# C to F

Whenever I need to convert Celsius to Fahrenheit, a few numbers go through my head. Five in total, including the source and the outcome. Let’s call these C and F respectively.

The five numbers are as follows:

• C
• round(C,5)/5
• 9*round(C,5)/5
• 9*round(C,5)/5+32
• 9*round(C,5)/5+32+(C-round(C,5))*2, Or F, if you like.

It’s way easier than it sounds, and takes less than two seconds from start to finish. Take 16 for example

• 16
• 3. That’s taken by rounding 16 to the nearest five (15) and dividing this by 5
• 27. That’s taking that three and multiplying it by nine
• 59. That’s adding 32 to the 27
• 61. The 16 was bigger than its rounded equivalent (15) by 1. Times this by 2 and add it to the 59. If it was less than its rounded equivalent, then reduce it accordingly.

Its results are perfect (always correct to the nearest degree Fahrenheit), and it’s a lot more reliable than the double it and add 30 estimate that works quite well for balmy weather, but gets unreliable for very hot and cold spells, and is downright wrong for oven temperatures.

• 30C gives you 90F instead of 86F
• 25C gives you 80F instead of 77F
• 16C gives you 62F instead of 61F
• 0C gives you 30F instead of 32F
• 200C gives you 430F instead of 390F

4 Responses to “C to F”

1. Mum on May 11th, 2008 09:32

Still prefer double it and add 30. Near enough for me!! Nearly rang about 1 hour ago but thought you might be in bed!

2. TimH on May 11th, 2008 10:25

Yeh but!

I never go from C to F, it is the Americans or old folk who keep using F dammit, do you have a similar easy way of changing F to C?

3. Kelvin on May 12th, 2008 05:52

Is there a method for converting F to C like this?

4. Dan on May 12th, 2008 08:11

To go from F to C, I guess you do the reverse. Take 32 off. Round to the nearest multiple of 9. Divide by 9. Multiply by 5. Adjust by half of the difference between the number that you got after taking off the 32 and the nearest multiple of nine.

Take 81 for example. Take off 32 to get 49. Round this to the nearest multiple of 9 (45). Divide by 9 to get 5. Multiply by 5 to get 25. Then add 2 (because the 49 is 4 higher than the 45) to get 27.