Hardware used in electronic devices always have a parameter by which we judge the quality. For processors we have clock speeds, cameras have megapixels and sensor size, while storage devices are judged by their capacity and read/write speeds. Such parameters give us a fair idea of how the device will perform. Though these are not the only things which we should take a look at, these are definitely among the first things we check out. Pixel density is one such number that is used to judge the quality of display panels.

We hear a lot about this, especially if someone is talking about devices such as smartphones. But they never care about explaining what exactly the term means. Neither do they provide any formula to calculate the pixel density. But, what exactly is pixel density? Why does it matter? How does one calculate this? Well, this article will provide you with answers for all the above questions, and some more!

So, pixel density actually means the number of pixels per unit length. Since the display sizes are usually measured in inches, the most popular unit is pixel per inches (PPI). But why should we bother about it?

First, to understand terms like this, we need to look at each word separately. Understanding what a pixel is, and understanding the importance of density is what we need here.

Pixel

Pixel, or picture element, is the smallest part of an image. This is the smallest controllable or addressable piece of information in a picture. A typical image contains a lot of colours – or picture information – in it. The pixels of the image stores this information, and each pixel can carry only one colour.

Density

The term density is technically related to mass and volume of a substance. The number shows the mass per unit volume of any substance. It shows how ‘dense’ the material is. There’s no use in going deep into this subject, but there are a few things which you need to understand with respect to density.

If a jar can hold 2 kilograms of water, it doesn’t mean it can hold 2 kilograms of every other material. Similarly, one cubic feet of water will be a lot lighter than one cubic feet of steel. But what does this mean with respect to display panels?

Why does it matter?

Now, here is the importance of pixel density. Imagine a 5 inch screen supports only 2×2 resolution – i.e. just 4 pixels. Since one pixel can show only one colour, you will be able to see only 4 different blocks of colours. Now imagine the same screen supports 200×200 resolution. This gives you 40,000 pixels in the same area that had just 4 pixels.

Pixel density is directly proportional to number of pixels and inversely proportional to size. Since the total area hasn’t changed, to accommodate 100 times more pixels on each side, the pixels will be 100 times smaller. Which means your eyes will not be able to distinguish the pixels as easily as it was with just 4 pixels. It may be visible, but the display will look a lot sharper.

Now, assume the same screen has 1920×1080 resolution, which means 2 million pixels within the same size. The individual pixels will be so small that they won’t be distinguishable by the naked human eye. The transition from one colour to another colour will be smoother, and hence the display will look better

Since we have tried looking at this by varying resolutions and keeping the display size same, let’s try it the other way. Imagine a 5 inch phone with 1280×720 resolution. The pixel size will be too small to be perceived easily. Assume that a 50-inch display has the same resolution. Here, the resolution is constant, and the size has increased, which means individual pixels will be a lot bigger. It will be big enough for the human eye to distinguish. This is the reason pixel density matters!

There are a lot of other factors too, like the viewing distance, but since we are talking about pixel density, let’s stick to this one.

pixel density
Top – Low Pixel Density
Middle – Medium Pixel Density
Bottom – High Pixel Density

How to calculate it?

Displays are usually rectangular in shape, except for a few smart watches. A typical display has right-angled corners and its size is measured diagonally. But the resolution gives us an idea about the number of pixels along its breadth and height, not along its diagonal (or hypotenuse). Oh, wait a minute! Right angles, breadth and height, hypotenuse – these terms remind us of Pythagorean Theorem! Remember the time when you asked “When would I use this formula in my life?” Well, now is the time!

The Pythagorean Theorem gives us the measure of the hypotenuse by using the length of other two sides. This gives us the number of pixels along the diagonal. Divide it by the diagonal length of the display, which we already know. Bingo!

So here goes the formula –

pixel density

Take out the calculator and calculate the PPI of all the displays you have in your home!

Myths about Pixel Density

Well, let’s wind up this article by changing some of the misconceptions one may have.

Myth 1 – Only Pixel densities matter

This is same as the megapixels in camera. The pixel density will only tell you how sharp the screen is. But if the manufacturer messes up the colour reproduction and white balance, it will still be a bad screen. A 440 PPI screen with neutral white balance will look better than 800PPI screen with awful white balance.

Myth 2 – Only Apple makes Retina Display

Well, this is partly true if we consider the term Retina display as a trademark. People started to talk about this topic after Apple coined the term ‘Retina display’. But, technically any display with a pixel density that’s high enough so that the retina of human eye can’t differentiate the pixels is a Retina Display.

Myth 3 – 326 is the magic number

Since we view different screens from different distance, this number tends to change. For a smartphone, which is usually viewed at about 12-15 inches, PPI of 300 is high enough. If it’s a tablet or a TV, the number will be lower. For huge screens the number will be even in double digits! Since the PPI of first iPhone with Retina display was 326, people believe it’s the magic number.

So, that’s the end of this long article. I hope that this one has answered all your questions about pixel density. Share this one with your friends if you learned something new here. Do let us know if you liked this article by commenting below.