## Introduction: Convert Centigrade to Fahrenheit Mentally

My first Instructable.

While on holiday in the Canary Islands we were kept informed of the time and temperature by a number of public digital displays. Naturally the temperature was in Centigrade. Fine, except that in spite of the UK going metric, atmospheric temperatures do not mean much to me unless they are in Fahrenheit (it is an age thing, you know!) so I got to thinking how best to convert them. The following was my train of thought.

The formula is, of course :

F = (9/5)C + 32

The problem is the fraction 9/5 until I realised that with a little elementary algebra, this could be transformed into a much simpler form.

Consider

9/5 = 18/10 = (20-2)/10 = 2 + 0.2

Wow ... Eureka!

## Step 1: Examples

So, in order to convert degrees Centigrade to Fahrenheit we have now a new formula:

F = (2C + 0.2C) + 32

... and that we can do in our head! We merely multiply the centigrade number by 2, subtract a tenth of that, and add 32.

Let's try some examples.

Firstly, something that we all know. 100C = 212F

100 x 2 = 200

1/10 = 20 (move the decimal point)

Subtract = 180

+32 = 212F

Let's try another one, 60C

60 x 2 = 120

1/10 = 12 (move the decimal point)

Subtract = 108

+32 = 140F

Or 25C

25 x 2 = 50

1/10 = 5

Subtract = 45

+32 = 77 (see below)

## Step 2: Finally

If you are only interested in an answer to the nearest degree then it is OK to round the tenth part to the nearest degree if it is not a whole number, to ease the mental strain.

i.e. 19C

19 x 2 = 38

1/10 = 4 (rounding 3.8)

Subtract = 34

+32 = 66F (instead of 66.2F)

Now it is your turn. Try 30C, 62C and 1,000C.

Easy, isn't it?

You can always check by entering "XXX Centigrade in Fahrenheit" into Google.

If only it was that easy the other way round.

If you have a mental method for converting degrees F to degrees C then let's hear it.

## 14 Comments

14 years ago

Hey, this is a great instructable and is very informative. Just one thing is missing... pictures! It really helps a lot when trying to follow directions so you should consider taking some photographs. Once you do that and leave me a message when you have so that we can publish your work. Thanks! Thanks for the cool instructable and we hope to publish this soon!

Reply 9 years ago on Introduction

As an admin you should be obligated to check the validity of such formulations before you give the kudos and publish an instructible like this. This comes up very near the top of a google search for C to F conversion formula, and it is dead wrong. And apparently only one other person bothered to even look at the math.

14 years ago

Hey, this is a great instructable and is very informative. Just one thing is missing... pictures! It really helps a lot when trying to follow directions so you should consider taking some photographs. Once you do that and leave me a message when you have so that we can publish your work. Thanks! Thanks for the cool instructable and we hope to publish this soon!

14 years ago

Hey, this is a great instructable and is very informative. Just one thing is missing... pictures! It really helps a lot when trying to follow directions so you should consider taking some photographs. Once you do that and leave me a message when you have so that we can publish your work. Thanks! Thanks for the cool instructable and we hope to publish this soon!

7 years ago

Very confusing writing. The author is mixing up addition and subtraction, as if they were the same thing. The text says 20/10 - 2/10 = 2 + 0.2. Note how the minus sign has changed to a plus sign. In the written description it uses the word “subtract” for the “+” operation. Very confusing.

8 years ago on Introduction

Typo ! F = (2C - 0.2C) + 32 is the right formula

9 years ago on Introduction

All this and only Phebra noticed that your formula is incorrect.

In the universe where I live (20-2)/10 = 1.8 not 2.2 as you claim.

You should either delete this instructable or correct your errors.

10 years ago on Introduction

That's a great way of thinking it, BUT the trick is not to convert at all. Just notice what the temperature feels like when you see it in centigrade and internalise that number - that way you will quickly 'know' centigrade. If you 'convert' then you will be forever 'converting': from the new, superior system to your old internal inferior system and keeping yourself rooted in the past. Give yourself a firmware upgrade and learn to *work with* metric!

14 years ago on Step 2

Like you, I am so used to fahrenheit that I find centigrade useless. No matter how many times I have tried to learn the calculation converter, yours is the very first I can remember, time after time. Thank you SO MUCH for publishing this as it has made such a difference when travelling.

14 years ago on Introduction

Nothing wrong with metric system. For example: water ices at 0°C and boils at 100°C (approximate, of course). If you prefer to convert it to °F, OK, but... what is the logic on indicating water icing at 32°F and boiling at 212°F?! I know a lot of people in "non-metric" countries are used to, and there's a scientific reason for Fahrenheit unit, but It will be always pretty weird to me, anyway. (PS: don´t ask me about inches x milimeters: I can be rude! >=[

14 years ago on Introduction

I am usually not that interested in being exactly correct with the conversion. I usually estimate the C to F conversion by doubling C and add 30. It is fairly close at normal temperatures .

25C is about 80F (25 x 2 =50 + 30 = 80)

when really 25C = 77F. I don't find the small error of 3F degrees critical.

F to C is estimated by subtract 30 from F and halve it. And don't worry about decimals.

77F - 30 = 47 / 2 = 23C or 24C (really 77F is 25C).

By the way, 'normal' body temperature is considered 98.6F. As body temperature will change throughout the day this always seemed silly. The real reason it is always stated so accurately is so the people using Celsius have it easier. 98.6F is exactly 36C. The Fahrenheit scale is inherently more accurate than the Celsius scale because 1 degree Celsius is almost 2 degrees Fahrenheit. Therefore it would make more sense to say normal body temperature is 98F and 36.66666666......C.

Reply 14 years ago on Introduction

Sorry, 98.6F is exactly 37C.

14 years ago on Introduction

Going the other way is also easy, but it requires an approximation.

C = (F-32) 5/9

which is approximately equal to

C = 0.55 F - 18

To further simplify:

C = 0.5 F (1+1/10) - 18

This suggests taking half of F, adding a tenth of that and subtracting 18.

Example: F = 112. Half of that is 56. A tenth of that is 5.6. Adding the two we get 61.6. Subtracting 18 (or subtracting 20 and adding 2) we get 43.6 or 44. The correct is 44.44. Close enough.

To summarize:

Take half of F, add a tenth of that, subtract 20 and add 2.14 years ago on Introduction

The formula is wrong!

It should be F=(2C-0.2C)+32 i.e. -0.2C and not +02C

e.g 45 C = (90 - 9) + 32 = 113 F

phebra