Well, when considering Metric, which uses Celsius, versus Imperial, which uses Fahrenheit, there really isn’t much argument about which is superior for scientific measurement. Metric is the International System of Units, SI.

But most of us learned about our world in one system, not the other. For example, if you grew up in the US, Liberia, or Myanmar, you likely think of the weight of something measured in pounds, someone’s height in feet and inches, and the outside temperature in Fahrenheit (abbreviated as F). If you grew up elsewhere, you may think of the weight of something measured in kilograms, height in meters, and the outside temperature in Celsius (abbreviated as C).

If you have a friend who uses a system with which you are unfamiliar, it would be nice to understand each other better when discussing the temperature.

In this post, I will share a simple math trick for estimating the conversion between Celsius and Fahrenheit. It doesn’t give you the exact conversion, F = 1.8C + 32, but it is close enough to be useful in everyday weather conversations. Below I’ll share, and then explain how and why it works.

## The Trick

To go from Celsius to Fahrenheit: Double the number and add 30.

To convert Fahrenheit to Celsius: Subtract 30 and divide by 2.

Yes, that is all there is to it.

**Note:** This is an approximate conversion method. A better conversion formula is developed later in this chapter.

**Examples:**

## Celsius to Fahrenheit:

25° C ⇒ double 25 to get 50 then add 30 to get ⇒ 80° F

5° C ⇒ double 5 to get 10 then add 30 to get ⇒ 40° F

-15° C ⇒ double -15 to get -30, then add 30 to get ⇒ 0° F

This trick is useful to give a fast, approximate temperature conversion and might be sufficient for casual purposes in everyday conversations.

## F to C

80° F ⇒ 80-30=50, then half of 50 is ⇒ 25° C

40° F ⇒ 40-30=10, then half of 10 is ⇒ 5° C

0° F ⇒ 0-30=-30, then half of -30 is ⇒ -15° C

These examples work fine with easily doubled and halved numbers, but the trick works with any number:

18° C ⇒ Double 18 to get 36, then add 30 to get ⇒ 66° F

66° F ⇒ Take 66 less 30 to get 36, then half 36 to get ⇒ 18° C

Voilà! That is all you need to do, given that you can double or halve two-digit numbers and add or subtract 30. If you can’t, then this is a great way to practice.

## What to Remember

If you grew up using the Imperial, or Fahrenheit, system, and your Metric, or Celsius, friend says: “It’s nice out, it’s 22!” then just double it and add 30. That way, you will know that, indeed, it is nice outside and not below freezing. Try it!

The actual conversion between Celsius and Fahrenheit is a linear function, much like the estimated conversion. Remember the formula

=+y=mx+b from Algebra class? This means that plotting all Celsius and Fahrenheit value pairs on a graph will result in a straight line.

The following graph demonstrates how closely the actual conversion line aligns with the estimated conversion line.

**Side Note**: This graph is in Celsius, on the x-axis and Fahrenheit on the y-axis, as associated with equation to convert C to F, but the closeness of the lines would be clear from any orientation of the axes.

The approximate conversion formula makes the math easier by replacing 1.8 by 2 (the slope ????) and 32 by 30 (the intercept ????:

b) If we round 1.8 up a bit to 2, and 32 down a bit to 30, then we get a very good model for the temperatures that we usually see.

But how close is “close enough” when it comes to weather? Note that the lines are very close together near the middle of this graph, which corresponds to the most common temperatures experienced on Earth. In fact, at 10°C (50°F), the estimate from the formula comes out the same as the value from the actual formula. At that temperature, the lines intersect. But the further the temperature is from that point of intersection, the worse the estimate is.

The table below shows the errors, in degrees Fahrenheit, between the estimated conversion and the actual temperature from-40°C (-40°F) to 50°C (122°F).

This table demonstrates how accurate the estimated conversion is for the typical range of temperatures used in weather discussions. Within this range, the estimated values deviate from the actual values by no more than a couple of degrees on either scale, making it a reliable model.

**Side Note:** You can solve the system of equations to find the temperature where the estimated conversion equals the actual conversion:

=1.8+32=2+302+30=1.8+320.2+30=320.2=2=10F=1.8C+32F=2C+302C+30=1.8C+320.20Thus,=1.8×10+32=50=2×10+30=50

**F=1.8×10+32=50**

**F=2×10+30=50**

## Closing Remarks:

After understanding that the actual conversion formula is a linear function, it is intriguing to consider why Daniel Gabriel Fahrenheit chose 1.8 and 32 for the constants

m andb in=+y=mx+b. Why not use simpler values like=2+30y=2x+30? Or even omit the conversion (using=y=x)? The Kelvin scale, for instance, has a straightforward and meaningful conversion:=+273K=C+273, which relates to absolute zero. But what purpose does the Fahrenheit scale serve? The Wikipedia article on Fahrenheit provides some interesting insights.

This trick is easy to remember and yields results very close to the actual values with minimal mental math. Give it a try next time you need to convert temperatures!

Thanks and happy temperature converting! For more information oxomagazine