Temperature converter
Celsius to Fahrenheit is × 9/5 then + 32; back the other way is − 32 then × 5/9. Kelvin is Celsius plus 273.15. Rankine is Fahrenheit measured from absolute zero.
How to convert temperature
Temperature is the odd one out among unit conversions, and the reason is that the scales do not share a zero. Every other conversion is a single multiplication, so ten of something is always ten times one of it; temperature needs an offset as well, which is why 20°C is 68°F but 40°C is not 136°F. It also means differences and absolute values convert differently: a rise of 10°C is a rise of 18°F, not 50°F. The single crossing point where the two scales agree is −40, which is genuinely −40 in both.
Questions
Multiply by 9/5 and add 32. Reverse it by subtracting 32 first, then multiplying by 5/9.
Minus 40. It is the only point where the two scales cross.
Because the scales start from different zeros. Every other unit conversion is a single multiplication; this one is not.
By the factor only, with no offset. A rise of 10°C is a rise of 18°F.
Fahrenheit-sized degrees counted from absolute zero, so 0°R is −459.67°F. It appears in some US engineering work.
Just kelvin, lower case, with no degree symbol: it was redefined that way in 1967.