I use "double and add 30" (or "minus 30 and halve" for F->C). The accuracy isn't that great, but it's within single digits between freezing and boiling. More th

I use "double and add 30" (or "minus 30 and halve" for F->C). The accuracy isn't that great, but it's within single digits between freezing and boiling. More than good enough to parse "oh my god it was like 40 degrees outside" as being a meaningful statement about the weather.