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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This method is Not correct!

To be more exact:

It should be

((Celsius X 2) - 10%) + 32

Eg 25F - 25 X2 = 50 - 5 + 32 = 77 Fahrenheit

Posted by Martin Stolzenberg on Wednesday, July 20, 2011 at 11:33 pm

Thanks, Martin. Certainly the formula you give is the correct and accurate one (as I noted above), and it is indeed easier to think of doubling and subtracting 10% than multiplying by 9/5. (Why didn't my elementary school teachers point that out?) But the point of my "quick conversion" formula is to give an approximation, not the exact value. For the purposes of deciding whether to wear a sweater or shorts, there is no difference between 80 degrees and 77.

Posted by SursumCorda on Thursday, July 21, 2011 at 6:12 am

Ah, thank you, just what I needed. I have a weather app on my Android that originated in Norway, and temps are only in Celcius.
I like this formula, nothing fancy, very quick and easy to remember.
Anne O, Philadelphia, PA

Posted by Anne O on Friday, September 16, 2011 at 10:09 am

Glad you find it useful, Anne. Thanks for letting me know.

Posted by SursumCorda on Saturday, September 17, 2011 at 7:53 am

I have a math test on Monday. we are using the method you provided. I cant find any math web pages containing practice work sheets with the formula. I really want to practice the conversions the way you've described. do you know where i can find some Q and A drills with this formula?
BTW I am going to school for the first time in 33 years and need all the help I can get with my math. Can you help?

Posted by lisa on Friday, October 21, 2011 at 10:07 pm

Hi, Lisa! You don't want to use this method on a math test -- it is only an approximation, and teachers generally require exact answers. You could, I suppose, use the approximation to check to see if your answer, after doing the more complicated arithmetic, makes sense.

But you don't need worksheets to practice converting between Fahrenheit and Celsius. Using the exact formulas,

F = 9/5 x C + 32 and

C = 5/9 x (F - 32),

pick any number, use one formula to convert it, and then use the other to convert it back and thus check your answer.

I find that the real trick is in remembering the formulas. If I'm not sure (as I might not be under the pressure of a test), I check out my guess at the formula using values I know, such as the freezing and boiling points of water (32 F = 0 C, and 212 F = 100 C). Also, at -40 degrees, the Fahrenheit and Celsius values are equal.

I had no idea that math classes still required knowledge of this conversion. I'm impressed. :)

Posted by SursumCorda on Saturday, October 22, 2011 at 6:56 am

Or, Lisa, you could go to or another weather website that allows you to switch between F and C. On that site, look up any city's forecast in F, and convert to C, then check your answers by switching to "metric". Pick another city, look up its forecast (now in C), and convert to F. Check your answers by switching back to F. Repeat with more cities until you feel comfortable.

Posted by Stephan on Sunday, October 23, 2011 at 6:19 am

Thanks so much! Goin to Japan soon and this will help

Posted by SeigiYuu on Thursday, October 27, 2011 at 1:02 am

I learned this quick calculation when I was in Australia last year. I remembered doubling the Celsius but forgot the number to add, and vice versa.
This answered my question quick and simple. And just in case I forget again, I have now Bookmarked this page for future reference.
Thanks so very much !!!

Posted by Pat on Saturday, March 31, 2012 at 1:40 pm

You're welcome, Pat.

Posted by SursumCorda on Sunday, April 01, 2012 at 6:46 pm

Hey, this works I am in year 6 and I needed this formula to figure out my formulas quickly for my physics test

Posted by Tarun on Tuesday, April 10, 2012 at 7:56 am

I hope you did all right on your physics test. This formula is only an approximation, and in my experience physics teachers tend to be a little pickier than that. It would be a good device for checking to be sure your answers make sense, though.

Posted by SursumCorda on Tuesday, April 10, 2012 at 11:12 am

Thanks, this is great! One suggestion to simplify the conversion formula for those visiting the United States (and I'm no mathematician, but think this is right): C = F - 30 2 Its just that some people dont do very well with fractions.

Posted by Natasha on Wednesday, June 20, 2012 at 1:19 pm

It is right, Natasha, if you add some parentheses: C = (F - 30) 2.

Posted by SursumCorda on Thursday, June 21, 2012 at 6:31 am

Thanks for the quick tip! It's just I wanted!

I'm going to visit the Unites States next month and was looking for quick way to convert F to C.

Posted by Yukari on Friday, July 13, 2012 at 3:17 pm

Thank you for the formula for F. I grew up thinking in Fahrenheit and, in these Centigrade/Celsius days, still do. I enjoyed your disclaimer. I have homeschooled and my grandmother's village is named for the limes that used to grow abundantly everywhere. Thanks again.

Posted by Limeville on Sunday, August 05, 2012 at 5:09 am

Welcome, Limeville. I do like your name!

Posted by SursumCorda on Sunday, August 05, 2012 at 8:03 am

(More) accurate method is (For F to C)

Subtract 32
Devide by 2 (remember this)
Devide by 10 (add to the number above)
50 / 2 = 25
25/2 = 2.5
25+2.5 = 27.5

Other way around
Double the number (remember this)
Devide by 10 (Subtract this from the above)
Add 32
15 * 2 = 30
30 / 10 = 3
30 - 3 + 32 = 59

Posted by max on Saturday, August 18, 2012 at 9:30 am

Max's C to F forumla is 100% accurate, just a little less intimidating than multiplying with 9 and dividing by 5. His first formula, which is equivalent to dividing by 20 and multiplying by 11, means you get a result that is always exactly 1% too small, which for most people in most situations is more than sufficiently accurate.

Posted by Stephan on Saturday, August 18, 2012 at 2:20 pm

thank you for easy conversion method.

Posted by Su on Thursday, September 27, 2012 at 11:06 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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