Though the terms DPI (dots per inch) and PPI (pixels per inch) both describe the resolution (or clarity) of an image, they’re not the same thing. PPI describes the number of square pixels that show up in an inch of digital screen (usually between 67-300). DPI, on the other hand, is a printing term referring to the number of physical dots of ink in a printed document.
Still not clear? 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.
Not to worry—by the end of this article you will have a thorough understanding of these terms and should feel comfortable enough to smugly correct any misuser of the terms. (And explain to them why they’re wrong! Because everyone loves that.)
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.
People say “DPI” when they really mean “PPI” all the time — so much so that it’s 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 to know is whether someone talking about DPI 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. These are also the smallest addressable unit of a digital image.
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.
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.
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).
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: 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.
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.
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.