For a display with horizontal resolution W, vertical resolution H and diagonal size D in inches:

This combines diagonal pixel count with physical diagonal size.

Enter screen width in pixels, height in pixels and physical diagonal size. The result can include PPI, PPcm, diagonal pixels, pixel pitch, megapixels and aspect ratio. Use the print mode when you know image pixel dimensions and want physical print size at a target PPI.

Pixel pitch is approximately 25.4/PPI millimetres per pixel for a display. Smaller pitch means more pixels fit into the same physical distance. “Dot pitch” is sometimes used loosely, though display subpixel structures can make the historical term more complicated.

PPI describes pixels in an image or display. DPI describes physical dots produced by a printer or output device. A printer may use multiple ink dots to reproduce one image pixel, so a 300 PPI image does not imply a 300 DPI printer mechanism.

Print width in inches is pixel width divided by target PPI, and height follows the same rule. A 4800×6000 image at 300 PPI prints at 16×20 inches without resampling.

Required pixels depend on target PPI. At 300 PPI, an 8.5×11 inch Letter page needs about 2550×3300 pixels before bleed or cropping. A 16×20 inch print needs about 4800×6000 pixels.

A higher resolution does not automatically mean higher PPI because physical screen size matters. A 4K television can have lower PPI than a high-resolution phone even though it has far more total pixels.

Whether extra PPI is visible depends on viewing distance, eyesight and content. Very high PPI can still improve typography and rendering, but “best” PPI is not a single universal number.

A browser can know its logical viewport and device-pixel ratio, but exact physical PPI generally requires the actual physical display dimensions or a reliable device model lookup. We do not label a browser-derived estimate as exact physical PPI.

A 24-inch 1920×1080 monitor has a diagonal pixel count of about 2203 pixels. Dividing by 24 inches gives roughly 91.8 PPI. A 27-inch 2560×1440 monitor is about 108.8 PPI, so the larger display is still sharper because resolution increased more than physical size.

An image file does not have one immutable physical PPI until you decide how large to display or print it. A 3000-pixel-wide image printed at 10 inches is 300 PPI; printed at 20 inches it is 150 PPI. The pixel count stays the same while physical output size changes.

Do not use browser CSS pixels as if they were guaranteed physical pixels, do not call PPI and printer DPI identical, and do not compare two monitors by resolution alone without considering screen size. A “4K” label describes pixel dimensions, not physical sharpness by itself.