Temperature cannot be converted by multiplying, because the zero points differ.
Celsius sets water's freezing point at zero and its boiling point at one hundred. Fahrenheit was built on different anchors, with water freezing at 32 and boiling at 212. In both scales, zero is simply a point on the scale rather than an absence of temperature. Saying twenty degrees is twice as hot as ten therefore does not hold.
Kelvin sets zero at absolute zero, where molecular motion stops. Because zero really means none, 200K is twice 100K. Its degree size matches Celsius, so adding 273.15 to a Celsius value gives Kelvin. That property is why science uses Kelvin.
Confusing the order of subtracting and dividing produces badly wrong values. Thinking of it as aligning the zero point first, then the degree size, keeps it straight.
Where precision is unnecessary, subtracting 30 from Fahrenheit and halving lands close to Celsius. 70°F is roughly 20°C and 90°F roughly 30°C. Going the other way, doubling Celsius and adding 30 gives a rough Fahrenheit. For reading a forecast this is enough.
A temperature and a temperature difference convert by different rules. To express a five-degree rise in Fahrenheit, you multiply by 1.8 only and do not add 32, because the zero points already cancel. This is a common error in calculations dealing with differences, such as heating and cooling settings.
Each button runs this keyword on that search engine