What is PPI in display?

You might have very often come across this term in Internet especially when reading about phones. Reviews say that higher the ppi, better the display sharpness. Although it is not always true, what exactly is this PPI?

Pixels in a displayA picture showing Pixels in a low PPI display –Photo by Sigmund on Unsplash

What is PPI?

PPI simply stands for Pixels per Inch. It is roughly the number of pixels per inch across an axis on a display. PPI is a measure of Pixel density of a display. It is a measure of the perceived sharpness of whatever is on the display. Higher the PPI often means, more the pixels and sharper the display. Read on to know how can you calculate PPI of a display. 

What exactly is a pixel

Pixel can be the acronym of ‘picture element’. It is the smallest assignable element in a display. Simply put, a point in a digital display that can show information about colors and brightness. Usually, each pixel contains three subpixels of red, blue and green color. Some displays have one more sub-pixel, white to display a brighter white image.

How to measure PPI in a display

Measuring PPI of a display involves dividing the resolution of one of the axes by its length. PPI is a two-dimensional measurement. Sometimes, there can be PPI measurements for both horizontal and vertical resolutions. In general, if the pixels are square and the screen size measurements are diagonal based, the PPI calculations are also diagonal based. That is, the total number of pixels across the diagonal divided by its length. This is exactly what manufacturers mention in brochures.

As per apple, a 16 inch new MacBook Pro screen with 3072 x 1920 resolution has 226 PPI. 
The calculation goes as follows.
Using the Pythagoras theorem you can calculate the diagonal resolution.

Diagonal = √(length2 + breadth2)

Number of pixels diagonally  = √ (3072 2 + 19202)

= √ (9437184 + 3686400)

= √ 13123584

Number of pixels diagonally  = 3622.65

Pixels per inch (PPI) = Number of Pixels diagonally / length of diagonal

= 3622.65 / 16

Pixels per inch (PPI)              = 226.41

Ppi and DPi in Displays

Just like how PPI stands for Pixels per inch, Dpi stands for Dots per inch. Although people use both interchangeably, they are inherently different. Dot is just a point, unlike a pixel which comprises of sub-pixels. Printing applications widely use dpi or dots per inch. The standard value for good printing is 300dpi. As written earlier, a single-pixel consists of subpixels like red, blue, green and white. In other words, a Pixel is 3 to 4 dots that work together. Theoretically, 300 PPI can be 900 dpi or 1200 dpi.

DPI in cameras

Dslr DPI and PPIA high resolution display of a canon Dslr – Photo by JESHOOTS.COM on Unsplash

Camera manufacturers usually mention the display resolution as dots. Generally, you can calculate the PPI of a camera display by dividing its DPI by the number of sub-pixels constituting a pixel.

Nikon d850 has a 2,359,000 dot display in 3.2 inches. 2359000/4. 589,750 pixels. That is roughly 335 PPI. Since the exact resolution is unknown, an accurate calculation of PPI is tough to make.

Where to use what?

It is commonplace to see both used interchangeably. However, Print media such as photos and vector images use DPI while Digital displays use PPI.

Thank you.

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