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What Is PPI?
Although we can define a LCD display with resolution, a Full HD resolution on screen size of a 5.5” mobile phone or a 27” monitor will show different. The screen “fineness” is very important for some application, like medical, our cell phone and more. If the display “fineness” is not enough, the display will look “pixelized” which is unable to show details. We define this “fineness” as PPI.
PPI is the acronym from Pixels Per Inch. It is a unit of measure used to quantify the number of pixels found on a square inch surface. To get a clear idea of what it means, imagine a square inch that’s divided and organized in a grid of cells. Each and every cell in that grid has a pixel inside. The number of cells inside the grid, also known as pixels, tells you the PPI.
Usually, the Pixels Per Inch value is used to measure the pixel density of displays, such as the monitor you have on your computer or laptop, on your TV screen, on your smartphone and any display devices.
There are three steps to calculating a screen’s PPI.
- Step One: Find the Screen’s Diagonal
The first step to calculate PPI is to measure the screen’s diagonal size in inches. Most displays, screens, monitors, and televisions are sold by their diagonal measurement, which should be marked on the screen or its documentation.
- Step Two: Find the Diagonal Pixels
Given the screen’s resolution, you can find the number of pixels along the diagonal using the Pythagorean theorem.
In other words, the diagonal pixels dp is equal to the square root of the width in pixels w squared plus the height in pixels h squared.
The width in pixels is equal to the first part of the screen’s resolution, and the height is the second. For instance, the width in pixels of a 1920×1080 screen is 1920, and the height is 1080.
- Step Three: Use the PPI Formula
Given the diagonal measurement in pixels and inches, use the following formula to calculate PPI.
PPI = dp / Inch
Thus, screen pixel density in pixels per inch is equal to the pixels along the diagonal dp divided by the diagonal in inches.
Example: Calculate PPI (Pixels per Inch)
You have a computer screen that is 1920 pixels wide by 1080 pixels high. The diagonal is 10 inches. You want to find the pixel density in one linear inch of display.
Calculate the diagonal length in pixels with the Pythagorean Theorem:
diagonalpixels = √(19202 + 10802)
diagonalpixels = √(3686400 + 1166400)
diagonalpixels = √4852800
diagonalpixels = 2202.91 pixels
Now divide:
PPI = diagonalpixels / diagonalinches
PPI = 2202.91 / 10
PPI = 220.29 pixels
So there are 220.29 pixels in a 1 inch line on the display.
