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Is this image big enough? Image resolution and DPI explained

Almost every time I receive a picture from a client, they ask, “Is this image big enough?” If it isn’t, the jargon begins to fly: resolution (or “res”), pixels, pixelated, screen resolution, print resolution, DPI, PPI … whew! Clients are often confused because the image looks fine on their computer and they can’t understand why it won’t look great when it is printed for the final piece.

Let me try to explain some of the technical jargon, as simply as possible, as it is related to the field of graphic design.

Resolution

Image resolution describes the amount of detail an image holds. Higher resolution images are crisper and more detailed. In a lower resolution image, the fine differences in color disappear, edges become blurred, etc. There are many kinds of resolution that can apply to film, television, etc., but the two types we are concerned with here are print resolution and screen resolution.

Screen Resolution

In general, resolution is measured in ___ per ___, with those blanks depending on the medium. Screen resolution is measured in pixels per inch (PPI). There’s another piece of jargon – a pixel is a tiny square of color. A monitor uses tiny pixels to assemble text and images on screen.

The optimal resolution for images on screen is 72 DPI. Increasing the DPI won’t make the image look any better, it’ll just make the file larger, which will probably slow down the website when it loads or the file when it opens.

Print Resolution

Print resolution is measured in dots per inch (or “DPI”), which means the number of dots of ink per square inch that a printer deposits on a piece of paper. So, 300 DPI means that a printer will output 300 tiny dots of ink to fill every square inch of the print. 300 DPI is the standard print resolution for high resolution output.

How the two work together

Documents begin on screen and either stay on screen, or they are printed. If the document will stay on the screen (like a website), you just need to worry about screen resolution, so your images should be 72 PPI.

An important note: Sometimes the terms DPI (print) and PPI (screen) are used interchangeably. So, don’t be confused if someone refers to a 300 DPI image that is on screen, because pixels per inch (PPI) translate equally to dots per inch (DPI).

If you’re going to print the document, you need to make sure the images are 300 DPI at 100% of the final output size. This sounds more complicated than it really is.

We already know that 300 DPI images have 300 dots per square inch. So, if we have an image that is 300 pixels by 300 pixels — we can print it at 1″ × 1″ at 300 DPI. If this is the final output size, we are good to go. If the image was intended to be printed as a 2″ × 2″ image, we would be in trouble, because it would output at only 150 DPI. If this was intended to be a 5″ × 5″ image, we are in trouble, because the image would output at only 60 DPI.

The bigger we try to print the 300px × 300px image (note: px is the abbreviation for pixel), the more pixellated it becomes. Pixellated is a term used to indicate the degradation of the image – the eye can start to see the individual pixels, and the edges become very jagged.

Here is how a 300px × 300 px image, printed at 1″ × 1″ at 300 DPI:

Here is how the same 300px × 300 px image, printed at 2″ × 2″ at 150 DPI would look (I had to crop in on it for the purposes of the post, but you can start to see the pixellation:

Here is how the same 300px × 300 px image, printed at 5″ × 5″ at 60 DPI would look (I had to crop in on it for the purposes of the post, but you can  really start to see the pixellation:

 

As I hope you can see, the original image size doesn’t matter as much as what the DPI will be when the image is printed out at 100%. The image is perfectly fine at 1 inch by 1 inch, but it looks terrible at 5 inches by 5 inches because the DPI is only 60.

How can I figure out the DPI of an image?

Math. And unfortunately, it has little do with the overall file size of an image (like whether the image is 1MB or 10MB).  In general, a bigger file size is better to give your designer, but there’s a more exact method to it. We need to find the overall dimensions of the image, and then do some math.

On a Mac

  • Right-click (or control-click) on an image.
  • Select “Get Info.”
  • Under the “More info” tab, look for Dimensions.

You should see a number like “1024 x 768” (some number x some number). These numbers show the number of pixels in the image (width x height)

On a PC

  • Right-click on image icon.
  • Select “Properties.”
  • Click the “Summary” tab in the properties window.

You’ll see values for the Width, Height, Horizontal Resolution and Vertical Resolution.
(You can see screen shots here). Ignore the Horizontal and Vertical Resolution values. Just pay attention to the overall Width and Height

So, if you want to print an image that is 1024 × 768 (listed as Width=1024px, Height=768px on a PC), you need to divide each value by 300 to see how many inches you can print at 300 dpi.

1024 ÷ 300 = 3.4133″ (width)

768 ÷ 300 = 2.56″ (height)

So, you could print this 1024px × 768px image at 300 DPI at a size of 3.4133″ × 2.56″ – any bigger than this, and you risk the image becoming pixellated. Sure, you can enlarge the image a teeny, tiny bit beyond this size, if you need to, but it’s best if you don’t.

That’s it!

Designers, did I forget anything? Non-designers, does this make sense to you? Is there anything that is still confusing you?

Editor’s note: The original version of this post was published May 25, 2010.

11 Comments

  1. Meg on February 5, 2011 at 5:21 pm

    I had a photographer take pictures of my kids and part of the package was a CD including all jpeg files. I want to make large prints but the width of one specific photo is only 853 and the height is 640. I used your formula and figured I can only make 2.8 x 2.1 prints so that’s not even 4 x 6 pictures. I am assuming i got ripped off? Can the photographer re-send me the CD with higher resolution photos? did they most likely upload higher resolution files to thier computer but then shrink them before putting them on my cd?
    THANK YOU!

  2. Alison on October 11, 2011 at 9:15 pm

    Hi there, thanks for the info, its really useful to me as I’m working as a graphic designer. Thanks alot! God bless ^ ^

  3. Ed on June 27, 2012 at 5:28 pm

    Very well explained. Thanks.

  4. Heiko Knoll on January 7, 2017 at 5:21 pm

    Hello,

    I think there is an error in your text. Under the header “Print resolution” you write: “. . .So, 300 DPI means that a printer will output 300 tiny dots of ink to fill every square inch of the print. . .” This is false. 300 DPI means that a printer will output 300 tiny dots of ink to fill EVERY LINEAR INCH of the print. If. A printer with 300 DPI actually prints 90,000 dots per square inch. (300×300).

    thanks,
    Heiko

  5. Witty on March 9, 2017 at 2:00 am

    Trawling through the internet looking for relevant info is not one of my favourite passtimes, so when I found your article Danielle – relief – I had to search no more. Thanks, compared to other things I found this explains it perfectly – I get it now.

  6. May on May 31, 2018 at 11:59 am

    So helpful and to the point. Many thanks!

  7. Gregory David on June 2, 2018 at 7:01 am

    The concept is explained, BUT you are still a 72 ppi myth believer. Very few displays, display at 72 ppi. From computers to phones they all vary and nearly all of them now have much higher ppi than 72. You can find your display’s ppi by looking up what the manufactures says the display resolution is, for example my laptop is 2880 x 1800, then measure the width of the actual display area on the screen in inches. Mine is very close to 14 inches. Now divide the width of the resolution number by the measurement – mine is 2880 ÷ 14 = 205. A lot more than 72! Of course What your eye perceives as sharp depends on a number of factors. How good your eye sight is, how far you are from the screen, how close you can focus etc. So this is what is important; on a small screen like a phone, just about nobody can see improvements with very high ppi, but on a 50 inch screen, it becomes much more important. So bear that in mind when creating those 72 ppi jpegs… oh, I mean dpi. No I don’t – dpi is a term for printing, ppi is for electronic displays. But Apple does not think so because it keeps correcting ppi to dpi as I write this. Sigh….

  8. Lydia a Kerverdo on February 26, 2019 at 6:06 pm

    Very well explained thanks !

  9. Gaby on May 20, 2019 at 3:09 pm

    Quoting:
    “300 DPI means that a printer will output 300 tiny dots of ink to fill every square inch of the print.”
    Actually there will be 300×300 = 90,000 dots per square inch.

  10. Dan on June 26, 2019 at 4:40 pm

    Hi

    DPI is not dots per square inch. It’s dots per linear inch. 300dpi will print 90’000 dots per square inch.

  11. Robin A Emura on July 2, 2019 at 9:27 am

    Love your explanation, I send your link to new clients all the time… it is spot on and they understand the principal, well most of them do… Thank you Danielle… your site is awesome!

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