# Easy math for F to C conversion? **URL:** \ **Category:** Factual Questions\ **Created:** [March 10, 2008, 2:15am UTC](https://boards.straightdope.com/t/easy-math-for-f-to-c-conversion/440790 "2008-03-10T02:15:03Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![BellRungBookShut-CandleSnuffed](https://avatars.discourse-cdn.com/v4/letter/b/bcef8e/32.png) [@BellRungBookShut-CandleSnuffed](https://boards.straightdope.com/u/BellRungBookShut-CandleSnuffed)\ **Post date:** [March 10, 2008, 2:15am UTC](https://boards.straightdope.com/t/easy-math-for-f-to-c-conversion/440790/1 "2008-03-10T02:15:03Z")
In another thread a little while ago, someone posted a method of coverting Celsius to Farenheit in your head. Multiply the C value by 2, subtract 10% of that number, and then add 32. I’ve been trying to come up with an easy way to do F to C, but I’m having trouble getting a formula. I know I suck at math, but I feel like I should be able to reverse this process somehow, and I’m frustrated that I can’t. Can anyone help? ---
**Author:** ![Lamar\_Mundane](https://avatars.discourse-cdn.com/v4/letter/l/85f322/32.png) [@Lamar\_Mundane](https://boards.straightdope.com/u/Lamar_Mundane)\ **Post date:** [March 10, 2008, 2:21am UTC](https://boards.straightdope.com/t/easy-math-for-f-to-c-conversion/440790/2 "2008-03-10T02:21:11Z")
(F x 5/8) - 32 = C Very accurate, although not completely easy to do in one’s head if math challenged. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [March 10, 2008, 2:30am UTC](https://boards.straightdope.com/t/easy-math-for-f-to-c-conversion/440790/3 "2008-03-10T02:30:01Z")
[QUOTE=Lamar Mundane] (F x 5/8) - 32 = C [/QUOTE] You mean 5/9, not 5/8. To reverse the easy math-in-the-head method in the OP, take half the F value, then add 10% of that figure and subtract 32. ---
**Author:** ![BellRungBookShut-CandleSnuffed](https://avatars.discourse-cdn.com/v4/letter/b/bcef8e/32.png) [@BellRungBookShut-CandleSnuffed](https://boards.straightdope.com/u/BellRungBookShut-CandleSnuffed)\ **Post date:** [March 10, 2008, 2:36am UTC](https://boards.straightdope.com/t/easy-math-for-f-to-c-conversion/440790/4 "2008-03-10T02:36:32Z")
Gah, I knew it had to be something simple like that. I don’t know what I was thinking. **Lamar** , I did know the formula for it, just was looking for a way to not have to divide by nine in my head. The formula I gave and the one **QED** provides here does give an exact temperature as well. There are some simple ways to get ballpark estimates, but it’s nice to have a way to do it exactly in your head. Thanks guys. Oh, and when did SDSAB become longer? ---
**Author:** ![lobotomyboy63](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lobotomyboy63/32/2900_2.png) [@lobotomyboy63](https://boards.straightdope.com/u/lobotomyboy63)\ **Post date:** [March 10, 2008, 2:40am UTC](https://boards.straightdope.com/t/easy-math-for-f-to-c-conversion/440790/5 "2008-03-10T02:40:19Z")
[QUOTE=Lamar Mundane] (F x 5/8) - 32 = C Very accurate, although not completely easy to do in one’s head if math challenged. [/QUOTE] I believe the correct method is: (F-32) x 5/9 E.g. 212-32=180 180 x 5/9 =100C Check…212F and 100C are the boiling points. If F=32, it will work out to 0C, both freezing. Ballparking, I keep in mind that 37C=98.6F, body temp. And the scales cross at -40C/F. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [March 10, 2008, 2:40am UTC](https://boards.straightdope.com/t/easy-math-for-f-to-c-conversion/440790/6 "2008-03-10T02:40:52Z")
[QUOTE=Q.E.D.] To reverse the easy math-in-the-head method in the OP, take half the F value, then add 10% of that figure and subtract 32. [/QUOTE] Actually, make that subtract 32 from the F value, take half the resulting value and then add back 10%. Sorry for the confusion. > [@BellRungBookShut-CandleSnuffed](#): > > Oh, and when did SDSAB become longer? Since some time today, because apparently the old way too confusing for some people. ---
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [March 10, 2008, 2:47am UTC](https://boards.straightdope.com/t/easy-math-for-f-to-c-conversion/440790/7 "2008-03-10T02:47:24Z")
Simplest formula to remember and use, to go either way: Add 40 Multiply by 5/9 for F -\> C, or by 9/5 for C -\> F [to remember which factor to use, think of the boiling points: 212F = 100C: F is “bigger” and needs to be made “smaller,”] Subtract 40 ---
**Author:** ![BellRungBookShut-CandleSnuffed](https://avatars.discourse-cdn.com/v4/letter/b/bcef8e/32.png) [@BellRungBookShut-CandleSnuffed](https://boards.straightdope.com/u/BellRungBookShut-CandleSnuffed)\ **Post date:** [March 10, 2008, 3:12am UTC](https://boards.straightdope.com/t/easy-math-for-f-to-c-conversion/440790/8 "2008-03-10T03:12:49Z")
[QUOTE=Q.E.D.] Actually, make that subtract 32 from the F value, take half the resulting value and then add back 10%. Sorry for the confusion. [/QUOTE] Wait, 212-32 is 180. Half of that is 90, and 110% of 90 is 99. What’s going wrong here? ---
**Author:** ![Giles](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/giles/32/60_2.png) [@Giles](https://boards.straightdope.com/u/Giles)\ **Post date:** [March 10, 2008, 3:22am UTC](https://boards.straightdope.com/t/easy-math-for-f-to-c-conversion/440790/9 "2008-03-10T03:22:24Z")
[QUOTE=Q.E.D.] Actually, make that subtract 32 from the F value, take half the resulting value and then add back 10%. Sorry for the confusion. [/QUOTE] No, you need to add back about 11% (1/9 to be precise). ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [March 10, 2008, 3:29am UTC](https://boards.straightdope.com/t/easy-math-for-f-to-c-conversion/440790/10 "2008-03-10T03:29:55Z")
[QUOTE=Giles] No, you need to add back about 11% (1/9 to be precise). [/QUOTE] Right, but it’s much easier to do 10% in your head (particularly if you’re math-challenged), and it’s close enough for government work over the range of everyday temperatures. ---
**Author:** ![Cunctator](https://avatars.discourse-cdn.com/v4/letter/c/43a26b/32.png) [@Cunctator](https://boards.straightdope.com/u/Cunctator)\ **Post date:** [March 10, 2008, 3:45am UTC](https://boards.straightdope.com/t/easy-math-for-f-to-c-conversion/440790/11 "2008-03-10T03:45:12Z")
It depends how accurate you want to be. Back in the 1970s when we metricised, my father’s ‘rough’ rule of thumb for C -\> F was “double it and add 30”. So for F -\> C, a simple approximation is “subtract 30 and halve it”. It’s reasonably accurate at lower room temperatures; increasingly less so as the temperature rises (say above 30C). ---
**Author:** ![mhendo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mhendo/32/3159_2.png) [@mhendo](https://boards.straightdope.com/u/mhendo)\ **Post date:** [March 10, 2008, 4:55am UTC](https://boards.straightdope.com/t/easy-math-for-f-to-c-conversion/440790/12 "2008-03-10T04:55:16Z")
I’m perfectly comfortable doing the calculations in my head, but when i’m trying to explain it to newbies, i usually just tell them to memorize the following: ```auto C F 0 32 10 50 20 68 30 86 40 104 ``` Remember those 5 pairs, and you can get pretty close on any conversion, even if you don’t remember the exact equation. ---
**Author:** ![mnemosyne](https://avatars.discourse-cdn.com/v4/letter/m/c4cdca/32.png) [@mnemosyne](https://boards.straightdope.com/u/mnemosyne)\ **Post date:** [March 10, 2008, 4:57am UTC](https://boards.straightdope.com/t/easy-math-for-f-to-c-conversion/440790/13 "2008-03-10T04:57:43Z")
[QUOTE=Q.E.D.] Since some time today, because apparently the old way too confusing for some people. [/QUOTE] Only now it’s distracting. Can’t win 🙂 ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [March 10, 2008, 5:00am UTC](https://boards.straightdope.com/t/easy-math-for-f-to-c-conversion/440790/14 "2008-03-10T05:00:11Z")
[QUOTE=mnemosyne] Only now it’s distracting. Can’t win 🙂 [/QUOTE] Yeah, but it’s long. What more could a guy ask for? 😃 ---
**Author:** ![Tim\_T-Bonham.net](https://avatars.discourse-cdn.com/v4/letter/t/46a35a/32.png) [@Tim\_T-Bonham.net](https://boards.straightdope.com/u/Tim_T-Bonham.net)\ **Post date:** [March 11, 2008, 2:59am UTC](https://boards.straightdope.com/t/easy-math-for-f-to-c-conversion/440790/15 "2008-03-11T02:59:12Z")
[QUOTE=lobotomyboy63] Ballparking, I keep in mind that 37C=98.6F, body temp. And the scales cross at -40C/F. [/QUOTE] Like this, and like mhendo says, just remember a few numbers and you can do well enough for most purposes: 175ºC = oven baking 100ºC = boiling 60ºC = hot shower 37ºC = normal human body temp 20ºC = comfortable room temp 0ºC = freezing -40ºC or F = bloody cold in either scale! (But not unheard of here in Minnesota winters!) ---
**Author:** ![Dr\_Paprika](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr_paprika/32/3042_2.png) [@Dr\_Paprika](https://boards.straightdope.com/u/Dr_Paprika)\ **Post date:** [March 11, 2008, 3:17am UTC](https://boards.straightdope.com/t/easy-math-for-f-to-c-conversion/440790/16 "2008-03-11T03:17:49Z")
Ten degrees Celsius is fifty degrees Farenheit. Every five degree chance in Celsius is a nine degree change in Farenheit. So 59F is 15C, 32F is 0C… Too easy? ---
**Author:** ![Mangosteen](https://avatars.discourse-cdn.com/v4/letter/m/6a8cbe/32.png) [@Mangosteen](https://boards.straightdope.com/u/Mangosteen)\ **Post date:** [March 11, 2008, 4:10am UTC](https://boards.straightdope.com/t/easy-math-for-f-to-c-conversion/440790/17 "2008-03-11T04:10:41Z")
[QUOTE=mhendo] I’m perfectly comfortable doing the calculations in my head, but when i’m trying to explain it to newbies, i usually just tell them to memorize the following: ```auto C F 0 32 10 50 20 68 30 86 40 104 ``` Remember those 5 pairs, and you can get pretty close on any conversion, even if you don’t remember the exact equation. [/QUOTE] This is the system I used when I lived in Taiwan. I just wanted to add that 18 degrees always separate the F numbers so if you wanted to know 15 C using this method you can just add or subtract 9 from the F column. ---
**Author:** ![don\_t\_ask](https://avatars.discourse-cdn.com/v4/letter/d/e68b1a/32.png) [@don\_t\_ask](https://boards.straightdope.com/u/don_t_ask)\ **Post date:** [March 11, 2008, 4:20am UTC](https://boards.straightdope.com/t/easy-math-for-f-to-c-conversion/440790/18 "2008-03-11T04:20:20Z")
I have always remembered the first conversion formula I was ever told: 5F = 9C + 160 and I still laboriously substitute in whatever value I have and work it out. ---
**Author:** ![Polycarp](https://avatars.discourse-cdn.com/v4/letter/p/82dd89/32.png) [@Polycarp](https://boards.straightdope.com/u/Polycarp)\ **Post date:** [March 11, 2008, 4:35am UTC](https://boards.straightdope.com/t/easy-math-for-f-to-c-conversion/440790/19 "2008-03-11T04:35:33Z")
[QUOTE=Gary T] Simplest formula to remember and use, to go either way: Add 40 Multiply by 5/9 for F -\> C, or by 9/5 for C -\> F [to remember which factor to use, think of the boiling points: 212F = 100C: F is “bigger” and needs to be made “smaller,”] Subtract 40 [/QUOTE] I’ve always liked this one, because it depends on a fun trivia fact and means you don’t have to remember whether you add or subtract 32 inside or outside the parentheses. 40 degrees below zero is 40 degrees below zero, in both systems. So if you add 40 to the temperature you start with, you have the number of degrees above minus 40 it is – whichever scale you’re working with. Then convert it to the other scale, remembering that a Celsius degree is larger than a Farehnheit degree … 180% the size, to be exact, so your result in C needs to be a smaller figure than your start in F, your result in F larger than your start in C. Then subtract the 40 to get back to the result system’s zero value. ---
**Author:** ![Leaffan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leaffan/32/299_2.png) [@Leaffan](https://boards.straightdope.com/u/Leaffan)\ **Post date:** [March 11, 2008, 8:52pm UTC](https://boards.straightdope.com/t/easy-math-for-f-to-c-conversion/440790/20 "2008-03-11T20:52:25Z")
If you want a close enough guesstimate simply subtract 30 then take half of that answer. So: 80 - 30 = 50/2 = 25 70 - 30 = 40/2 = 20 60 - 30 = 30/2 =15 50 - 30 = 20/2 = 10 40 - 30 = 10/2 = 5 30 - 30 = 0/2 = 0 20 - 30 = -10/2 = -5 10 - 30 = -20/2 = -10 0 - 30 = -30/2 = -15 For most ambient ranges this gets you within a degree or two every time. Close enough for me. In fact that’s the reciprocal of how we learned to convert from Celsius to Fahrenheit in the first place in Canada way back when. We simply doubled the Celsius temperature and added 30. Now we’re totally bilingual and don’t need to convert. [Next page](https://boards.straightdope.com/t/easy-math-for-f-to-c-conversion/440790.md?page=2)