text: The answer "dbl and add 30" is incorrect.\n\nA simple way that is 100% accurate is to double the C temperature, then subtract 1/10 of that number, then add 32.\n\nFor instance: 18 degrees C; double that and you have 36; subtract 1/10 of 36: 36 - 3.6 = 32.4.  Now add 32.4 + 32 = 64.4.