Do you know your monitor's resolution?

"Sure I do," you might answer, "it's 1024 x 768," or whatever the height and width in pixels happen to be. That's an easy number to get, and it's what most Web designers mean when they talk about monitor resolution. Looking at the W3C's browser display statistics, you can see that "resolution" is something they keep careful tabs on.

But "resolution" is a misnomer. Your monitor's dimensions in pixels are not really its resolution. If the word means anything, "resolution" should mean how big, or small, the pixels themselves are. I'm talking here about what is sometimes called pixel density. That's a more difficult number to get. Unlike the pixel dimensions, which are sent by your computer with every HTTP request, the pixel density is impossible to measure from the server side.

Every textbook or reference site I've seen says that the "average" monitor has a resolution of 72 pixels per inch. I have my doubts about the accuracy of that number, which probably hasn't been right since 1994. But even if 72 is the average, that tells us nothing about your monitor! Don't you want to know how big your own pixels are?

This simple test will show you how many pixels per inch you're looking at right now. Use the left and right arrow buttons to re-size the picture of the credit card, until it is exactly the size of an actual credit card. You can hold one up to the monitor while you click.

If you're curious, my own monitor right now measures 95 pixels per inch.

I chose a credit card as the standard, because credit cards are made to standard dimensions worldwide, and they are common enough that you probably have one handy. If you don't, there are still many sorts of cards made to standard credit card size: gift cards, library cards, food stamp cards, video rental cards -- just use whatever you've got. (This size is technically known as the ISO/IEC 7810-2003 size, and it is 3.370" × 2.125".)

In the future, I plan to have a test such as this built in to anaglyph games, to optimize the graphics for every player. (To see just how this value can optimize anaglyph displays, see experiment 05.) For now, it's just for your edification.