Chapotraphouse

For Fahrenheit to Celsius conversion, you only need to calculate with twos:

• Divide by two
• Add two
• Subtract two from the tens (i.e., subtract 20).

For extra accuracy, after adding two (in step two 😉), slide that number over to the right (i.e., divide by 10) and add that on. Do this as many times as you are comfortable with (which can be none for rough calculations).

For example:

• 90ºF divided by two is 45
• 45 plus two is 47
• Subtracting two from the 4 in 47 leaves 27

This is quite rough so round it off: 30ºC

More accurately:

• 90ºF divided by 2 is 45
• 45 plus two is 47
• Sliding 47 over to the right makes 4.7
• 47 plus 4.7 is 51.7
• Subtracting two from the 5 in 51.7 leaves 31.7

This is a little rough so round it off: 32ºC.

Probably unnecessarily accurate:

• 90ºF divided by 2 is 45
• 45 plus 2 is 47
• Sliding 47 over gives 4.7, and again is 0.47
• 47 + 4.7 + 0.47 = 52.17
• Subtracting twenty makes 32.17ºC

There's still an infinity of sliding that could go on so, once again, round off: 32.2ºC.

The usual (F - 32) * 5 ÷ 9 formula gives 32.222…ºC.