A shockingly useful "quick and dirty" estimate for C to F for temps humans are likely to encounter is 2x + 30. It's not precise at all, but for purposes of "what does that feel like outside" or "should I bring a sweater" it works pretty well.
21C would, by this estimate, be 72F. The true conversion is just shy of 70F, so, again, it's not correct but it's close enough for this kind of conversation.
In this HN subthread: users slowly converge on the conversion formula for Celsius to Fahrenheit (32+9C/5) in greater and greater precision while calling it an “approximation.”
21C would, by this estimate, be 72F. The true conversion is just shy of 70F, so, again, it's not correct but it's close enough for this kind of conversation.