What Does a Photo's Pixels Per Inch Mean? (MacMost 2697)

Sometimes when you provide a photo to be printed or used in a publication, you will be asked about pixels per inch, or PPI. But PPI isn't really important. Only the number of pixels in the image matters, PPI is just a setting in the file.

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Video Transcript: Hi, this is Gary with MacMost.com. Let's talk about Pixels Per Inch. 
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Now occasionally I'm asked questions about pixels per inch or PPI. This is usually something you don't need to worry about. But occasionally you'll find your self uploading a photo or document to a printing service or handing it over to somebody to print and they'll ask you about PPI and you need to figure out whether you meet their requirements. In most cases you do and PPI isn't really something that you need to even look into. But it is useful to understand PPI so you don't make some critical mistakes like sending in an image that is too low in resolution.
So let's take a typical photograph. This is one taken with an iPhone. It has 4032 pixels horizontally and 3024 pixels vertically. So if you add that all up it's more than twelve million pixels making up this photo. But what is a pixel? Well, a pixel is a single square representing a single color. This image is made up of more than twelve million pixels. If we look at a small portion of this image here, just this one right here, zoom in on it, we can see all the individual pixels. Each one of these squares is just one single color. Put all together they make this entire image.
So what is pixels per inch? Well, pixels per inch is the number of pixels contained in each inch horizontally or vertically of the image. But the key thing is here that it is per inch. Inch is a physical measurement. So when you're dealing with a digital file it has no physical size. It is no set number of inches wide. It has a set number of pixels, like 4032, but without actually showing it in the physical world, like on a printed piece of paper or on a physical screen, there is no inch to measure. So it is important to realize that a pixel is a virtual square of color that's in the image. An inch, of course, is a physical measurement. So in order to have pixels per inch we need to have both of these. The easiest way to think about this is by taking the obvious physical object of a piece of paper. Here we have an American standard sheet of paper, 8½ inches wide and 11 inches tall. Now if we put our image on this it has to fit in the paper. We can scale it to be the entire width of the paper or just a portion of it. In this case we're just putting it in the middle at about this size.
The image itself is still 4032 pixels across. That doesn't change. But now we're putting it on a piece of paper and we can actually measure the width. It may be easier to think of this inside of some other document like maybe a newsletter like this. Now it actually has a physical width because it's printed on a piece of paper. We can measure that width and we can find out it's 4 inches. As a matter of fact the software you're using whether it's Pages or InDesign or something like that will tell you that when you print this is going to be 4 inches wide. You could size it for that. So you know it's 4 inches wide.
So now that we have inches and pixels we can tell the pixels per inch. In this case if we take 4032 divided by 4 we get 1008 pixels per inch. But that actually doesn't tell us too much because it can be 1008 pixels per inch doesn't mean it will be when you print it. It depends on the printer. For instance a printer may print at 300 DPI, D for dots because printers print in dots not pixels. So a printer that is 300 DPI can only print 300 dots per inch. Our image is more than that. It's more than three times that which is a good thing. That means for every dot there's going to be plenty of information in the photo to know what color to make that dot to make a good image. On the other hand let's say this image was not something taken straight from an iPhone but maybe somebody texted it to us or sent it in an email and compressed it, made it a really small image. Maybe it was only 240 pixels wide and 180 pixels high. In this case 240 divided by 4 is going to give us 60 pixels per inch. So the data is only 60 pixels per inch. But the printer is 300 DPI meaning that every pixel information is going to have to stretch across 5 dots on the printer making the image kind of blocky and low resolution. You're going to be able to tell when you print this image that it's not a high resolution image unlike the image that was 4032 pixels across which will look really good and sharp when printed.
Now screens are another physical object. Here's a typical screen. It could be 3840 across and 2160 vertically. That's a typical 4K screen. Let's say it's 24 inches wide and 13½ inches high. Those seem like odd measurements because we usually talk about screens in vertical measurements. In this case a 27 inch screen. So not all 27 inch screens are 24 by 13½ but it's a pretty typical size. So the screen has a number of pixels across and also a physical size in inches so we can actually calculate the PPI of the screen as 160 PPI. Now what happens when we put an image on the screen. Say we take our 4032 pixel wide image and we put it on the screen. There are more pixels in this image than the entire screen and we're not even using the whole screen. We're only using about 1000 pixels in the middle of it to actually put the image. So what's going on here? How can there be a higher PPI right here in the middle of the screen where this image is than the entire screen in general.
Well, it's not. It's going to be re-rendered on the screen to actually fit the pixels better there. So it's probably going to be about a 1000 pixels by 750 pixels. So we have plenty of information in this high resolution photo to create a nice looking 1000 by 750 image to go on the screen at this position. Now with the screen we don't think of it as printing but we do publish. So say you publish a webpage like this. You expect people to be looking at the webpage across their entire screen. You're got some text on one side and you've got your image taking up maybe a third, maybe a quarter of the size of the screen. So you can estimate that it's going to be about a 1000 pixels wide for a lot of people viewing this image. Of course what screen that people have are going to vary. They are going to have 5K screens. They are going to have 2K screens. They are going to have all sorts of different setups. So you basically have to come up with an estimate of how many pixels is this going to take up on the screen. In this case you can come up with a pretty safe estimate that's going to be about a 1000 pixels wide. You image is 4032 wide. So it should look great on the screen.
However you may not want the image to be full size because it's a larger file to download. So if you're a web developer you may look at this image and say I'm going to resize that down to about a 1000 pixels wide so it will look good on most screens but the file size isn't any larger than it needs to be. A lot of modern web software, say like WordPress will actually create multiple versions of the image. Store them on the server and it will send the appropriate one to the person who is viewing the site depending upon the resolution of their screen. So it's important to understand that pixels per inch is just a setting in a file. Now you can actually set an image to a specific number of pixels per inch. But it's basically just a number saved in the file. It doesn't actually change the data in the file. So think of it as a suggestion. When you're going to use this image you should be using this many pixels per inch.
Let's use PhotoShop but just about any graphics app will do the same here. We create a new document here. You can see I've got it 432 pixels across, 3024 vertically, and I'm going to send it at a resolution of 100 pixels per inch. I'll create this document here. I'm going to drag in the image to fit here. Because the image is exactly 432 by 3024 it's going to fit right in here. Now let's go and create another image here. But this time I'm going to set it to be 300 pixels per inch and I'll create this one and I'm going to drag the same image in here and place it. I've got the second document here at 300 pixels per inch and the first document at 100 pixels per inch with exactly the same image inside. So let's go and export and do a quick export here and save it out as a png. I'll call this one 100 DPI and I will export this one using the name 300 DPI. Now I've got these two files here on the desktop. Let's take a look at the first one. It's 24 megs and the dimensions are 4032 by 3024. The second one is exactly the same size and exactly the same dimensions. So what's the difference between the two of these.
Well, if I open this one up in Preview and I get information on it, it will show here that it is, in fact, 100 pixels per inch. This one it says is 300 pixels per inch. So it does remember that number. But nothing else about these two files is different. So let's put them side-by-side like this and let's go and print this one here. I'm going to turn off Auto Rotate. Turn Off Scale here and have it scaled to 20%. If I go here and I print and I turn off Auto Rotate, I turn Off Scale and I set it to 20% as well you could see this takes up more of the page than this does because I'm saying 100 pixels equals an inch here whereas 300 pixels equals an inch here. However, these are exactly the same image. So it's just numbers. If I were to actually change this to 60% I would get exactly the same result. The pixels per inch is just a suggestion as to how to print this particular image.
So if you're sending an image file to somebody to print they may use the pixels per inch to determine what size it should look like on the paper. However if you're imbedding an image inside of a document, like a newsletter or a website, then you're actually scaling it to fit the space you want. The pixels per inch in that case don't matter. Both of these documents are exactly the same. They just have a different suggestion as to how many pixels to use when scaling the image for printing.
So if it still sounds a bit confusing then you're not alone. A lot of people really get caught up on this. The important thing to realize is when you're giving a document or image to somebody and they bother you about pixels per inch or dots per inch usually it doesn't matter as long as you provided them with a high enough resolution image. Here's another way to look at it. You can take exactly the same image, 4032 pixels across, if you want to consider it as 1008 pixels per inch at 100% scale you're going to get an image that's 4 inches across on a sheet of paper, approximately 4 inches across on the screen. If you take exactly that same image but let's say you set the image to 5004 pixels per inch if you make it 4 inches across that's just 50% scale. The number of pixels is the same. The result is the same. PPI in that case is just a number. It's still the same number of pixels. It's still the same resolution when see by the eye on a page or on the screen.

Comments: 2 Responses to “What Does a Photo's Pixels Per Inch Mean? (MacMost 2697)”

Jim S.

3 years ago

Why is the vertical dimension ignored? Seems like a 100ppi image (width) with a one inch vertrical (length) would contain less pixels than a 100ppi image width that is 10 inches vertically. Doesn't pixels per square inch matter?

Gary Rosenzweig

Jim: PPI is both horizontal and vertical. I use horizontal in my examples to simplify things. But if you are using 100 PPI then that is 100 PPI horizontally and 100 PPI vertically. I've never seen an image format that does anything else. That would mean rectangular pixels instead of square ones.

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What Does a Photo's Pixels Per Inch Mean? (MacMost 2697)

Comments: 2 Responses to “What Does a Photo's Pixels Per Inch Mean? (MacMost 2697)”

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