Pixels to Inches Converter

Someone with 20/20 vision can separate detail about one arcminute apart. Beyond that, extra pixels are wasted. That gives a rule of thumb: PPI needed ≈ 3,438 ÷ viewing distance in inches. It's why billboards print at a handful of pixels per inch and still look sharp from the road.

CSS defines 1in as exactly 96 CSS pixels, and 1pt as 1/72 of an inch, so 12pt text is 16px. A CSS pixel is a layout unit, not a hardware dot. On a high-density phone one CSS pixel may be drawn with three device pixels in each direction.

The 96 comes from Windows, which assumed 96 dots per inch for screens. The original Macintosh used 72, matching the 72 points in a printer's inch. That history is why images still get saved "at 72 dpi for the web". On a website the setting does nothing: browsers lay out images by pixel dimensions and ignore the PPI stored in the file. It only matters when the image is placed in a print layout.

A screen's real density is its diagonal resolution in pixels divided by the diagonal in inches. That's what the Screen PPI mode computes, and it rarely equals 96. A 1-inch CSS box will only measure an inch with a ruler if the operating system's scaling happens to cancel out the difference, which it usually doesn't.

PPI is image pixels per inch of output. DPI is the printer's ink dots per inch. An inkjet rated at 1,440 or 4,800 dpi uses many tiny dots to build each image pixel, so don't enter the printer's dpi as the PPI.

Changing an image's PPI without resampling only changes its print size. In Photoshop, that's Image Size with Resample unticked: 3,000 px at 300 PPI becomes 10 in, or 20 in at 150 PPI, with the same pixels. Resampling up adds pixels by interpolation, which smooths edges but can't add detail the camera didn't capture.