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Four scales, one number
Celsius and Fahrenheit both have arbitrary zero points, so converting between them needs a shift as well as a scale factor: °F = °C × 9/5 + 32. Kelvin and Rankine start at absolute zero instead, which is why they never go negative.
Where −40 comes from
There is exactly one temperature where Celsius and Fahrenheit agree, and it is −40°. Below that, Fahrenheit numbers are smaller; above it, larger. It is a handy sanity check when you are not sure a conversion went the right way.
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Frequently asked questions
Q. Why does Kelvin have no degree sign?
Kelvin is an absolute unit, not a degree on a scale with an arbitrary zero. Since 1968 the correct form is “300 K”, not “300 °K”.
Q. What is Rankine used for?
Rankine is Fahrenheit measured from absolute zero. It still turns up in some US engineering work — thermodynamics of engines and air conditioning — where Fahrenheit is the working unit.
Q. Is absolute zero really −273.15 °C?
Yes, by definition. Since the 2019 SI redefinition the kelvin is fixed via the Boltzmann constant, and 0 K corresponds to exactly −273.15 °C.