−40 (number)

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Decimal	−40
Binary	−101000 or 11011000 in two's complement in a signed byte
Hexadecimal	−28, or D8 in two's complement in a signed byte

Minus forty is the unique temperature (in degrees) at which the Celsius and Fahrenheit scales read the same; it is often referred to as 40 below, which is short for "40 degrees below zero"

To convert from Fahrenheit to Celsius, one subtracts 32, then multiplies by 5/9. This is expressed mathematically as C = (5/9)(F − 32), or F = C(9/5) + 32. It is easily checked that if C = −40 then F = −40 and vice versa.

Using −40, the Fahrenheit and Celsius temperature conversion formulae can be represented by the following:

and

which may be easier for mental calculation.

To prove that F = C, when either are equal to -40,

Let's let x = F and C

9x + 160 = 5x

4x = − 160

x = − 40