The 72dpi Myth
True or false: your monitor will display web graphics at 72 dpi.
Fact is, this commonly held view is actually quite false. Keep reading for the real scoop.
Early in the history of computers, there was no way to change the resolution of your monitor, and often the monitor was built in so you didn't have a choice in that either. Somewhere along the line, someone figured out that monitors did in fact display at 72dpi at the time, and it got passed down as gospel ever since.
A "normal" monitor always has width to height dimensions that have a ratio of 4 to 3. Today, this could mean 640x480, 800x600, 1024x768 or even higher. These numbers are known as the monitor's resolution. Regardless of the physical size of your monitor, you can now set the resolution to be whatever you want (from a list of choices that is). With the same monitor, raising the resolution just makes everything smaller so you can fit more on the screen.
Monitor sizes are measured diagonally, but in a truly marketing driven fashion, they tend to exaggerate a bit. They quote the size by measuring the picture tube corner to corner, including the part you can't even see or use that is within the case. The visible size is typically an inch smaller than what is quoted, making a 17" monitor (for example) actually closer to 16" diagonally.
Finally finding a real use for what you learned in high school math class, you can use the Pythagorean Theorem to figure out that the ratio of width to diagonal measurements will always be 4 to 5 (the hypotenuse of a right triangle such that 32 + 42 = 52). So, four-fifths of 16 gives us 12.8" for the visible width of our example 17" monitor. Doing a bit more arithmetic allows us to determine what the true dpi of our monitor is. If we run it at 640x480, it will be 50dpi (640 dots divided by 12.8 inches); running at 800x600 would give us 62.5 dpi (800 divided by 12.8), and at 1024x768 it would come to 80dpi (1024 divided by 12.8). If you don't like my hypothetical 17" monitor, take a ruler to your real-life monitor, get out your calculator, and figure out what it actually is if you want. Doing so on mine gives me 88.3dpi.
So, what happened to 72dpi?
Your monitor really has no concept of inches (or centimeters for that matter); all it knows is pixels. 72dpi can be used as a good working average, but that's really about it is (if even that). If you display an image that is 800x600 on a monitor running at a resolution of 800x600, it will fill the entire screen, no matter what the physical dimensions are. Such images are known as wallpaper <grin>.