PPI calculation


Have the screen resolution (width and height) in pixels and the diagonal length in inches, and want to calculate the PPI? Pretty simple really. Let’s use a concrete example. The iPad 4 has a screen resolution of 2048 by 1536 pixels, and a diagonal length of 9.7 inches. Thus we calculate its PPI as

20130222-212656.jpg

The calculation above was performed using the wonderful MyScript Calculator, and the result captured by taking a screenshot.

Here’s the same formula using WordPress’ \LaTeX renderer, in a more generalized form: \sqrt{w^2 + h^2} \over L, where w is the screen width in pixels, h is the screen height in pixels, and L is the diagonal length of the screen in inches.

One thought on “PPI calculation

  1. A good point indeed. The formula comes from the Pythagorean Theorem (a^2 = b^2 + c^2), which lets you calculate the hypotenuse of a triangle based on the height and width. Then, it’s as simple as divide by the number of inches.

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