Well, I admit I happen to remember the conversion through using it, but that's if you want it exact which normally you don't. Key is simply that 0°C = 32°F.

To do it simply, take °C, double it and add 30 and that gives you a good feel for how hot it is if you normally think in °F. Or take °F, subtract 30 and halve the result if you normally think in °C. That's good enough for normal domestic use!

So 20°C, double it to 40, add 30 gives 70°F, comfortably warm. Almost exact. Easy peasy .