalent to -40°C.