Image glossary
PPI
Also known as pixels per inch
PPI is the number of pixels per inch in an image or on a display, and it is the measurement people usually mean when they say DPI.
The distinction
Strictly, PPI describes an image or a screen and DPI describes a printer. Pixels are the square coloured units a file is made of; dots are ink droplets, and a printer uses several droplets to reproduce one pixel's colour.
In everyday use everybody says DPI for both, and when a print shop asks for 300 DPI they mean 300 PPI in your file. Insisting on the distinction helps nobody in that conversation.
The one place it earns its keep
Printer specifications quote enormous numbers — an inkjet advertised at 1440 DPI, for instance. That number is about ink placement, not about what you should supply. Sending a 1440 PPI image achieves nothing except a very large file and a long wait.
The rule is that the printer's DPI figure is the printer's business and the image's PPI figure is yours. They are not comparable and the first is not a target for the second.
Display PPI
Screens also have a PPI, and it is why the same image looks different on different devices. A typical desktop monitor is around 110 PPI; a high-density laptop or phone display is 220 PPI or more, packing two device pixels into each CSS pixel.
For image assets this is the whole argument for supplying a 2x version of anything prominent. An image displayed at 800 CSS pixels wide on a high-density screen has 1600 physical pixels available; supply only 800 and the browser scales it up, which looks soft next to the crisp text beside it.
It is not an argument for supplying 2x everything. Doubling the width quadruples the pixel count, and for a thumbnail nobody is examining, that is bandwidth spent on nothing.
Scanning
When scanning, the resolution setting is not metadata — it genuinely controls how finely the sensor samples the original, and it is the one context where the number does real work. 300 for a text document, 600 for a photograph you may enlarge, and 2400 or more for 35 mm film, which is tiny and needs the sampling density to yield a usable pixel count.
Tools that work with this
Related terms
- DPIDPI is the number of ink dots a printer places per inch, and in everyday use it is the ratio that converts an image's pixel dimensions into a physical printed size.
- Bicubic interpolationBicubic interpolation is a resampling method that estimates each new pixel from a weighted cubic function of the sixteen surrounding source pixels, producing smoother results than simpler methods without adding any real detail.
- Super-resolutionSuper-resolution is the use of a trained model to increase an image's resolution by generating plausible detail consistent with its training data, rather than by interpolating between existing pixels.
- Aspect ratioAspect ratio is the proportional relationship between an image's width and height, expressed as two numbers such as 16:9, and it is what determines whether a platform crops your image.
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