Why Pixel Density Matters More than Just Screen Size or Resolution

Computer screens aren't getting any bigger — in fact, they've only continued to shrink. While larger displays continue to dominate the upper end of the spectrum, an increasing number of manufacturers are producing netbooks and tablets more suitable for Middle Earth than daily desktop usage. But when it comes to our eyes, these new, minuscule screens aren't as bad as you think. 

While physical sizes may be shrinking, the resolution of these displays is most definitely not. Where a 17" screen may have once displayed just 1440x900 pixels, those same screens are being built today to handle 1920x1080 resolutions or higher. How, then, do those all-important resolutions keep on increasing? 
 
It all comes down to something called pixel density, and how many of those little lights we can cram into our favorite displays. Let's examine how this works and how the pixel density of popular devices compare.

The progression of technology naturally means that we've become better at manufacturing higher quality LCDs. New processes have, in some cases, even doubled the pixel pixel count of a display without increasing it's physical size — something we may even see with the rumored new iPod's 960x640 pixel screen, while still maintaining it's 3.5" form. 

But if we're not increasing the size of these screens, what's the point of increasing the number of pixels? Ironically, as is often the case with many digital technologies, it's all about replicating the functionality of the analog world. With print, words and letters appear as...well, words and letters. We don't see any of the individual fibers that make up the ink. That level of density is far greater than the anti-aliasing effects or individual pixels clearly visible on a smartphone or laptop screen. 

To figure out the pixel density of a screen, we need to crunch a few numbers. For most screens we already know the diagonal size in inches, but we still need the diagonal pixel resolution. Luckily, there's a nice and simple formula with which to figure that out. Square the display's resolution in width, then in height, and add the two numbers together. Find the root of that resulting number, and you have your display's diagonal resolution in pixels.
 
By diving the diagonal pixel resolution by your display's physical size, you can easily find out the pixel density, or pixels per inch (PPI) of a given device. For example, the current iPhone, with its resolution of 320x480 has a PPI of about 163. But if the iPhone HD rumors prove to be true, the next generation iPhone would have a PPI of around 330.

By applying this model to other screens, it's easy to determine how legible a particular device may be. For example, the Nexus One has a PPI of around 252 based on a 3.7" screen, whereas the Kindle, with it's 6" display, only has a PPI of 167. One of the few commercial devices to even come close to the iPhone HD's projected display is the Sony Xperia X1, a 2008 Windows Mobile phone with a 3" screen, and a PPI of 310.

What's interesting is how these measurements work on a larger scale. A 46" HDTV with a 1920x1080 resolution has a PPI of just 52. At first glance, that might seem low compared to today's mobile screens; however, televisions are rarely viewed at the same distance as a mobile phone or laptop. When sitting on your couch, individual pixels are almost impossible to discern, making PPI far more important for small-screen legibility than big-screen action. 
 
The real question is, how does your display stack up? 
 

Device	Screen Size	Screen Resolution	Pixel Density (PPI)
iPhone HD/4G	3.5 inches (rumored)	960x640 (rumored)	330
Nexus One	3.7 inches	800x480	252
HTC Evo 4G	4.3 inches	800x480	217
Palm Pre	3.1 inches	320x480	186
Kindle	6 inches	800x600	167
Zune HD	3.3 inches	480x272	167
iPhone 3GS	3.5 inches	480x320	164
Macbook Pro 17"	17 inches	1920x1200	133
iPad	9.7 inches	1024x768	132
PSP	4.3 inches	480x272	128
Macbook Pro 15"	15.4 inches	1440x900	110
Typical 30" Monitor	30 inches	2560x1600	101
Typical 20" Monitor	20 inches	1680x1050	99
Typical 24" Monitor	24 inches	1920x1200	94
Typical 46" HDTV	46 inches	1920x1080	48

 
Images via Flickr user ChicagoSage, California State University, Wikipedia, and XKCD.