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?
rumored new iPod's 960x640 pixel screen, while still maintaining it's 3.5" form.
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.
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.
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.
|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|
|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.