PPI vs. DPI: what’s the difference?

facebooktwittergoogle_plusredditpinterestlinkedinmail

If you’ve ever worked on a print design, used Photoshop or any other such program for manipulating raster images, you’ve surely come across the terms DPI (dots per inch) and/or PPI (pixels per inch). Chances are, you’ve also received some confusing DPI/PPI-related requests from clients who don’t understand what these terms mean. Before you start griping, though, pause and ask: how well do you yourself know your way around these terms?

dpi ppi screen shot2

There’s no shame in feeling lost on the DPI/PPI subject; the world seems to have conspired to make it as confusing as humanly imaginable. Among other reasons, this is largely a result of people (and some software manufacturers) using the two terms interchangeably when in fact they refer to completely different things. Not to worry, though: by the end of this short article you will have a thorough understanding of these terms and should feel comfortable enough to enlighten your future clients.

Pixels Per Inch (PPI)

We’ll start here because, if you’re a designer, your primary concern is going to be PPI. DPI, which we’ll discuss next, refers to a technical aspect of printing devices that doesn’t directly concern you — it’s the print shop’s domain. Now, that being said, people say “DPI” when they really mean “PPI” all the time — so much so that it has become an established convention that you have to put up with (Apple, Microsoft and Adobe have all been guilty of this improper usage). The important thing is to be able to know whether someone is referring to actual DPI or really means PPI. Read on and you will.

PPI’s digital basis

First off, what is a pixel? It seems rudimentary but for many the confusion begins here. Pixel stands for “picture element”. It’s the smallest physical element of a digital display device that the eye can discern. Zoom in close to the photo on your computer screen and you’ll see them: rows and rows of tiny little squares. As such, they are also the smallest addressable unit of a digital image.

pixels

Application: PistoCasero (via flickr)

In fact, pixels are actually made up of “sub-pixels” — red, green and blue light elements that the human eye cannot see because additive color processing blends them into a single hue which appears on the pixel level – but this fact is not directly relevant to designers.

Confusion point:  Regrettably, some manufacturers refer to these sub-pixels as “dots” because they are (roughly) analogous to the CMYK dots of a printer, which function in a similar way but by subtractive color processing (more on this later). These manufacturers then boast of the “DPI” of their screens. If you see this, ignore it! It is an annoying misuse of terms and probably an attempt to overcharge you.

subpixels

Video screen: BruceTurner (via flickr); iPad 4: citoki0815 (via flickr)

Note that pixels are physical things of a fixed size (albeit not a standard one; different devices have pixels of different shapes and sizes, the smallest known being a microscopic 11¼ μm). Hence, the number of pixels per inch (PPI) on your screen is a fixed quantity — not something you can adjust by typing in a new number somewhere. Most LCD monitors are in the neighborhood of 67 – 130ppi.

200pixels

Diagram: Wikipedia

What does this mean? If you’re only going to look at an image on a screen, its PPI doesn’t matter because the PPI of your monitor is already fixed. So next time someone tells you to upload images to a website at 72ppi because that is “web resolution,” you can tell them that they have simply added a ridiculous extra step. Unless they are concerned with visitors taking the images from the website and then printing them, the PPI doesn’t matter. A 72ppi image and a 3,000ppi image will appear exactly the same on screen.

What you need to understand: PPI and a printed target

So we’ve established this much: setting PPI only matters for printing — the transfer of a digital image onto a non-digital surface. “But wait…” you say, “we’re talking pixels per inch but printing paper doesn’t have pixels!” Yep, it’s confusing. Read on.

In the printing process, all the physical pixels that composed the image on screen are translated into little squares of different hues on paper. Obviously these are not pixels in the sense of the light-emitting mechanical device but “pixels” in the more abstract sense of a square picture element (we’ll use quotes around this abstract usage from now on to help keep things clear).

printed pixels

Print: Укларочить (via flickr)

What does this mean? “Pixels” on paper have no fixed size. If you increase the size of your image by 300%, the “pixels” on the paper will become three times as large, resulting in a bigger but more rough-looking image.

And how do you increase or decrease print-out size in this way? By adjusting the number in the PPI (or, depending on your software, DPI) field.

Suppose you have a 300 x 300 pixel image. If you set the PPI to 10, this is going to make the print out relatively large: at 10 pixels per inch, it will be 30 x 30 inches (300 divided by 10 is 30). If you set the PPI to 300, this is going to make the print out relatively small: at 300 pixels per inch, it will by 1 x 1 inches (300 divided by 300 is 1). Make sense?

Takeway: think of the PPI input as a way to adjust the physical size, not the resolution, of the eventual print-out. Decreasing the PPI, thus increasing the size of the printout, may seem to produce a lower quality image because the pixels are larger and more visible. But remember, this is only a relative gauge of quality; if you were to stand further away, the image would appear as clear as it did before. The absolute resolution of the image has not changed; there are still as many “pixels” relative to the picture as there were before. So the way to increase the resolution of an image is to produce an image with more pixels, not increase the PPI.

Note: simply re-sampling an image at a higher number of pixels (inputting a new number into the pixels field after the image is already made) is generally not a great way to go about increasing quality, because the computer will likely cram the image full of pixels in weird places.

billboard

Billboard: Friction NYC (via flickr)

Looking at this billboard close up, the pixelation is obvious (so are dots, which we will discuss in a minute). But at the distance from which most passersby will see it, it will look crystal clear. 

Dots Per Inch (DPI)

This section is going to be shorter because if you’re a designer, DPI barely concerns you. Still, it’s an important concept to understand.

8292725849_593d6bd2dd_z

Print: Nick Sherman (via flickr)

Printers do not reproduce an image by tiling pixel squares directly on top of one another. Rather, they reproduce an image by spitting out tiny dots consisting of a mix of four colors, Cyan, Magenta, Yellow and Key (black), which combine to create a range of hues by the subtractive color model. There is bound to be some space between these dots, and this is what DPI measures: their density.

dpi

Diagrams: Wikipedia

For example, if you are printing a 150ppi image at 600dpi, each “pixel” will consist of 16 dots (600 dots/150 “pixels” = 4 rows of 4 dots per “pixel”).

This matters to the client because, as a rule of thumb, the higher the DPI, the better the image’s tonality and the smoother its color blending will be (it will also use more ink and take longer to print, so keep that in mind for personal home printing). 150dpi is generally considered the minimum standard for high quality photographic reproduction in books and magazines. Newspapers often use 85dpi and the effect is clear: individual dots are visible and some detail is lost. Billboards go as low as 45dpi, but you can’t tell because you’re typically viewing from very far away. Typical dot matrix printers are capable for 60 – 90dpi, inkjet printers 300 – 600dpi, and laser printers 600 – 1,800dpi.

Note: higher dpi does not necessarily equate to higher quality because there is no standard dot size or shape, meaning that one manufacturer’s dots might look as good at 1200dpi as another manufacturer’s dots do at 700dpi. Anyway, that’s not really your problem.

Takeaway: DPI is just a technical aspect of an individual printer, like the pixel resolution of your computer monitor. As a designer, you have no control over this. All you can do is recommend your client to a professional print shop and have the shop, which will know the specifications of its machines, take over from there.

Do you have any questions or tips about PPI vs. DPI?

facebooktwittergoogle_plusredditpinterestlinkedinmail

Alex contributes from New York City on topics ranging from branding and typography to the history of design.

  • http://www.swfx.de swint

    Displays are usually specified in MDPI, HDPI, XHDPI, XXHDPI – 160 is the logical density for MDPI devices, right? A Nexus 5 is specified with 445 ppi, which should or could be the same as 320 DPI / XDPI or 480 / XXHDPI? Is there any formula to convert ppi into dpi?
    Image based on: https://www.youtube.com/watch?v=zhszwkcay2A

  • studio5

    Actually dpi/ppi DOES have an impact on the screen.

    If you create a PDF with images of identical pixel extents but differing ppi metadata the result is a mess of different sized pages.

    To see an image at its best it must be displayed at a pixel per pixel match… on a Macpro super retina 13″ for example this requires a ppi resolution of 227. Otherwise it will interpolate.

    The latter is a real problem to do with the pixel doubling of the Retina solution which to prevent teeny tiny icons and so forth is treated for much of the time as a 1440×800 monitor with the applications graphics then pixel doubled.This really messes up video. If you want to zoom 1:1 on a video player on a Macpro retina…all I can say is good luck. This is proving a nightmare in my work. As far as I know there is no way to add ppi data to video and no player would recognise it if there were.

    • Jim Mulvaney

      PDF stands for Portable Document Format. As it stands today, it is used
      so that regardless of what platform you viewed it on it would still be
      accessible (not pixel perfect but accessible). Originally it was
      primarily used in desktop publishing, ie; print purposes. The fact that
      it can be viewed on screen does not detract from its original core
      purpose in printing. It is merely a nicety to user experience. So
      although you are right about viewing pdfs on screen, you are still
      incorrect about PDFs being an exception because you are ignoring its
      core purpose for use in printing. In which case PDF is not solely for on
      screen, which is what the article said. “If you’re only going to look at an image on a screen, its PPI doesn’t matter because the PPI of your monitor is already fixed.”

      • studio5

        Jim I repeat. Create a sequence of images of matching resolution, lets say 1929×1080 but set their DPI differently, some 72dpi some 300. Use adobe acrobat pro to assemble a PDF of these.

        Now view the result in multiple operating systems. The result is a disaster. You have to match dpi across all images.
        .

  • Drew Goldsberry

    Converting images to web optimized is really more to reduce the size of the image to 72 at the size you need to make it so the client can download the image quickly. This is not a worthless step, serving only what is needed not what is available can provide a better user experience. Nice post had some really great content- Web Developer

    • Johan Dahl

      I suggest you try for yourself; make an image 72dpi and another 3000 dpi. Check the file size. It is the exact same. You are not providing a better user experience, you are indeed just adding an extra unnecessary step. The DPI/PPI setting only tells the printer how big you wan’t to print the image. If the image is shown on the web, it doesn’t matter the slightest.

      • Drew Goldsberry

        The assumption in my previous post was you were using the image inside a website. For instance I have space for a 400px wide image and i am starting with a 2000+ wide pixel image. I could allow the browser to resize the image for me or i could send a 400px wide 72 dpi image. Both would achieve the same effect the difference is the browser does not need to load the large image if it is only 400 px wide for instance. I understand where the confusion is i was not clear on my post.

        Yes re-sampling an image without keeping the dimensions the same will result in the same size image.

        • Johan Dahl

          I’m not sure you understand. Serving a 2000+ wide pixel image would indeed be bad, since it would undoubtedly be of bigger size than a 400px image. BUT, the DPI doesn’t matter when it comes to web images. You don’t need to set your 400px wide image to 72 DPI (it can be anything). The DPI of that image is totally irrelevant when it comes to displaying it on the web.

          • Александр Компанец

            How can the DPI of an image be changed? Article says “DPI is just a technical aspect of an individual printer”.

            At the same time author says “the PPI doesn’t matter” (not DPI but PPI). He says you may do your site using 3000 PPI images and it wil look the same. It will. But the size of that images will be too large.

          • Johan Dahl

            No it won’t. The size of the image will not change at all, which is easy to test for yourself! I suggest you do that.

          • Александр Компанец

            Got it! PPI affects only printed image. If we don’t change the resolution it will take the same disk space. Thank you. Really didn’t know that we may leave the resolution while changing PPI setting.

  • John Marovino

    Great article Alex . well written and badly needed. There’s so much confusion over a topic that shouldn’t be that complicated. Thanks.

  • Donna Compton

    Am I getting this correct?___if my MX860 color res is 9600×2400….the Epson color res of 5760×1440 might be just as good or better?

  • Javier

    Great article, thanks!!