Using mental math to convert Celsius to Fahrenheit

[Bushman]

I go to Europe about every 4 years and am always trying to convert temperatures in my head. This trip my wifes cousin who is a math professor at the University of Tronheim gave me this simple method.

I used to use the standard one listed below.

How to convert Celsius temperatures to Fahrenheit
Multiply the Celsius temperature by 9/5.
Add 32 to adjust for the offset in the Fahrenheit scale.
Example: convert 37 C to Fahrenheit.
37 * 9/5 = 333/5 = 66.6
66.6 + 32 = 98.6o F

There is a mental math method to convert from Celsius to Fahrenheit. The ratio of 9/5 is equal to 1.8.

If you just remember that 10C is 50 Fahrenheit
Every 10 C you add 18 thus 20C becomes 68 degrees. 30C you would be 86, etc
Every 5 C you add 1/2 or 9 thus 25C becomes 77, 35C becomes 95, etc

With this it is easy to get exact answers since every C=1.8F
26C would be 78.8

Simple! Hope this helps some of you that don't have quick access to a conversion chart and want to avoid fractions.

[Odin]

Thank you BM,

This does help!Got confused a few weeks ago about temps & pitched too hot.

Odin.

[Bushman]

You're welcome, I immediately upon reading a few strands saw a cross-over to what we do with members all over the world using different measuring systems!

[AKAAB]

I do this every day because our airport weather is reported in C.

Double C
Subtract 10%
Add 32
TADA!

Ex:

25 x 2 = 50
50 - 10% = 45
45 +32 = 77 degrees

[Bushman]

I do this every day because our airport weather is reported in C.

Double C
Subtract 10%
Add 32
TADA!

Ex:

25 x 2 = 50
50 - 10% = 45
45 +32 = 77 degrees

I found that after posting works on the same principle and may even be easier! Thanks.

[J3ff]

I am glad I found this thread - I am clueless when it comes to C - this will save me time asking google to convert for me!!!

[Rod]

I remember the old formula

9C=5F-160

a few to remember

68°F is 20°C 9*20=5*68-160

32°F is 0°C

212°F is 100°C

[KY1792]

I also use the times 2, minus 10%, then add 32 for C to F but going from F to C requires a bit more work and not so easy to do in the head.

25C = 25 x 2 = 50
50 x 10% = 5
50 - 5 = 45
45 + 32 = 77F

conversely

77F = 77 - 32 = 45
45 x 10% = 4.5
45 + 4.5 = 49.5
49.5 / 2 = 24.75C (close enough but more difficult to think out)

Sometimes knowing why you do what you do makes it a bit easier to understand,

1) the F scale starts at 32 whereas C starts at 0, hence the reason to either add or subtract 32
2) the C scale has 100 divisions between boiling and freezing (100 - 0 = 100)
3) the F scale has 180 divisions between boiling and freezing (212 - 32 = 180)

This means that F has a resolution of almost twice that of C (1.8 or 9/5) per degree
Or that C has only has about half the resolution of F (0.55 or 5/9)

I also try to remember certain way points such as:
-40C = -40F - the only time the scales match but is quite useless for us
4C = 39F - highest density of water ( 4C = 40F is close enough)
25C = 77F - room temperature (a bit warm for me)
40C = 104F - ambient motor temperature

As a side note you can thank nature or God for making 39F water denser than 32F water/ice or all of our lakes would freeze from the bottom up, killing all the vegetation and fishy (just a thought to ponder). Ice in general is about 10% lighter than water and ice cubes will sink in a good strong drink.

It is also why a beer can explodes in the freezer when the water tries to expand by 10% its volume, or when held sufficiently under pressure will not freeze until the pressure is released by pulling the tab - then instant ice beer (jacked off).

[Bushman]

Thanks ky1792, I have learned more than I thought I would when I first posted this thread.