What Is -40 Celsius In Fahrenheit?

It is the only temp that is the same for both.  :)
-40C = -40F

There are 4 easy ways to convert from one temp to another using a calculator (unless you can multiply and/or divide in your head).

C: Celsius
F: Fahrenheit
+: Plus
-: Minus
x: Times
/: Divided By
=: Equals

To convert from C to F:
C x 9 / 5 + 32 = F
C + 40 x 9 / 5 - 40 = F
C x 1.8 + 32 = F
C + 40 x 1.8 - 40 = F

To convert from F to C:
F - 32 x 5 / 9 = C
F + 40 x 5 / 9 - 40 = C
F - 32 x .5555556 = C*
F + 40 x .5555556 - 40 = C*

*The last 2 equations will never give you an exact temp.  The larger the number, the less accurate it is.  This is because .5555556 is actually .555...  For practical purposes with normal temps, just round C to the nearest number.

-40 Celsius is equal to -40 Fahrenheit.

-40 Celsius= -40 Fahrenheit

Whereas 40 Celsius is equal to 104 Fahrenheit.

-40° C  =  -40° F