DPI vs PPI: What the Difference Actually Is

2026-10-09

DPI and PPI get used as if they were the same measurement, and they are not. PPI belongs to the image and counts the pixels in one inch of it. DPI belongs to the output device and counts how many ink dots a printer puts in one inch. A screen only cares about PPI. Print is the one place where the two meet, and what you are deciding there is how many dots the printer should spend rendering each pixel.

The two abbreviations measure different things

• PPI (pixels per inch) is stored in the file and changes whenever the output size changes
• DPI (dots per inch) is a printer specification and has nothing to do with the image file
• A 3000 pixel wide image printed 10 inches wide is 300 PPI, and printed 20 inches wide it is 150 PPI
• The same file has different PPI at different finished sizes while the pixel count stays put
• The resolution field in the Photoshop image size dialog is PPI, despite what people call it

The pixel count is the number that matters. PPI is just that count divided by the output size, so changing the size changes PPI without adding or removing a single pixel.

Why DPI turns up everywhere

• Scanner specifications say DPI while sampling pixels, which PPI would describe more accurately
• System DPI scaling in Windows and macOS is unrelated, referring to how much interface elements are enlarged
• A print shop asking for 300 DPI wants 300 pixels per inch at the finished size
• Plenty of viewers label PPI as DPI, since that is the word people recognise
• PPI in a phone specification means screen pixel density, which is yet another meaning

The sloppy terminology causes real mistakes. Changing the resolution field from 72 to 300 edits one number in the file and adds no pixels, and with resampling left on it shrinks the dimensions at the same time, so the whole exercise costs you detail instead of gaining it.

How the conversion works for print

• Fix the finished size first: a 6 by 4 inch print has a 6 inch long edge
• Work backwards at 300 pixels per inch: 6 inches needs 1800 pixels and 4 inches needs 1200
• Below 300 PPI grain starts to show, and 150 PPI is already visible on coated stock
• An inkjet uses 1200 DPI or more to render a single pixel, so dots far outnumber pixels
• Posters and display boards viewed from a distance are usually fine at 150 PPI
• Backlit displays and large format banners can go lower, since every doubling of viewing distance relaxes the requirement

The conversion is one division: pixels divided by inches. Going the other way, multiply the finished size by the PPI you want and you know whether the source file is big enough.

Which one to watch depends on where the image ends up

• Web and apps: only pixel dimensions count, and both PPI and DPI are ignored by the browser, which lays out in CSS pixels
• High density screens: supply twice the display width, which is still a pixel dimension question rather than a PPI one
• Printed photos and booklets: work out PPI at the finished size, with 300 as the usual floor
• Scanning old prints: the 600 on the scanner is sampling precision, and archiving at that setting leaves room to crop later
• Before compressing, decide first: print work should not lose resolution, so save on encoding and format instead

How resolution relates to file size is covered in /blog/image-resolution-vs-file-size, the resize-then-compress order in /blog/resize-and-compress-image, and hitting a hard size cap in /blog/compress-jpg-under-100kb.

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