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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You're welcome; I'm glad you like it.

Posted by SursumCorda on Friday, September 28, 2012 at 8:01 am

Really helpful. Thank you

Posted by Arshad on Sunday, December 30, 2012 at 11:49 pm

This is great, easy to remember/figure out...

Posted by Jason on Tuesday, January 15, 2013 at 6:18 am

try to convert using this formula
32 F to C
should be o C, but it comes 1 C

Posted by jon on Wednesday, February 06, 2013 at 10:54 am

Correct: the quick conversion is almost always wrong, but - and that's what matters - almost always close.

Posted by Stephan on Wednesday, February 06, 2013 at 4:34 pm

Really, the number of people who read this post but don't understand it at all is rather scary. What are schools teaching if so many people fail at such basic reading comprehension!

Posted by Janet on Thursday, February 07, 2013 at 2:01 am

Why don't we just start using metric system - and Celsius - as any other good boy on this planet?? Easier and more effective...

Posted by Luca on Thursday, May 02, 2013 at 7:21 am

Will this work for my tests in math

Posted by Christa on Monday, May 13, 2013 at 5:50 pm

Not unless you're asked for an estimate, Christa. Most math teachers want the exact answer.

Posted by SursumCorda on Tuesday, May 14, 2013 at 5:45 am

Thanks! Leaving for Australia next week and this is a hot tip! Gracias, gracias!

Posted by Donna Hanna on Thursday, August 08, 2013 at 9:24 am

Another couple of good ones to remember are 61F = 16C and 82F = 28C.

Together with 50 = 10, you should be able to make an educated guess when trying to decide what the temperature is..


Posted by Wendy on Thursday, October 10, 2013 at 7:48 pm

Thanks, Wendy. They're not exact, but absolutely close enough for these purposes—and easy to remember.

Posted by SursumCorda on Friday, October 11, 2013 at 6:30 am

Great. The wife is always bugging me but now I can do it in my head. Much thanks for this simple formula.

Posted by JOhn on Tuesday, January 21, 2014 at 6:29 pm

You're welcome, John.

Posted by SursumCorda on Wednesday, January 22, 2014 at 7:08 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...
Weblog: Lift Up Your Hearts!
Date: February 8, 2013, 6:31 am
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