The Retina MacBook Pro comes in 13 and 15-inch sizes. Both of them have similar pixel density: 227 and 220 pixels per inch, respectively. The 13-inch screen has an actual resolution of 2560 x 1600, while the 15-inch screen’s physical resolution is 2880 x 1800 (OS X scales to lower resolutions so you can actually see what you’re doing).
The 5K iMac is the second Mac model to get the Retina display treatment. This premium 27-inch iMac model gets a screen with 5120 x 2880 resolution — higher even than the brilliant 4K televisions now gracing the media centers of well-heeled consumers. That display has a PPI measurement of 218.
The new MacBook, meanwhile, comes with a 12-inch display, with 2304 x 1440 physical resolution. It supports a scaled resolution as high as 1440 x 900, higher than the old standard-resolution MacBook Pro, but not quite as high as the 13-inch Retina MacBook Pro, which maxes out at 1680 x 1050. The MacBook’s Retina display sports an actual PPI measurement of 226, just a single pixel per inch lower than the 13-inch Retina display.