Pixel Density: Pixels Per Inch (PPI) Explained

[#1]

One of the key battlegrounds in mobile technology this year has been the race to produce smartphones and tablets with higher pixel densities. The latest smartphones have pixel densities of up to 468ppi. What does this mean and should pixel density be an important consideration when buying your next smartphone?

What Is Pixel Density and PPI?

A mobile phone display has thousands (or even millions) of individual pixels. Pixels stand for picture elements – they’re the basic building blocks on a smartphone display. Images are formed by directing the millions of pixels to change colour in unison. By changing each pixel to give the desired colour, smartphones can produce almost any image.

There are many factors to consider when comparing smartphone displays. A popular point of contention has been the way pixels produce light. Organic LED displays and LCD displays produce light in a different way. This leads to differences in how the displays look – OLEDs have a higher contrast ratio whereas LCDs give a brighter image. Other people will look at how individual pixels are formed – the subpixel arrangement can greatly affect the readability of a display. Finally, there’s pixel density. This has captured the most attention in the past year and it’s where we’ve seen the biggest developments.

Simply put, pixel density is a measure of how close together the pixels are packed on your smartphone’s display. The most popular way to measure pixel density is “pixels per inch”. This is often abbreviated as ppi.

The following diagram illustrates the meaning of “pixels per inch”. We’ve considered a hypothetical display that measures 1-inch in each direction. Pixels are shown as a grey square. Starting from the leftmost example, we have a display with just one pixel. The one pixel takes up the entire display. In this example, there’s one pixel for one inch of display. We say the display has a pixel density of 1ppi.

Moving rightwards to the next image, the number of pixels has been doubled in both directions. The pixel density has also doubled to 2ppi. The pattern continues as we move to 4ppi and 8ppi.

As pixel density is increased, the display is able to show more details in an image. The following diagram shows the effect of increasing pixel density. We’ve shown a heart on our one-inch square display. As pixel density is increased to higher ppis, finer details can be seen. The image becomes progressively clear.

An increased pixel density allows for more detail to be shown. Displays with a higher pixel density will give clearer and sharper images.

A key thing to note is that when pixel density is doubled, the amount of information shown is quadrupled. Taking an example from the diagram above, as we go from 1ppi to 2ppi the pixel density doubles. At the same time, the pixel count quadruples. From 2ppi to 4ppi, we see the same pattern. A doubling of pixel density leads to a quadrupling of pixel count. As we move towards smartphones with a higher pixel density, the amount of information shown increases even faster.

Pixel Density (Pixels Per Inch/PPI)	Pixels Per Square Inch
1ppi	1
2ppi	4 (double ppi = quadruple pixel count)
4ppi	16
8ppi	64
218ppi (Galaxy S II)	47,524
306ppi (Galaxy S III)	93,636
326ppi (iPhone 5 Retina Display)	106,276
441ppi (Galaxy S4)	194,481
468ppi (HTC One)	219,024

“Pixels per square inch” is a much better indicator of image quality. When pixel density doubles, the “pixels per square inch” will quadruple. Compared to the iPhone 5 (326ppi), the HTC One (468ppi) offers 43% more pixels per inch. However, the “pixels per square inch” has actually increased by 106%. Picture quality has improved by more than is expected.

How to Calculate Pixel Density

To calculate the pixel density of your smartphone, start by taking its resolution. You can look at the resolution in either the horizontal or vertical directions. Once you have the resolution, measure the length of the display. This should be recorded in inches (2.54cm = 1 inch). By dividing the two numbers, you’ll end up with the “pixels per inch” (ppi).

Here’s an example where we’ve calculated the pixel density for the Galaxy S4:

An example of how to calculate the pixel density on the Galaxy S4.

If you don’t have a ruler to hand, a quick search online will normally tell you the size of your smartphone’s display. Please note: this is usually given as a length along the diagonal. To give an example, the Galaxy S4 (pictured) has a display that measures 4.35x2.45 inches. Its size is usually stated as 4.99-inches: this is the length along the diagonal. You can use Pythagoras' theorem to convert this to a horizontal & vertical length – alternatively use an online calculator that can do it all for you.

Pixel Density: Today’s Smartphones & The Limits of Human Vision

Over the past few years, smartphone manufacturers have worked on integrating displays with an ever-higher pixel density.

The battle for pixel density began in 2010 when Apple released the iPhone 4. The iPhone 4 had a Retina display with 326 pixels per inch. Under normal viewing conditions, Apple claims the Retina display is sharp enough that most people would be unable to distinguish individual pixels. With magazines typically printed at 300 dots per inch, Apple’s Retina display was comparable in quality.

In the years since, rival manufacturers have pushed the boundaries even further. Earlier this year, Samsung and Sony introduced flagship devices with a 441ppi display. HTC took things even further: the HTC One packs a massive 468 pixels per inch. Comparing the same area like-for-like, the HTC One packs twice as many pixels as the iPhone 5.

Research has suggested that at a typical reading distance of 1 foot (30cm), humans can see up to 720 pixels per inch. At the average viewing distance for a computer monitor (2.5 feet or 76cm), this drops to a maximum of 300 pixels per inch.

Handset	Pixel Density (ppi)
Newspaper Quality	200ppi
Samsung Galaxy Mega 6.3	233ppi
Samsung Galaxy S4 Mini	256ppi
Samsung Galaxy Note II	267ppi
Printed Magazine	300ppi
Viewing Limits of 20/20 Human Eye (2.5ft/76cm)	300ppi
Samsung Galaxy S III	306ppi
Apple iPhone 4S	326ppi (“Retina”)
Apple iPhone 5	326ppi (“Retina”)
BlackBerry Q10	330ppi
Nokia Lumia 925	334ppi
Samsung Galaxy S4	441ppi
Sony Xperia Z	441ppi
HTC One(2013)	468ppi
Viewing Limits of 20/20 Human Eye (1ft/30cm)	720ppi

The human eye can only resolve details up to a certain limit. Someone with 20/20 vision could resolve details up to 300ppi when their phone is held at a distance of 2.5ft (76cm). At a viewing distance of 1ft (30cm), this increases to 720ppi.

When choosing your next smartphone, it’s important to look at the pixel density on the display. A smartphone with low pixel density may give poor-quality images. Text legibility will also be limited – particularly on webpages where text is small. On smartphones with a higher pixel density, the legibility of text improves and further details can be seen in a photo or video. A high-end smartphone should have a pixel density of at least 300ppi.

Your Thoughts…

In this article, we’ve looked at pixel density on smartphone displays. Over the past few years, manufacturers have raced to develop smartphone displays with higher pixel densitys. The latest smartphones can have pixel densities of up to 468 pixels per inch. Compared to a printed magazine, the latest smartphones pack twice as much information into the same space.

Is pixel density an important consideration when buying a new smartphone? Can you tell the difference between a Retina display on Apple’s iPhone and the higher pixel density displays from HTC, Sony and Samsung? Do you think we’ll continue to see displays with an even higher pixel density? We’d love to hear your thoughts… please drop us a comment below and let us know what you think!

Ken Lo writes about mobile technology and the mobile industry at Ken's Tech Tips.

[#2]

that was very detailed,thanks for a good read. i know my eyesight is pretty bad lol. was intresting

[#3]

Good blog. How come Apple claim their Mac Books have "retina" screens with a PPI of 226 PPI (or something very similar)?

[#4]

Interesting article, thanks  Could you give us a comparison for tablets as well please?

[#5]

Interesting article thanks. It could be argued that having more ppi than the user can appreciate at normal viewing distance is actually counter-productive - more screen res. means more battery power and more CPU power.

According to the article link the average visual acuity of the human eye is one arc minute which means the limit of resolution is around 300ppi at 1 foot range. For a given screen size, going from 300 ppi to say 425 ppi means double the number of pixels, i.e. double the graphics processing - but for the average owner, this will have zero benefit as compared to 300 ppi because the average owner's eye sight will not be able to see any difference.

[#6]

Thanks for an interesting read.

It's also the first time I've seen Pythagoras' Theorem have any practical use, since leaving school.

[#7]

@seang The rationale for this is that the typical viewing distance for a laptop is greater than for a smartphone/tablet. At greater distances, the eye will perceive less fine detail and hence will still be unable to distinguish individual pixels. This is why, despite the lower ppi, Apple calls it a Retina display. Aside from that, it's worth mentioning "Retina" is a marketing term. Hence Apple could apply it to whatever they want!

@cim You make a really good point, and today's 1080p smartphones will certainly consume more power than its predecessors. I think you'll notice the biggest difference on 3D graphics e.g. when playing a video game. The handset needs to work out the colour of each pixel based on calculations (ray tracing, etc). With double the number of pixels, this would certainly add a lot more stress to the CPU/GPU. If you're just browsing the web, the effects will probably be less significant (e.g. a white background is a white background regardless of the number of pixels you need to use). I haven't seen figures/calculations on this but it sounds really interesting!

[#8]

Great detail to this post and very interesting!

[#9]

very disapointed to find out that the retinia display is lless quality than it advertises. however - they are very clever for marketing it like that! 

i wouldnt have guesed htc one screen is better 

[#10]

Question is. Do we really need that much ppi. I mean i own a 4S and I am very happy with the display, specially as it is Retina (looks same from any angle). But phones like HTC have got over 400 ppi like you mentioned. Do we really need that much ppi??

[#11]

It is going the same way as audio where top end products have more capability than a human can distiguish. People who buy these products then pretend to see the benefit to justify the wasted extra cost.