Why this one is hard to do in your head
Most unit conversions are a single multiplication. Temperature is not, because the two scales disagree about both the size of a degree and where zero sits. A Fahrenheit degree is five ninths of a Celsius degree, and the Fahrenheit zero is 32 degrees below the Celsius zero. Both corrections have to be applied, in the right order.
Where the scales came from
Fahrenheit set his zero at the coldest temperature he could reliably reproduce — a brine of ice, water and ammonium chloride — and calibrated body temperature near 96. Celsius anchored his scale to water: zero at freezing, one hundred at boiling, at standard atmospheric pressure. Neither zero is physically special, which is why Kelvin exists. Kelvin uses the Celsius degree but starts at absolute zero, so 0 K is −273.15 °C and there is nothing below it.
The useful anchors
Minus forty is the crossing point — −40 °C and −40 °F are the same temperature, which makes it a handy sanity check on any conversion. Zero Celsius is 32 °F, sixteen is 61, twenty-eight is 82. Normal human body temperature is 37 °C or 98.6 °F, though that famous figure is a nineteenth-century average and modern studies put the typical value slightly lower.
The quick approximation
Double the Celsius figure and add 30. At 20 °C that gives 70 against a true 68; at 30 °C it gives 90 against a true 86. It is good enough for deciding what to wear and useless for cooking or medicine, where a couple of degrees changes the answer.