# Using percent key on calculator **URL:** \ **Category:** Factual Questions\ **Created:** [March 27, 2012, 3:29am UTC](https://boards.straightdope.com/t/using-percent-key-on-calculator/616923 "2012-03-27T03:29:46Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![awottokat](https://avatars.discourse-cdn.com/v4/letter/a/bc8723/32.png) [@awottokat](https://boards.straightdope.com/u/awottokat)\ **Post date:** [March 27, 2012, 3:29am UTC](https://boards.straightdope.com/t/using-percent-key-on-calculator/616923/1 "2012-03-27T03:29:46Z")
My calculator has a percent key. I thought that using it to add sales tax to a number would yield the correct total but it gives me a higher amount than it should be. For example if I key in 100+8.25% (using the percent key), the answer should be 108.25, but instead the total appears as 108.99. Why am I getting this incorrect total? ---
**Author:** ![Inner\_Stickler](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/inner_stickler/32/318_2.png) [@Inner\_Stickler](https://boards.straightdope.com/u/Inner_Stickler)\ **Post date:** [March 27, 2012, 3:40am UTC](https://boards.straightdope.com/t/using-percent-key-on-calculator/616923/2 "2012-03-27T03:40:26Z")
Have you tried other amounts? Are they correct? Or are they off by 74 cents as well? [As far as I can tell,](http://www.straightdope.com/columns/read/1539/how-does-a-calculator-percent-key-work) you’re using it correctly. Perhaps, it doesn’t like the decimal point? Have you tried 100+825%? That should be calculated as .0825 unless I miss my guess. ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [March 27, 2012, 3:46am UTC](https://boards.straightdope.com/t/using-percent-key-on-calculator/616923/3 "2012-03-27T03:46:07Z")
On my calculator if I key in 100+8.25% I get 108.25. If I key in 50+6% I get 53. (Actually I never realized you could do this at all with addition.) So are you sure you are keying it in that way? I cannot figure out any method of interpreting that which could yield 108.99. ---
**Author:** ![CurtC](https://avatars.discourse-cdn.com/v4/letter/c/ce73a5/32.png) [@CurtC](https://boards.straightdope.com/u/CurtC)\ **Post date:** [March 27, 2012, 3:50am UTC](https://boards.straightdope.com/t/using-percent-key-on-calculator/616923/4 "2012-03-27T03:50:43Z")
[1] [0] [0] [Enter] [8] [.] [2] [5] [%] [+] ---
**Author:** ![awottokat](https://avatars.discourse-cdn.com/v4/letter/a/bc8723/32.png) [@awottokat](https://boards.straightdope.com/u/awottokat)\ **Post date:** [March 27, 2012, 4:00am UTC](https://boards.straightdope.com/t/using-percent-key-on-calculator/616923/5 "2012-03-27T04:00:47Z")
@Inner Stockler 8.25%=.0825 825% would equal 8.25 Keying in 100+825% yields -13.79 for some reason??? All amounts are proportionately off. If I calculate for 50 instead of 100, the total is off by .37, i.e., I get 54.50 instead of 54.13. I’m confused. ---
**Author:** ![Inner\_Stickler](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/inner_stickler/32/318_2.png) [@Inner\_Stickler](https://boards.straightdope.com/u/Inner_Stickler)\ **Post date:** [March 27, 2012, 4:02am UTC](https://boards.straightdope.com/t/using-percent-key-on-calculator/616923/6 "2012-03-27T04:02:21Z")
I’ve arrived at a diagnosis. Someone spilled soda on your calculator. ---
**Author:** ![awottokat](https://avatars.discourse-cdn.com/v4/letter/a/bc8723/32.png) [@awottokat](https://boards.straightdope.com/u/awottokat)\ **Post date:** [March 27, 2012, 4:10am UTC](https://boards.straightdope.com/t/using-percent-key-on-calculator/616923/7 "2012-03-27T04:10:01Z")
@CookingWithGas Yes, I’m sure I’m entering it correctly. If I key in 50+6%, I get 53.19 instead of 53. There must be some strange setting that I don’t know about. It’s a fairly ancient Casio calculator (model HR-8TE) and I have not been able to locate a user manual for it on line to explain all the possible settings. ---
**Author:** ![Pai325](https://avatars.discourse-cdn.com/v4/letter/p/c68b51/32.png) [@Pai325](https://boards.straightdope.com/u/Pai325)\ **Post date:** [March 27, 2012, 4:59am UTC](https://boards.straightdope.com/t/using-percent-key-on-calculator/616923/8 "2012-03-27T04:59:43Z")
I have found different calculators seem to treat the percent key in different ways, so I found it easier to change the percent to a decimal and work that way. Even if I was working with fractions the calculator would accept a decimal in the calculation with no problem. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [March 27, 2012, 5:45am UTC](https://boards.straightdope.com/t/using-percent-key-on-calculator/616923/9 "2012-03-27T05:45:34Z")
> [@](#): > > Keying in 100+825% yields -13.79 for some reason??? I don’t know what the problem is, but I do know what my next step would be in trying to figure it out. Somewhere in between 8.25% and 825%, it’s transitioning from an increase to a decrease. I’d try to figure out where that transition is. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [March 27, 2012, 5:52am UTC](https://boards.straightdope.com/t/using-percent-key-on-calculator/616923/10 "2012-03-27T05:52:14Z")
> [@awottokat](#): > > For example if I key in 100+8.25% (using the percent key), the answer should be 108.25, but instead the total appears as 108.99. Why am I getting this incorrect total? It sounds like an odd way to implement a percent function, but I’ll point out that (100% - 8.25%) of 108.99 is almost exactly 100. It is as if instead of multiplying by 1.0825, it is dividing by 0.9175. Same goes for your other example: 50\*1.06 = 53, while 50/0.94=53.19. ---
**Author:** ![Cabbage](https://avatars.discourse-cdn.com/v4/letter/c/f07891/32.png) [@Cabbage](https://boards.straightdope.com/u/Cabbage)\ **Post date:** [March 27, 2012, 6:08am UTC](https://boards.straightdope.com/t/using-percent-key-on-calculator/616923/11 "2012-03-27T06:08:39Z")
I think I may have figured out how the calculator is calculating it: The calculator seems to be basing the percentage on the result, rather than the initial value. For example: > [@](#): > > Keying in 100+825% yields -13.79 for some reason??? Working backwards: Take 825% of -13.79: -113.7675 Now subtract that from -13.79: 99.9775 Close enough to 100 that it makes me think the difference is due to a calculator rounding error. The other examples in this thread fit this pattern, too. ---
**Author:** ![spanna](https://avatars.discourse-cdn.com/v4/letter/s/a6a055/32.png) [@spanna](https://boards.straightdope.com/u/spanna)\ **Post date:** [March 27, 2012, 8:10am UTC](https://boards.straightdope.com/t/using-percent-key-on-calculator/616923/12 "2012-03-27T08:10:55Z")
I have never used the % calc on my calculator so I tried this out of curiosity 100 + 8.25 % ans displays on screen as 100+8.25% and the answer is 1312.121212 What the heck is that (100+10% gives 1100 if that helps and 100\*8.25% gives 8.25) ---
**Author:** ![jjimm](https://avatars.discourse-cdn.com/v4/letter/j/ba8739/32.png) [@jjimm](https://boards.straightdope.com/u/jjimm)\ **Post date:** [March 27, 2012, 8:23am UTC](https://boards.straightdope.com/t/using-percent-key-on-calculator/616923/13 "2012-03-27T08:23:13Z")
On my Android calculator app, 100 + 10% = 100.1 WTF. Not exactly intuitive. ---
**Author:** ![Jragon](https://avatars.discourse-cdn.com/v4/letter/j/e19b73/32.png) [@Jragon](https://boards.straightdope.com/u/Jragon)\ **Post date:** [March 27, 2012, 8:26am UTC](https://boards.straightdope.com/t/using-percent-key-on-calculator/616923/14 "2012-03-27T08:26:22Z")
Well, the multiplication in spanna’s case is obvious. 8.25% = .0825 .0825 \* 100 = 8.25 I have no idea what the hell it’s doing with addition. ---
**Author:** ![jjimm](https://avatars.discourse-cdn.com/v4/letter/j/ba8739/32.png) [@jjimm](https://boards.straightdope.com/u/jjimm)\ **Post date:** [March 27, 2012, 8:30am UTC](https://boards.straightdope.com/t/using-percent-key-on-calculator/616923/15 "2012-03-27T08:30:19Z")
It seems to think (on mine) that the % button just means “divide this number by 100 before doing anything else”. Whereas one would expect it to mean “apply this percentage of the preceding number please”. ---
**Author:** ![Dervorin](https://avatars.discourse-cdn.com/v4/letter/d/eb8c5e/32.png) [@Dervorin](https://boards.straightdope.com/u/Dervorin)\ **Post date:** [March 27, 2012, 8:40am UTC](https://boards.straightdope.com/t/using-percent-key-on-calculator/616923/16 "2012-03-27T08:40:58Z")
> [@jjimm](#): > > On my Android calculator app, 100 + 10% = 100.1 > > WTF. Not exactly intuitive. On RealCalc, on Android, 100 + 10% = 110 But yes, the native Android calculator gives me 100.1. WTF? However, 100 x 10% = 10, which makes sense. After more experimentation, I think I know what it’s doing. When you do 100 + 10%, it’s treating the 10% as a fraction, which is 1/10, hence giving you 100.1. That’s mathematically correct, I suppose, but entirely idiotic, because in what circumstances would you want to treat a percentage as a pure fraction? It’s also consistent with the behaviour of the multiplication, since 100 \* 0.1 = 10. 100 + 100x10% = 110, which is again mathematically more accurate but so much more convoluted. ---
**Author:** ![Jragon](https://avatars.discourse-cdn.com/v4/letter/j/e19b73/32.png) [@Jragon](https://boards.straightdope.com/u/Jragon)\ **Post date:** [March 27, 2012, 8:44am UTC](https://boards.straightdope.com/t/using-percent-key-on-calculator/616923/17 "2012-03-27T08:44:49Z")
100+10% = 100.1 is exactly what I would expect. Maybe it’s my programmer brain, but if I wanted 110% or 1.10 as my answer, I would have entered 100%+10% ETA: Though I can see a case to be made for a button that’s essentially a macro such that: 100+10(MACRO%) = 100 + (10% \* 100) but I don’t think that button should be the default percent key. ---
**Author:** ![spanna](https://avatars.discourse-cdn.com/v4/letter/s/a6a055/32.png) [@spanna](https://boards.straightdope.com/u/spanna)\ **Post date:** [March 27, 2012, 8:48am UTC](https://boards.straightdope.com/t/using-percent-key-on-calculator/616923/18 "2012-03-27T08:48:16Z")
ok I just googled my calculator and got the manual looks like to add 8.25% I should have keyed the following 100\*8.25%+ not exactly intuitive ---
**Author:** ![Mr.Greenjeans](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mr.greenjeans/32/551_2.png) [@Mr.Greenjeans](https://boards.straightdope.com/u/Mr.Greenjeans)\ **Post date:** [March 27, 2012, 12:04pm UTC](https://boards.straightdope.com/t/using-percent-key-on-calculator/616923/19 "2012-03-27T12:04:54Z")
Not really. I typically wouldn’t use the % key at all for this calculation. To add 8.25% to 100, I would enter: 100 \* 1.0825 I confess I never really understood the results I got with the % key, but this method always got the job done, so… ---
**Author:** ![srzss05](https://avatars.discourse-cdn.com/v4/letter/s/898d66/32.png) [@srzss05](https://boards.straightdope.com/u/srzss05)\ **Post date:** [March 27, 2012, 1:40pm UTC](https://boards.straightdope.com/t/using-percent-key-on-calculator/616923/20 "2012-03-27T13:40:26Z")
> [@Mr.Greenjeans](#): > > Not really. > > I typically wouldn’t use the % key at all for this calculation. To add 8.25% to 100, I would enter: > > 100 \* 1.0825 > > I confess I never really understood the results I got with the % key, but this method always got the job done, so… That works for 100, but what would you do for, say, 8.25% of 67? I just enter 67+67\*.0825, and get the correct results. I never found a calculator that used the percentage key the same way each time, and never bothered to use it when it had one. [Next page](https://boards.straightdope.com/t/using-percent-key-on-calculator/616923.md?page=2)