Ok, so I'm doing the printable version of a webpage layout for a client. The "tarket" is older middle-aged men and this particular page is printed a lot. I started with the assumption that browsers print by default at 72 ppi unless "shrink to fit" is turned on, which is a logical assumption considering how large browsers typically print things. But then I got to thinking, "is that a valid assumption? It's been a long time since I've measured it…"
So I do some print preview screen captures in Firefox 3 and IE 7, and am plesantly surprised that they both print at higher resolutions than I thought. Unfortunately, they don't both print at the same resolution. I also tested in Firefox 3 on Mac OS X, and it's different than either Windows browser.
IE 7 seems to print at 96 ppi, which is pretty logical, but Firefox 3 only prints at 91.5 ppi…? What's that about? Mac Firefox was higher than both, but I didn't bother measuring it since I only cared if it was lower. The other bizarre thing is the default margins. They tend to be larger than I expected, around 0.6-0.8 inches on the left and right, leaving a lot less printable area than I thought based on my first screen captures.
So the result: the optimal printable width seems to be 660 pixels wide, since the lowest resolution browser (Firefox 3) seems to print 664 pixels total, but I just rounded it down. Now, most of the time this doesn't matter since a lot of people just use "shrink to fit" all of the time. But if you have a page that just has to be guaranteed to print correctly, aim for 660 pixels or narrower and you'll be golden.