How it works
Every conversion goes through Celsius as a common reference point. Celsius and Fahrenheit each have their own zero point and degree size, so converting between them needs both a scale factor and an offset — not just a simple ratio like most other unit conversions.
Why -40 is special
Negative 40 is the one temperature where Celsius and Fahrenheit agree, since it is exactly where the two scales’ lines cross. Every other temperature reads as a different number in each scale.
When to use Kelvin
Kelvin starts at absolute zero and uses the same degree size as Celsius, which is why it is the standard scale in physics and chemistry — formulas involving temperature and energy work out more simply when temperature can never go negative.