I am sometimes confused with the two scales of temperature: Fahrenheit and Celsius.

You know Fahrenheit is used in a few countries including the U.S. I don't know why Fahrenheit is still commonly used in the U.S. even though many other countries use Celsius. Every time I talk about weather with them, I have to "translate" Celsius to Fahrenheit, and vice versa.

But it is ridiculous to always hold a pen and notes (or a calculator) to use this formula: (F=C*9/5+32). So I have several steps to easily calculate scales in my head.

**Celsius to Fahrenheit**:

You have a value C in Celsius.

1. Double C

C * 2

2. Subtract 10% of it

2C - 0.1 * 2C

3. Add 32

2C - 0.1 * 2C + 32

Then you get an exact value in Fahrenheit.

It is easier than expected.

For example, let's say the temperature is 30 deg. C.

So you can calculate as the below:

30*2=60

60-6=54

54+32=86 The temperature is exactly 86 deg. F.

In case of 20 deg. C.:

20*2=40

40-4=36

36+32=68 The temperature is 68 deg. F.

The reason can be derived from the formula.

F=C*9/5+32

=C*2/2*9/5+32

=C*2*(9/10)+32

=C*2*0.9+32

=C*2-C*2*0.1+32

**Fahrenheit to Celsius**:

You have a value F in Fahrenheit.

1. Divide F by 2

F/2

2. Subtract 16 from it

F/2 - 16

3. Add 10% of it

F/2-16 + 0.1 * (F/2-16)

In this case, you get an approximately value in Celsius.

For example, let's say the temperature is 70 deg. F.

So you can calculate as the below:

70/2=35

35-16=19

19+1.9=20.9 The temperature is about 21 deg. C.

The reason can be derived from the formula.

C=(F-32)*5/9

=(F-32)/2*2*5/9

=(F/2-16)*10/9

=(F/2-16)*1.11111...

≒(F/2-16)*1.1