Screen sizes

The page compares screen sizes available on eBook Readers. This display size is usually given as a single number that represents the diagonal dimension of the screen. Next to the size of the screen the next important number is related to the number of pixels on the screen. This is usually summarized as a PPI (pixels per inch)

aspect ratio

While the diagonal number is a convenient number to use for display size the important screen size is actually the area of the screen which determines the total number of letters and thus words that can appear on the page. The area is related to the diagonal size by the aspect ratio. This is a term that describes the relationship of the height of the display to the width of the display. For example a 5" screen that was 4" high and 3" wide would have a ratio of 4:3 or sometimes represented by dividing the smaller number into the larger as 1.33:1. The area of such a screen would be 12 square inches. A narrower screen would have a less reading area. For example a 5" screen with a ratio of 16:9 would be 4.35" high and 2.45" wide and have a reading area of only 10.6 square inches. Some PDAs and cell phones use a ratio of 3:2 on small pocket size devices although 4:3 is also popular.

4:3 ratio

This ratio is called the golden triangle as a triangle with sides 3", 4", and 5" produces a triangle with one edge at 90 degrees (a right triangle). This is the ratio used on most CRT monitors and is considered pleasing to look at although for eBook readers you are likely to prefer portrait mode. It is often used with eBook readers. The computer monitors have standardize the pixel sizes for this ratio as: 640 x 480, 800 x 600, 1024 x 768, 1280 x 1024, 1600 x 1200 and 2048 x 1536 for square pixels. Many eBook readers use one of these standard values but may have screens that are not exactly the 4:3 ratio or may use pixels that are not exactly square. Computed PPI values may be different for horizontal and vertical for these reasons and an average may be shown.

approx 4x3 ratio

Size	Dimensions	Pixels	PPI	Area
13.3"	271 x 203 mm, 10.67" x 8.0"	1600 x 1200	150	85.3
10.2"	207 x 155 mm, 8.15" x 6.1"	1280 x 1024	158	49.7
9.7"	203 x 140 mm, 8.0" x 5.5"	1600×1200	200	44
		1200 x 825	150
	197 × 148mm, 7.8" x 5.8"	2048 x 1536	264	45.2
		1024 x 768	132
9.0"	183 × 137 mm, 7.2" × 5.4"	1024 × 768	142	38.9
8.0"	163 x 122 mm, 6.4" x 4.8"	1024 x 768	160	30.7
7.9"	160 x 119 mm, 6.3" x 4.7"	1024 x 768	163	29.6
6.0"	122 x 91 mm, 4.8" x 3.6"	1024 x 768	213	17.3
		800 x 600	167
5.0"	101 x 76 mm, 4.0" x 3.0"	800 x 600	200	12
		640 x 480	160
4.3"	88 x 66 mm, 3.46 x 2.60"	800 x 600	232	9
3.5"	71 x 53 mm, 2.80" x 2.10"	320 x 240	114	5.9

16:9 ratio

Also known as widescreen, this is the standard for the new widescreen TV's and has also been used on many of the new computer screens. It is also known as 1.77:1. However computer screens are sometimes actually 16:10 (8:5) or have pixel counts that are slightly higher than would be normal for a true 16:9 screen. True wide screen TV's are either 720p (720 x 1280 pixels) or 1080p or i (1080 x 1920 pixels). A small computer screen of 800 x 480 would actually be 800 x 450 if it were true 16:9 ratio. Either the screen is slightly wider (16:9.6) or the pixels are not square to accomplish this difference. Computed PPI values may be different for horizontal and vertical for these reasons and an average may be shown.

approx 16x9 ratio

Size	Dimensions	Pixels	PPI	Area
10.1"	214 x 135 mm, 8.53" x 5.33"	1280 x 800	150	45.5
10.1"	220 x 132 mm, 8.68" x 5.1"	1024 x 600	118	44.3
7.1"	154 × 90 mm, 6.1" × 3.5"	1024 × 600	167	21.4
6.9"	152 × 86 mm, 6.0" × 3.4"	800 × 480	138	20.4
5.1"	110 × 66 mm, 4.35" × 2.62"		184	11.4
4.3"	95.3 x 54 mm, 3.75" x 2.13"	480 x 272	128	8.0
4.0"	88.9 x 49.3 mm, 3.5" x 1.94"	1136 x 640	326	6.8

3:2 ratio

This ratio is used on some pocket sized devices such as PDAs and cell phones with screens smaller than 4" although 4:3 screens are also used on these devices. It is sometimes shown as 1.5:1 and termed wide screen. The pixels on screens with a 3:2 ratio are often not square.

approx 3x2 ratio

Size	Dimensions	Pixels	PPI	Area
3.9"	82 x 55 mm, 3.22" x 2.15"	480 x 320	149	6.9
3.5"	73.9 x 49.3 mm, 2.91" x 1.94"	320 x 240	116	5.6
		480 x 320	165
		960 x 640	326

eReader Screens

click above for a comparison of the screens on various eReading devices.

PPI

PPI, pixels per inch, is the second number that is used to assess the readability of a screen. Generally the more ppi the better although there is a trade off of performance and battery requirements in generating the screen image when it contains more pixels.

PPI and DPI (dots per inch) are related values and for a monochrome screen they are usually the same number. For color, typically 3 or 4 dots are needed to represent a single pixel. Some reading systems take advantage of the fact that the human eye cannot detect really small color pixels so they use sub-pixel sampling as a form of anti-aliasing to increase the apparent resolution of text. See Fonts for more information on this topic.

The term retina display is used to identify a high ppi screen, usually 250ppi or higher. It attempts to identify the upper limit of useful ppi values and considers the viewing distance as well as the ppi value. A small screen device would typically be held closer to the eyes than a large screen and would thus need a higher ppi value to be termed a retina display.