This is for my dear friends who will soon be travelling to Spain:

When I was in elementary school, we were taught, drilled and tested on the formulas for converting temperature measurements between Fahrenheit and Celsius. Celsius was known as Centigrade back then, but they both begin with C so it doesn't matter. :)

F = 9/5 x C + 32

C = 5/9 x (F - 32)

It was an all but useless exercise. How often do most people need to do those conversions? In the science lab, we use Celsius; otherwise, Fahrenheit. Except at our house, when I was young. The thermometer that my father bought and installed outside our window read only in Celsius, so I was kept busy converting it into the more familiar numbers. Even so, I never really learned the conversion formulas; I never could remember which way they went. How liberating it was, many years later, when I realized that I could easily figure that out, knowing 0C = 32F (water freezes) and 100C = 212F (water boils).

It wasn't until I was more than 30 years outside of elementary school, vacationing in New Zealand, that I discovered even greater freedom. All temperatures there are in Celsius (as they are in most of the world), and those old formulas were just too clumsy. So I amused myself by developing a much handier formula that was just fine for my purposes.

When you are visiting the Celsius world, and you hear that the outside temperature is 25 degrees, and you're wondering if you should wear short sleeves or a heavy jacket, just multiply that temperature by 2 and add 30. Easy to do in your head, and it quickly tells you that the temperature is about 25 x 2 + 30 = 80 degrees. Definitely short sleeves.

F = 2 x C + 30

It's easy to remember, easy to work out in your head, and accurate to within a few degrees over the range a tourist is interested in. (Just don't plan to visit Antarctica.) If you want to be more accurate, just remember that the result of this formula is exact at 50 degrees Fahrenheit, a little too high for higher temperatures, and a little too low for lower ones.

For Celsius users visiting the United States (is there anywhere else that uses Fahrenheit?), the formula C = 1/2 x (F - 30) is almost as easy to use.

Posted by sursumcorda on Saturday, October 30, 2004 at 9:33 pm | Edit
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Thanks! This was really helpful. I will definitely use this when I am in Australia in a week! Thanks again.

Posted by Sarah on Thursday, February 02, 2006 at 12:24 pm
You're welcome! Have a wonderful time.

Posted by SursumCorda on Thursday, February 02, 2006 at 3:07 pm
I recently needed to convert between .1 degrees Kelvin and degrees Fahrenheit. It was fun figuring out the formula to make it work as efficiently and with as little rounding error as possible. I don't usually need to do any math at work.

Posted by jondaley on Saturday, February 04, 2006 at 3:26 pm
Hey, I just moved to Canada from the states and this helped me tremendously!! thank you!

Posted by Marie on Saturday, April 01, 2006 at 11:13 am
That's great to hear; thanks for letting me know it was useful.

Posted by SursumCorda on Saturday, April 01, 2006 at 5:42 pm
Belize does.

Posted by Stephan on Friday, February 29, 2008 at 6:52 pm

I am constantly figuring the F to C when spending months each year in a C speaking country!!! For example if a recipe calls for 350*F - I subtract 32 which gives me 318* and then divide by 1.8 which gives me 176.66 or 177*.

As the saying goes "That is close enough for government work"!

Posted by Karen on Thursday, February 05, 2009 at 6:27 am

I wish the government worked that precisely...

Anyway, for those who spend their vacations abroad online, here's a unit converter that can convert Fahrenheit to Celsius and much more.

Posted by Stephan on Friday, June 26, 2009 at 6:18 am

That's a cool converter. I enjoyed seeing all those different units. What a pity, however, that my father's favorite speed measurement is not among them: furlongs/fortnight. It does have both furlongs and fortnights, however.

Posted by SursumCorda on Tuesday, June 30, 2009 at 4:44 pm

Excellent. Very useful (and seems to impress as well!! LOL.
:) thank you

Posted by Jon Robinson on Sunday, May 23, 2010 at 7:32 am

Cool! A "good post" comment that's not spam. :) Thanks, Jon. I'm glad you found it helpful.

Posted by SursumCorda on Sunday, May 23, 2010 at 11:38 am

Another easy one is that you picks temperature you know, the simplest being 0 C and 32 F (or 50=10), and knowing that adding 5 to the Celsius = adding 9 to the Fahrenheit, so if 32=0, 41=5, 50=10, 59=15, 68=20, 77=25, 86=30, 95=35, 104=40... and so on and so forth.

Posted by pat on Thursday, September 30, 2010 at 7:17 pm

That's cool! I like the idea of remembering 50=10, too.

Posted by SursumCorda on Thursday, September 30, 2010 at 7:48 pm

what about if the temperature is -23C? what's the formula then?

Posted by Louise on Thursday, November 18, 2010 at 9:39 am

The quick calculator only works well for the mid-range. That's why I said it's not good for tourists in Antarctica. :)

Posted by SursumCorda on Friday, November 19, 2010 at 7:42 am

This formula is superb. Even Kindergartens can use it conveniently. I like it thanks for the research

Posted by Memuna on Wednesday, December 29, 2010 at 5:53 pm

Thanks, Memuna. I apologize for the long delay in moderating your comment; I quite neglected the blog over Christmas.

Posted by SursumCorda on Saturday, January 08, 2011 at 8:02 am

If your more familiar with Fahrenheit than celsius just remember the basics for celsius 15 and up you don't need a jacket, 5 and below you're gonna need a jacket, anywhere under -15 pull out your winter jacket haha

Posted by coldcanadian on Tuesday, March 08, 2011 at 4:53 am

Terrific answer. Thank you. I learned this formula a few years ago, as a tourist in Australia.
Now, I'm making plans to go to travel Canada, and I think knowing this bit of information, will be helpful.
Thanks again,,,

Posted by Jim on Sunday, July 03, 2011 at 12:27 am

You're welcome!

Posted by SursumCorda on Sunday, July 03, 2011 at 5:41 am
Excerpt: A comment made by Janet to my Quick Tourist's Conversion between Fahrenheit and Celsius post inspired these thoughts, and it seemed better to give them their own post rather than to comment on that one.: The Orlando Sentinel of January 31 contained an...
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