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.

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

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.

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

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

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?

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. :)

Or, Lisa, you could go to www.weather.com 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.

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

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 !!!

You're welcome, Pat.

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

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.

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 It’s just that some people don’t do very well with fractions.

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

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.

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.

Welcome, Limeville. I do like your name!

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

Subtract 32

Devide by 2 (remember this)

Devide by 10 (add to the number above)

E.g

82-32=50

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

E.g.

15 * 2 = 30

30 / 10 = 3

30 - 3 + 32 = 59

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.

thank you for easy conversion method.

**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