The iPhone 4 got me thinking about pixel density a couple months back. I feel like I’ve mislead, thinking that “72 DPI” was the standard “resolution” for digital screens. Of course I know now this metric is merely a misunderstood concept I must have inherited from print designers trying to equate screens to physical paper.

At work, I typically have 3 screens at my disposal: a 15.4” screen on my 15” MacBook, a 3.5” screen on my iPhone 3GS, and 23” Cinema Display (2006 Model). Looking at the same view between these screens, the difference in pixel density is fairly visible. If you use an additional display with your laptop, try dragging a window in between the screens and compare how the window lines up.

Pixels per inch (Shouldn’t it be pixels-per-inches-squared?) is calculated by the square root of horizontal pixels squared plus vertical pixels squared divided by the diagonal screen size. Or in Javascript terms:

Math.sqrt( px*px + py*py ) / diag

With that, you can calculate the PPI for a number of devices.

• iPhone 3G: 165
• iPhone 4: 330
• iPad: 132
• 15” MacBook Pro: 110
• 23” Cinema Display (2006 Model): 98
• Motorola Droid & Droid 2: 265
• HTC Evo 4G: 217
• BlackBerry Storm: 185

So yeah, you can forget about 72 DPI for screens.

Odd that Apple advertises the iPhone 4 as having PPI of 326. I’m not sure where the other 4 PPI went, unless the screen diagonal is actually greater than 3.5”.