What Is the Difference Between PPI and DPI in Digital Imaging? (Clarity Guide)

PPI, pixels per inch, describes images and screens: how many of an image's pixels are spread across each inch when it is printed, or how tightly a display packs its pixels. DPI, dots per inch, describes printers: how many tiny dots of ink or toner the machine can place in an inch. The two are related but they are not the same thing, and most software uses "DPI" when it means PPI.

The practical rule follows from that. On a screen, only the pixel dimensions of an image matter, and the ppi number stored in the file is ignored. On paper, pixel dimensions divided by ppi give the printed size in inches, so the ppi value decides how large the picture comes out and how much detail each inch carries.

PPI belongs to the image and the screen

A digital image is a grid of pixels, say 3000 x 2000. That is all the detail it has. Its ppi value is a single number saved alongside the grid that says how densely to print it. Tagged at 300 ppi, the image prints at 10 x 6.67 inches. Tagged at 150 ppi, the same pixels print at 20 x 13.3 inches, with each pixel twice as wide.

Screens have a ppi too, but it is a physical property of the panel rather than a setting. A 24-inch monitor showing 1920 x 1080 pixels has about 92 pixels per inch across its diagonal. Current flagship phones pack well over 400. The image file has no say in this: the screen draws each image pixel on its own pixels, scaled by the operating system.

DPI belongs to the printer

A printer cannot make one dot in any color it likes. An inkjet has a handful of inks, and each dot is either there or not. To reproduce a single pixel of a particular shade of orange, it places a pattern of many smaller dots of yellow and magenta, with gaps of white paper, and your eye blends them. That is why a photo printer is sold as 4800 x 1200 dpi while the photo it prints only needs around 300 ppi: dozens of printer dots go into each image pixel.

Commercial presses work in a related way, using a halftone screen measured in lines per inch (lpi). A common rule of thumb in print shops is to supply images at 1.5 to 2 times the screen ruling, which is where the familiar 300 ppi target comes from for a typical 150 lpi magazine screen.

Why the terms get mixed up

• ●Software labels. Windows shows "Horizontal resolution: 96 dpi" under File Properties > Details, and many export dialogs say DPI. Both mean the ppi value stored in the image.
• ●Scanners. A scan at 600 dpi samples 600 pixels per inch of the original, so the result is a 600 ppi image at the original's size. Here dpi and ppi are effectively the same number.
• ●Print shops. When a printer asks for "300 dpi artwork", they mean 300 pixels per inch at the final printed size.
• ●The old 72 dpi rule. Early Macintosh screens showed 72 pixels per inch, matching the 72 points in a typographic inch, so a 72 ppi image appeared on screen at its print size. Modern screens vary widely, and the rule no longer means anything for the web.

Why changing 72 to 300 does not sharpen anything

Changing the ppi value without resampling rewrites one number in the file. The pixel grid stays exactly the same, so the image looks identical on every screen. Only its intended print size changes: an image that was 41.7 inches wide at 72 ppi becomes 10 inches wide at 300 ppi, with the same detail packed into a smaller area.

The stored value does matter in a few places on a computer. Word and many layout programs read it to decide how big a picture appears when you first insert it, which is why two images with identical pixels can land on the page at very different sizes. Web browsers ignore it completely. CSS defines its own reference pixel of 1/96 inch, and phones with dense screens use a device pixel ratio of 2 or 3, which means an image shown 600 CSS pixels wide needs 1200 or 1800 real pixels to look crisp.

How many pixels a print needs

Multiply the print size in inches by the ppi you want. 300 ppi is the usual request for photos, brochures and anything held in the hand. Around 150 ppi is acceptable for posters and banners viewed from a few steps away, because the eye cannot resolve as much detail at that distance.

If your image has fewer pixels than the table asks for, raising the ppi value will not help, and upscaling only invents pixels by interpolation. Print smaller, accept a lower ppi for a large poster, or find a bigger original.

Change the print resolution without touching the pixels

1. 1.In Photoshop, choose Image > Image Size (Alt+Ctrl+I on Windows, Option+Cmd+I on a Mac).
2. 2.Untick Resample. The pixel dimensions are now locked.
3. 3.Type the new value in Resolution, for example 300 Pixels/Inch. Width and height in inches update to the new print size.
4. 4.Click OK and save.

In GIMP the same setting is under Image > Print Size. In Preview on a Mac, choose Tools > Adjust Size and untick Resample image before changing the resolution.

For screens, size by pixels

For a website, email or slide, decide the pixel width the image will occupy, double it for high-density screens, and resize to that. Resize Image sets exact pixel dimensions in your browser, and Compress Image then trims the file size. Browser-based editors re-encode the picture, so the stored ppi value may not survive. That makes no difference on screen; if a print shop insists on a particular value in the file, set it in a desktop editor at the end.