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A Little Trick for Converting Centigrade to Fahrenheit

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Nov 11th, 2009, 07:53 AM
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A Little Trick for Converting Centigrade to Fahrenheit

If you're like me, you still can't help wanting those temps in Fahrenheit. But if mentally trying F = 9/5C + 32 doesn’t work for you, here’s a tip on quickly making the conversion in your head when in Africa (or most places in the world).

Some years ago I was visiting a colleague in Canada, who saw me staring at a temperature sign given in Centigrade. He must have figured I was trying to use the standard formula (F = 9/5C + 32) to work out the Fahrenheit value, so he gave me this tip: double the Centigrade temp, subtract the first digit of the result from the result and add 32. It works pretty well; you’re usually right on but you’ll never be more than 1 degree Fahrenheit off.

For example, 23 C equals 74 F.
23 C x 2 = 46;
46 – 4 = 42;
42 + 32 = 74 F

or (an actual temp during a Christmas visit in Kruger)

42 C x 2 = 84;
84 - 8 = 76;
76 + 32 = 108 F

Last edited by ibtyen; Aug 16th, 2017 at 01:15 PM.
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Nov 11th, 2009, 08:39 AM
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Neat, I like it.
Now, there is a temperature at which the reading (degrees number) is the same in -both- centigrade (Celsius for us old timers) and Fahrenheit. What is that temperature???

regards - tom
ps - not a trick question, for real. Hint - it would be feel cold.
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Nov 11th, 2009, 08:46 AM
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Just between us old timers, that would be -40.

Best, Steve
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Nov 11th, 2009, 08:51 AM
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How nice that that works for you, sdb. However, with all respect, anyone who can do the arithmetic in your "shortcut" ought to be able to do the arithmetic to get the actual value. Also, your shortcut needs to include the notion that the Centigrade temperature must be expressed as a 2-digit value -- if you're using your formula to "convert" 4C to Fahrenheit, you need to think of the doubled 4 as "08," and thus subtract 0 (not 8) before adding 32.

The shortcut is actually calculating a value for Fahrenheit that's equal to (1.9C + 32), rather than the actual (1.8C + 32).

And I did not write this reply to be informative, but rather to show off that I passed algebra class several years ago, at Newton High School.
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Nov 11th, 2009, 09:17 AM
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Rizzuto, Newton High apparently has done an excellent job.

Yes, what you say for Centigrade values 1-4 is correct. I assumed the zero was intuitive but we all know about assuming. One must also remember that when working with minus values that minus a minus is a plus.

The tip I offer is meant to allow the calculation, in one's head, in a matter of a few seconds and to arrive at a Fahrenheit value that is, if not exact, useful as a reference---to those ends I believe it's helpful. My hat is off to those who can easily in their heads work 9/5C + 32 or 1.8C + 32.

Best, Steve
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Nov 11th, 2009, 09:52 AM
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9/5C or 1.8C - have no idea what this even means, so unless it's a test, do I really care whether it's 74 or 78-degrees? Don't think so.

Take the C, double it and add 32... close enough.

23C x 2 = 46 + 32 = 78... 4/degrees won't make me change what I'm wearing.

When it's hot, it's hot; when it's cold, it's cold... and you know it. Anything in-between doesn't much matter - it's comfy!
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Nov 11th, 2009, 11:56 AM
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All this show that the US really missed the boat when they failed to go metric in the last century.
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Nov 11th, 2009, 01:12 PM
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this is making my head hurt--better idea, just bring a thermometer with you I have a tiny one on a keychain which hooks to my purse, and also my alarm clock has one built in!

but @NoFlyZone is absolutely right, we are clueless in the US about metric. I have also printed up cheat sheets to convert cm to inches and made them into bookmarks to put in my bird guide, because all the sizes in the guide are listed in centimeters!
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Nov 11th, 2009, 01:12 PM
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nofly - i have to agree. I wish we'd gone metric back when it was being talked about. I was in junior high at the time and probably could have made the shift, although even then it would have been tough. Now (as an old dog) it would be much harder.
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Nov 11th, 2009, 01:14 PM
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and sandi - the shortcut i always heard is to double the number and add 30, rather than 32. I think it gets you a little closer. Still not quite accurate, but as you say, close enough for what most of us would need.
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Nov 11th, 2009, 01:29 PM
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althom - 30 also works for me.

Metric/schmetric... anything with a decimal point will further mess me up! Numbers/Me = dense! Unless there are lots of "Benjamins"
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Nov 11th, 2009, 03:23 PM
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I always double it and add 30. The math is a little easier that way.

The same works in reverse for our non-American friends when you come to visit. If the temp is 70°F, you can subtract 30 and divide by 2. Hence the temp in Centigrade is 20°C.

I remember in the 70's when we were learning the metric system and they were telling us we will switch someday soon. I wish that had happened but somebody must have dropped the ball on that.

Metrics are used in science and medicine here in the US, and to the best of my recollection that has always been the case.
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Nov 11th, 2009, 03:58 PM
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Okay let me get in on this !

To show that -40 C = -40 F :

F = 9/5 C +32
then we have

5F = 9 C +160

But since F has to be equal to C we have

5F = 9F +160
-4F = 160
F = 160/(-4) equals -40

So at minus 40 both C and F are equals ( Brrrr !)
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Nov 11th, 2009, 05:13 PM
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Excellent Percy, you get full credit for showing your work!!!!
Now for EXTRA credit - why is that fraction 9/5, 9/5C?????

regards - tom
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Nov 11th, 2009, 06:18 PM
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I immigrated in 1966 to the US. I never bothered to learn the US system of Farenheit and inches, because I was told that we (the US) would switch very soon and be "equal" to the rest of the world!!! Yeah, right .... I'm still waiting and I'm still ....

Clueless in Seattle!
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Nov 11th, 2009, 07:16 PM
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Thanks cary999 !!

Reminds me of the old University days...."full marks for showing all your work" and "2 marks out of 10 for only the correct answer."

safarimama:

I have never gotten used to the metric system.. You cannot teach an "Old " dog new tricks.

When I see a distance in Europe or Canada that reads 90Km , I quickly multiply by .6 to get 54 miles. !!!
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Nov 11th, 2009, 07:54 PM
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You're most welcome. But you're not going for the EXTRA credit Percy??
Anyone?...Bueller?...Bueller?... Anyone?
Ok, a hint then. 9/5 is the simplification of the original fraction which is 180/100. Why it is 180/100 well be good enough for the EXTRA credit

regards - tom
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Nov 11th, 2009, 09:42 PM
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Metric is so easy; everything divides by 10 or 100.

How many inches in a foot? How many feet in a yard? What the he...ll!!!

Percy, what's there not to get used to?
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Nov 12th, 2009, 12:56 AM
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tom: 180 and 100 are the number of degrees, in F and C respectively, to go from the freezing point of water to the boiling point.

As for metric vs English system, it boggles the mind that the U.S. remains a 3rd world country -- in fact, the world's only 3rd-world country -- in this respect. Not only is our weird little system difficult to calculate, it is profoundly confusing to most people. Especially the very bizarre notion that the word "ounce" has two entirely different meanings. So 1 ounce of pea soup does not equal 1 ounce of pea soup.
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Nov 12th, 2009, 08:57 AM
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In my travels I have developed a "feel" for kilometers, so I don't need to convert them. But for temperature, I haven't been able to develop a feel, so I do what's described here, with a slight difference. I double the Celsius number, subtract 10% (that's where I differ from others) and add 32. That's close enough to let me know whether I want a swim suit or a snow parka.

Now, for extra credit, who can explain the difference between Centigrade and Celsius, and tell us why one term is preferable in some situations, and the other in others? (Not me, I'm clueless on this one.)

Celia
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