PPI vs. DPI: what’s the difference?

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?

Alex Bigman is liaison to 99designs' awesome community of graphic designers. He is a recent grad of UC Berkeley, where he studied history of art and cognitive science.
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12 Comments

  1. GORDON JENKINS

    Having read your article I am still confused. I have a Canon 40D camera and when I import my photos into Photoshop the resolution is always 72 pixels/inch.
    3 questions:
    Is it possible to improve the quality of these photos by changing the no of pixels in PS?
    Is it possible to change the Resolution of 72 ppi and if so would it change the quality of the photo? I have tried changing the settings in the camera from high quality to raw but they still appear in PS at 72ppi.
    Why do the photos appear is PS with huge dimensions e.g. 137cm x 90cm but with a resolution of 72ppi? Why is it not set with a higher resolution and smaller dimensions?
    Thanking you in advance

    Reply June 18, 2014 at 5:58 am
    • Alex Bigman

      Hi Gordon,

      I’m not a camera expert, so I can’t tell you why your camera’s default setting is 72ppi rather than, say, 300ppi. It doesn’t really matter, though, because yes, you can simply change the resolution in photoshop. Your camera is producing extremely high-quality (high pixel count) images, so you have plenty of leeway to increase resolution while still keeping the images a reasonable size. If your image is 137cm x 90cm at 72ppi, then it will still be about 33cm x 22cm at 300ppi.

      Remember that quality equates to pixel count (the total number), not pixel density (ppi). So you can’t increase the “quality” of an image in photoshop because you can’t add pixels; you can only decrease its quality by removing pixels. You can change the ppi in either direction, but this will only change the image’s resolution. As I wrote in the article, it is best to associate resolution with size rather than quality. A small 300ppi image may seem to be of higher quality than a huge 72ppi image when at close range, but if you stand far enough back, the huge 72ppi image will read as just as high-quality. Make sense?

      Reply June 18, 2014 at 10:01 am
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  3. Sanjeev

    A great article and nice share, lot of useful information that we usually miss or don’t consider.

    Reply June 24, 2014 at 11:23 pm
  4. John “jaQ” Andrews

    So, TL;DR:

    PPI or DPI matter *only* to software programs that rely on PPI or DPI to determine print size.

    - If you’re using your image on a computer screen, you can ignore them.
    - If you’re printing by defining a final output size (5″x7″, 8″x10″, Letter, etc.), you can ignore them.
    - If someone requests a digital image of a certain size, you just need to know the pixel dimensions.
    - You can change PPI or DPI over and over again without ever affecting the quality or pixel dimensions of a digital image.

    Please correct me if I’m wrong.

    Reply June 27, 2014 at 9:27 am
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  6. Norval Aslan

    I self-publish and upload images to create covers. I get warnings telling me that the DPI is too low. I just bought a photoshop app, and they use PPI not DPI. The photoshop app has two places to resize: inches and PPI. If I keep the size the same but increase PPI, am I increasing resolution? This is my goal — to reach the printer’s 300 DPI minimum for good printing.

    Reply July 23, 2014 at 11:54 am
    • John “jaQ” Andrews

      Norval – does the printer give you any guidelines or requirements for the images you upload? It would be something like this:

      Minimum size: 2400 x 3000 pixels

      OR

      Minimum size: 8″ x 10″ at 300 DPI

      Both of those sizes are EXACTLY THE SAME. The proper term would be PPI rather than DPI, but just assume they’re being used interchangeably.

      300 pixels per inch x 8 inches = 2400 pixels
      300 pixels per inch x 10 inches = 3000 pixels

      In most programs, changing the PPI will automatically change the print size, because both of them are parts of the same equation. It’s just algebra!

      (pixels per inch) x (inches) = (pixels)
      (pixels / inch) x (inches) = (pixels)

      See how the inches cancel out?

      So, long answer short, YES, increase PPI to 300. You’ll probably see the print size (in inches) automatically decrease. If the image still does not meet the printer’s requirements, you’ll need an image with more pixels to begin with.

      Reply July 23, 2014 at 1:52 pm
      • John Williams

        John, great info there. So, given your example, would it hold true that if you decreased the image size from 8″ x 10″ to 4″ x 5″ that the DPI would correspondingly increase to 600?

        Great article Alex, and thanks in advance John for your response.

        Reply July 23, 2014 at 8:54 pm
        • John “jaQ” Andrews

          Correct!

          To be certain I just created a file in the free program GIMP (File -> New in its menu bar). Under Image Size, I set “Width” to 2400 pixels and “Height” to 3000 pixels; under Advanced Options, I set both the “X resolution” and “Y resolution” to 300 pixels/in. (It is very rare that anyone would want to have the horizontal X and vertical Y resolutions be different.)

          Then, after creating the file, I selected Image -> Print Size in the menu bar. I changed the unit of measure from the default “mm” to “in” so that “Width” was 8.000 and “Height” was 10.000. I then changed “Width” to 4 and “Height” to 5, and sure enough, both “X resolution” and “Y resolution” changed to 600.

          Reply July 24, 2014 at 11:02 am
          • John Williams

            Great stuff here, and thanks again John!

            July 25, 2014 at 1:24 am

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