Computer monitors and other displays are frequently measured in terms of "pixel density," which is the number of dots within a square inch of the display. This is called "pixels per inch" and is abbreviated ppi. Ppi can be calculated if you know the physical size of the display, and the number of pixels displayed horizontally and vertically.
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Calculate the aspect ratio of the display by dividing the number of pixels displayed horizontally by the number of pixels displayed vertically. You can find your monitor's pixel count in the Display or Monitor control panel or system preference pane; use the highest available resolution. Most computer monitors are in the ratio of 4 x 3 (1.333 horizontal pixels for every vertical) or 16 x 9 (1.777 horizontal pixels for every vertical).
Determine the horizontal and vertical measurements of the display from the diagonal measurement of the screen. A 15 or 19 inch display is measured across this diagonal, which is the hypotenuse of a right triangle formed by the sides of the screen. The Pythagorean Theorem, "a squared + b squared = c squared," provides the measurements, where c is the diagonal, and a and b are the sides of the screen. For example, to find the horizontal length of the display (a) on a 19-inch, 16 x 9 monitor, you would use the following calculations:
(a x a) + (b x b) = c x c
(a x a) + (9/16 x a x 9/16 x a) = 361
(1 + 81/256) x a x a = 361
a x a = 274.23
a = 16.56
Divide the horizontal number of pixels in the display by the result from Step 2 to determine the ppi of the display. For example, if the 19-inch monitor had a maximum horizontal resolution of 1440, it would have a ppi of 1440/16.56, or 86.95, which rounds up to 87 ppi. If its resolution were 1920, its ppi would be 115.94, which rounds up to 116 ppi.
Tips and warnings
- If you prefer to skip the math, you can use a pixel density calculator available online.
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