Resolution vs DPI vs PPI

Three numbers that get used as one

A digital image has one fixed physical property: its pixel dimensions, the count of pixels across and down. That count is what resolution means in most design conversations. It says nothing about size on paper until you decide how large to print, and that decision produces the second number, pixels per inch. Greenerprinter, a print company, defines PPI as the pixel density of an image at its printed size, and DPI as the tiny dots of ink or toner a printer places on paper. The printer's dot count is a property of the machine, often 600 or 1200 or more, and has little to do with the file you supply. When a shop asks for a 300 DPI file, it almost always means 300 pixels per inch at the finished size.

The arithmetic

Pixels per inch is simply the pixel count divided by the printed length in inches. Greenerprinter's own example is a 3000 pixel wide image: printed 10 inches wide it lands at 300 PPI, and printed 20 inches wide the same file drops to 150 PPI. The table on this page does the same for a 2480 by 3508 pixel image across the A sizes. Working backwards is just as useful. To know how many pixels a print needs, multiply the finished size in inches by the density the shop asks for; A4 is 8.27 by 11.69 inches, so at 300 PPI it needs 2480 by 3508 pixels. Metric sizes need converting first, at 25.4 millimetres to the inch.

The DPI field inside a file

Image files carry a density value in their metadata, often 72 or 300. It is only a note about the intended print size. Changing it from 72 to 300 without resampling leaves every pixel untouched and simply tells software to print the image smaller. Resampling is different: enlarging an image to create more pixels invents them by interpolation, so the file gets bigger but no real detail is added, and a soft photograph stays soft. This is why a request for a 300 DPI file cannot be met by retyping a number. What matters is whether the pixel dimensions are enough for the finished size, and that is a question of the source photograph or the export, not of the setting.

Common mistakes

The first is judging a file by its DPI field instead of its pixel count, and sending a 1080 pixel social image tagged 300 DPI for an A4 print that needs more than twice the width. The second is upscaling a small photograph to hit a number, which satisfies an automated check while the print still looks blurry. The third is applying the same density rule to every size, when viewing distance changes what is needed; Vistaprint's poster guide itself asks for 300 for close viewing and 150 to 250 for larger posters. The fourth is confusing screen resolution with print resolution, since a monitor's pixel density has no bearing on paper. The last is forgetting that vector text and icons are not limited by pixels until they are exported.

Pixels in the flyer builder

The flyer builder has no DPI setting. Every canvas is defined in pixels, A4 portrait at 1240 by 1754, which is A4 at 150 pixels per inch, and every export renders at exactly twice the canvas, so an A4 page leaves as 2480 by 3508 pixels, 300 per inch at A4, and an Instagram post as 2160 by 2160. The A0 poster canvas of 1656 by 2340 exports at 3312 by 4680, about 100 per inch at full A0. The number that most often decides print sharpness is the photograph you upload. Uploads up to 10 megabytes are cropped to fill their box, so a small image stretched across a large box will print soft whatever the export size. For how much density a flyer actually needs, see the page on flyer DPI.

Questions people ask