How to Calculate Pixel Density

By C. Taylor

Everyone has probably seen advertisements of an LCD screen's resolution or screen size, but sometimes that doesn't clearly tell you which screen is better. A better comparison is the number of pixels per inch (ppi), or pixel density. A pixel is a the smallest individual unit which makes up a display, so higher pixel density equates to better clarity. The information you need is already present. This calculation relies only on resolution and physical screen size.

Step 1

Determine the resolution and physical screen size. As an example, a smart phone's LCD screen might list a resolution of 480 pixels by 800 pixels with a screen size of 3 inches. The screen size is the diagonal measurement between opposite corners. The resolution is the number of horizontal and vertical pixels, respectively. In the example, one row would possess 480 pixels in a horizontal line, and one column would possess 800 pixels in a vertical line.

Step 2

Calculate the diagonal resolution using Pythagorean Theorem, which lets you calculate the hypotenuse of a triangle from the length of the other two sides. This formula is:Diagonal resolution = square root of [ (width x width) + (length x length) ]In the example:Diagonal resolution = square root of [ (480 x 480) + (800 x 800) ]Diagonal resolution = square root of [ 230,400 + 640,000 ]Diagonal resolution = square root of [ 870,400 ]Diagonal resolution = 933 pixelsTherefore, the equivalent resolution from corner to corner is 933 pixels.

Step 3

Divide the diagonal resolution by the diagonal screen size to calculate pixel density in ppi. In the example, 993 pixels divided by 3 inches would give you a pixel density of 311 ppi.