Dpi to Pixel Size Calculator

The core relation is trivial arithmetic, pixels equal inches multiplied by resolution, but the terminology hides a distinction worth keeping straight. An image file has pixels per inch, while a printer has dots per inch, and an inkjet lays many tiny ink dots down for every image pixel, which is why a printer advertising 1440 by 5760 dpi still wants only 300 ppi of image data. A US Letter page at 8.5 by 11 inches and 300 ppi is 2550 by 3300 pixels, or 8.4 megapixels, which uncompressed at 24 bit RGB occupies about 24.1 megabytes and at 32 bit CMYK about 32.1 megabytes. The conventional thresholds are 300 ppi for offset and photographic printing, 150 ppi as an acceptable floor for large format viewed at arm's length or further, and 72 or 96 ppi as screen-only conventions. Two practical points cause most of the confusion in production work. First, the resolution figure stored in a file is metadata only; changing it without resampling alters nothing about the pixels and only changes the default physical size when the image is placed. Second, the right target resolution depends on viewing distance rather than on any absolute rule. Human acuity resolves roughly one arcminute, which works out to a requirement near 3438 divided by the viewing distance in inches, so a page held at 12 inches justifies about 290 ppi while a poster read from 6 feet needs only about 48 ppi and highway billboards are routinely produced between 10 and 20 ppi.

The number 72 that anchors screen resolution is a typographic inheritance. The Anglo-American printer's point was standardized in the 1880s at about one seventy-second of an inch, and when Adobe released PostScript in 1984 it rounded the definition to exactly 1 over 72 of an inch to make the arithmetic clean. Apple designed the original Macintosh screen of that same year, 512 by 342 pixels across a 9 inch diagonal, at approximately 72 pixels per inch so that a point on the display would measure the same as a point on paper, and the convention outlived the hardware. Desktop publishing fixed the numbers that software still defaults to. Adobe PostScript in 1984, the Apple LaserWriter in 1985 with an output resolution of 300 dots per inch, and Aldus PageMaker in the same year put professional composition on a desk. The LaserWriter figure is the direct reason 300 became shorthand for print quality even though commercial imagesetters and platesetters have run at 2400 dpi and beyond for decades. Screen density then broke free of the old convention, with Microsoft settling on 96 ppi as its logical baseline for Windows and contemporary panels ranging past 400 ppi on a phone. Modern operating systems and browsers responded by decoupling the logical pixel from the physical dot through a device pixel ratio, which is why a CSS pixel and a hardware pixel are no longer the same thing.