every 10 degrees Celcius is 18 Fahrenheit.

0c is 32

10 c is 32 + 18 or 50f

20c is 50 + 18 or 68f

30c is 68 + 18 or 86f

40c is 86 + 18 or 104f

Congrats you can now dress comfortably when you travel.

Why would I want to convert TO Fahrenheit? =/

It's even easier to skip the "reduce by 1/10" step and you still get close. It gets less and less accurate the larger the number becomes, but most people are doing this for day-to-day outdoor temps and it's more than good enough for that to get close. Maybe don't rely on it for baking, though.

edited to add, also instead of "adding 32" it's often easier to think about just adding 30.

That's neat but I definitely won't remember

Is searching for "25 C in F" considered cheating?

I always multiply by 2 and add 30

I rarely care about the actual number. "Cold", "Not bad", "Warm", and "Hot" cover 99% of my weather related temperature interactions. I have also memorized 3 easy conversion points to estimate from if I care. If I really care...well I don't think that I ever have. Maybe cooking instructions, then I might care, but even that is usually just by feel. I'm not a baker.

10C = 50F 
28C = 82F
40C = 104F

Just double it

What about negative numbers