A Staff Report from the Straight Dope Science Advisory Board

How does a calculator percent key work?

Dear Straight Dope:

Hi! I am a dedicated reader of your column, and I hope it continues for many more years to come. I have a question for you. I was sitting in class today, bored, and looking at my calculator, and I wondered about the function of one of the keys. Every calculator I've seen, down to the cheapest $2 version, has a percent key. What is the use of this key? I've asked everyone I know but no one knows its function, or has ever used the key. Yet I see it on every calculator, even ones without a square root key, which I find infinitely more useful. Thanks for the help!

The percent key is for those of us that get confused by decimal places. (I, for instance, use it all the time.) What it does is just what it looks like it should do--give you a percent. For instance, if you want to know what 27% of 400 is, you just punch in 400 X 27% and it spits out 108. Hitting the percent key apparently tells the innards of the calculator to make whatever number you typed in right before you hit the percent key a two place decimal number (so if you type in 2%, it'll calculate as .02). After some experimentation with my own cheapie calculator ($2.98, no square root key), I have discovered that you must enter the percent last. So 27% X 400 ignores the percent and just multiplies 27 by 400. Also, it comes in pretty handy if you want to add the percent to the original number, as you can just punch in 400 + 27% and come up with 508 (which would be 400 plus 108 and we discovered earlier that 108 is 27% of 400), which is, of course, 127% of 400.

Perfectly clear now, right?

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