Celsius-Fahrenheit Conversion Lite

I was talking to an American at QCon about how cold Sydney gets in "winter", and I realised I didn't know how to readily express 5 degrees Celsius in Fahrenheit, which is what the Americans understand (to go with their other antediluvian measures like gallons, pounds and miles). Of course, I know that you multiply by 9, divide by 5 and add 32, but it's not an operation you can do when you're talking ("in clickstream", to use an expression I learnt at the conference).

This guy told me an approximation that works pretty well:

Celsius to Fahrenheit: Double the value and add 30.

Fahrenheit to Celsius: Subtract 30 and halve.

So 5 degrees Celsius is approximately 40 degrees Fahrenheit ((5 x 2) + 30), which is close to the actual number, which is 41 degrees Fahrenheit.

Sometimes, "timely and close enough" are better than "accurate but too late".