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What exactly is PPI in a display?

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

What is PPI in a display?
A picture showing Pixels in a low PPI display – Photo by Sigmund on Unsplash

PPI stands for Pixels per inch. It is a measure of pixel density on display. So, PPI is an important term in the mobile world. A lower or higher PPI determines how good or bad a display is. PPI is not at all the same as DPI. We will see them detailed below.

What is PPI in a display?

As you know, PPI in a display stands for Pixels per Inch. It is roughly the number of pixels per inch across an axis on display. PPI is a measure of the Pixel density per inch of a display. It is a measure of the perceived sharpness of whatever is on display. The higher the PPI often means, the more pixels and sharper the display because of higher image resolution.

Usually, for a 1080P or 4k computer monitor, anything above 100 ppi is more than enough. For mobile displays, anything above 350 PPI is enough. Higher pixel density than the said above might give more sharpness if you read the screen very close to your eyes.

Read on to learn how can you calculate the 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 colours 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. Image file usually contains the number of pixels in that digital image.

How to measure PPI in a display

Measuring PPI or pixels per inch or pixel density in 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

DPI and PPI in a Displays

Just like how PPI stands for Pixels per inch, Dpi stands for Dots per inch in a digital image. Both are pixel density measurement. Although people use both interchangeably, they are inherently different. Dot in a digital image 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 for image quality. The greater pixel density often translates to a better image quality.

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, a 300 PPI resolution can be 900 dpi or 1200 dpi of the printed image.

PPI depends upon pixel dimensions too. Usually, the lower the pixel dimensions higher the PPI.

DPI in cameras

A picture showing PPI in digital camera
A high resolution display of a canon Dslr – Photo by JESHOOTS.COM on Unsplash

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

A Nikon d850 has a 2,359,000 dot display in 3.2 inches. Individual pixels use four dots. Hence, to find PPI, the operation will be 2359000/4. The result is 589,750 pixels. Therefore the pixel density is roughly 335 PPI. Since the exact resolution is unknown, an accurate PPI calculation is tough.

But in digital photography, professionals use both PPI and DPI.

Where to use what?

It is a commonplace to see both used interchangeably. However, Print media such as photos and vector images use DPI, while Digital displays use PPI. Though both are pixel densities, it should not be used interchangeably.

Related post: What is gamma?

Practical use case

When you buy a new mobile phone, you compare a lot of technical terms of one phone with other phones. PPI or pixels per inch might be one of those terms. Please choose the phone with a higher PPI. But don’t go overboard and get the highest because, beyond a certain limit, you don’t see the difference.

Apple considered PPI to be very important and hence coined the term Retina display. It was showcased with iPhone 4, claiming that beyond that PPI, you don’t likely see a difference. The PPI of the iPhone 4 is 330. Hence, beyond 350 PPI or even 400 PPI, there is no point in buying those gadgets for sharper screens.

In fact, the unnecessary addition of PPI will increase resolution, which in turn increases the battery consumption, lags the processor, over heats, and even manufacturing such panels will be tough.

The same applies to TV and laptop/computer screens. Normally, for a computer screen, anything above 100 PPI is not needed as we place the screen far away from our eyes than a typical cellphone. Again, more the PPI, more strain on the GPU of the computer.

If you watch a 65-inch 4k TV (67.78 PPI) beyond 5 feet, you don’t see the pixels. Meaning you don’t need to get a higher 8K display (at 135.56 PPI) if your usage is larger than 5 feet from a 65-inch television. Unless you pixel peep, going any higher is pointless.

Conclusion – What exactly is PPI in a display?

PPI thus stands for pixels per inch, diagonally of the display. Pixel density or PPI is used to know about the sharpness of the display of any kind. It matters a lot that it is best to avoid a display with low PPI.

FAQs – What is PPI in a display?

Thank you for reading