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.

In general, resolution is measured in ___ per ___, with those blanks depending on the medium.

An important note: Sometimes the terms DPI (used in print) and PPI (used for screens) are used interchangeably. So, don’t be too confused if someone refers to a 300 DPI image that is digital-only. I have a bad habit of using dpi for everything.

Screen Resolution

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.

Back in 2010, when I wrote the first version of this article, the standard rule of thumb was that 72 dpi was for screen and 300 dpi was for print. Increasing the DPI beyond 72 wouldn’t make the image look any better, it would just make the file larger, which will probably slow down the website when it loads or the file when it opens.

Fast forward to 2021… most of our devices are capable of much higher resolution than they were back in the olden days of 2010.

In 2010, the most popular screen size was 1024 x 768. In 2021, it’s 1920 x 1080.

We no longer have to worry about measuring PPI — the new rule of thumb is to focus on making the image the size it will be displayed. Most web templates or social media sites provide these recommendations.

For example, your website template might have space for a hero image that is 1600px wide and 400px tall (in shorthand, that looks like 1600 x 400). You would create (or resize) your image to meet those dimensions.

One curveball is retina displays, which generally have 2x as many pixels, vertically and horizontally. So, you’d need to double your measurements to account for retina displays. In the example above, that’d be 3200 x 800.

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, 300dpi means that a printer will output a “line” that contains 300 tiny dots of ink horizontally, as well as 300 tiny dots of ink vertically to fill every square inch of the print.(1) This means 300 x 300 = 90,000 tiny dots of ink per square inch.

300dpi is the standard print resolution for high resolution output. “High resolution output” means that your image should look like and crisp, as expected.

But what if your image is only 200dpi instead of 300dpi? That would mean 200 x 200 = 40,000 dots of ink per square inch. Which is actually less than half of the 90,000 you’d get at 300dpi.

Sometimes 200dpi can work well enough. Sometimes you can even squeeze by with 150dpi. It depends on a lot of factors. Your graphic designer + your printer will be able to advise you, but you may just need to try a test print and find out.

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 or social media), you just need to worry about screen resolution. Size your image according the specifications for your website or Instagram or whatever.

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 90,000 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 BIG 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 300dpi might look:

Here is how the same 300px × 300 px image, printed at 2″ × 2″ at 150 DPI might 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 × 300px 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.

First, 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.
• Look at the values for Width and Height

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?

(1) Editor’s note: The original version of this post was published May 25, 2010. The original article falsely stated that “300 DPI means that a printer will output 300 tiny dots of ink to fill every square inch of the print.” Many thanks to the readers who corrected me in the comments. The correct answer is that a printer prints 300 x 300 = 90,000 dots of ink per square inch.