Set your document DPI to 94, 96 or 72 DPI - whatever your system DPI actually is.
I would like the inches on the ruler to appear as the actual length of an inch on my screen.
Then you'll have to figure out a zoom factor at which your particular monitor on your particular computer causes Illustrator's scale to match true measure. Different monitors do not all have the same-size hardware pixels. That's why, for example, a 1024 x 768 netbook monitor can be physically smaller than a 1024 x 768 laptop monitor; or why you can buy flat-panel TVs at multiple sizes, yet they are all the same "high def" resolution.
In other words, there is no one-size-fits-all answer to your question. One would have to know the physical dot pitch of your monitor, the default PPI of your OS, and the PPI of Illustrator (which will be different for other programs).
Even if you gather the info and do the math, the result will likely be inaccurate. Why do you think you need your onscreen display to actually match true measure?
Set your document DPI to 94, 96 or 72 DPI
An Illustrator document has no DPI.
As the pevious poster writes you have to really thik in relative terms about what you see on a computer monitor screen.
Matching is going to be very hard if at all possible.
I've been wondering the same thing. My OS knows the resolution of my monitor; why can't Illustrator just ask for that, and render the results accordingly?
My OS knows the resolution of my monitor...
Your OS knows the resolution of your monitor in the true sense of "resolution": The actual amount of information (i.e.; the number of pixels).
But your OS does not necessarily know the scale of those pixels (i.e.; the physical measure of the monitor pixels). For example, Windows 7 works on a tiny netbook monitor which has the same number of pixels as a cheap bulky desktop monitor, on which Windows 7 measures much larger.
Or, think of a presentation projector. It projects a fixed number of pixels onto the meeting room screen. Windows 7 may know how many pixels are being projected, but it has no idea how far away the screen is, therefore no idea how large those projected pixels actually are. An inch of Illustrator's rulers at 100% may very well measure 6 inches on the projector screen.
Since the beginning of desktop publishing, popular use of the term "resolution" as if it is synonomous with PPI (Pixels Per Inch), has led to much confusion. Imagine a satellite photo of a license plate. Let's say only 24 pixels span the width of the license plate, so the license number is not legible. Will doubling the PPI of that image (i.e.; scaling or zooming to 50%) make the license number more legible? Of course not. "Per Inch" is nothing but a scale factor. The actual "resolution" of that photo is all about count of pixels per license plate, not their size. Only enabling the satellite camera to capture more discrete color samples (pixels) at the same distance will increase the photo's resolution.
Huh, you're right. I thought for sure that information would be part of the monitor driver (it knows the display model number, after all) but System Profiler (Mac) doesn't say anything about the physical display size.
Interestingly, "Actual Size" comes out way too small on my laptop monitor, but when I plug it into my external monitor it's bang on. I installed Illustrator using this monitor so maybe it was able to figure something out after all. *shrug*
Still surprised there's no way to calibrate it, though -- actually _tell_ Illustrator how big an inch is on the screen. (Or is there?)
Adobe should be able to address this in the Preferences. I also use QuarkXpress, and it properly displays 100% at the real physical size after I set my monitor DPI in Quark's preferences. Why doesn't Adobe do this basic function?