Image resolution, DPI and PPI explained
Last reviewed 2026-09-17. Every competitor figure links to the vendor's own page.
Pixel dimensions are the only measure of how much detail an image contains. PPI describes how densely those pixels are laid out when printed, and DPI describes ink dots a printer places. Changing the PPI value in a file does not add or remove a single pixel.
The three terms
| Term | Measures | Property of the file? | Affects detail? |
|---|---|---|---|
| Pixel dimensions | Total pixels, width by height | Yes | Yes, this is the only one that does |
| PPI | Pixel density when printed | Metadata only | No |
| DPI | Ink dots the printer lays down | No, a printer setting | No |
Pixels needed by output size
| Output | At 150 PPI | At 300 PPI |
|---|---|---|
| 4 x 6 in print | 600 x 900 | 1200 x 1800 |
| 5 x 7 in print | 750 x 1050 | 1500 x 2100 |
| 8 x 10 in print | 1200 x 1500 | 2400 x 3000 |
| A4 document | 1240 x 1754 | 2480 x 3508 |
| 11 x 17 in poster | 1650 x 2550 | 3300 x 5100 |
Posters and banners viewed from a distance are routinely printed at 150 PPI or lower without visible loss, because perceived sharpness depends on viewing distance as much as density.
The 72 DPI myth
The idea that web images should be 72 DPI is a leftover from early Macintosh displays that happened to render roughly 72 pixels per inch. It has had no practical meaning for decades.
Screens draw pixels, not inches. An image 1200 pixels wide occupies the same screen space whether its metadata says 72 or 300. If a client asks for 72 DPI web images, they are asking for a sensible pixel width in outdated language.
Working out what you need
- For screen, work in pixels only, and double for high-density displays
- For print, multiply physical inches by target PPI to get required pixels
- Check that number against your source file's actual pixel dimensions
- If the source falls short, upscale, accepting that added detail is reconstructed
- Set the PPI metadata last; it changes the intended size, never the content
Frequently asked questions
What is the difference between DPI and PPI?
PPI, pixels per inch, describes a digital image's pixel density when printed at a given physical size. DPI, dots per inch, describes ink dots a printer actually deposits. The terms are used interchangeably in everyday practice, but only PPI is a property of your file.
Does changing DPI improve image quality?
No. The PPI or DPI value stored in a file is metadata describing intended print size. Changing it from 72 to 300 without resampling alters no pixel data at all; it only changes how large the image is meant to print.
Why do people say images need to be 300 DPI?
Because 300 PPI is the common standard for quality print, and it is shorthand for having enough pixels for the intended physical size. A 300 PPI image is only sufficient if its pixel dimensions match the size being printed.
Does DPI matter for web images?
No. Screens render by pixel, so only pixel dimensions and the device pixel ratio matter. A 72 PPI and a 300 PPI file with identical pixel dimensions display identically on any screen.
How many pixels do I need for a print?
Multiply the physical size in inches by your target PPI. A 6 by 4 inch print at 300 PPI needs 1800 by 1200 pixels. Large prints viewed from a distance, like posters, work acceptably at 150 PPI.
Can upscaling fix an image that is too small for print?
Partly. AI upscaling reconstructs plausible detail and usually beats simple resampling, but it invents rather than recovers. It works well for moderate increases on photographic subjects and poorly on fine text or precise line art.