PPI, retina, and when sharpness stops mattering

PPI — pixels per inch — is resolution divided by physical size. A 24-inch 1080p monitor is about 92 PPI; a 6.1-inch phone at 1179 × 2556 is 460 PPI. Same idea as resolution, but corrected for how big the pixels actually are, which is what your eyes respond to.

The 'retina' threshold Apple popularized was roughly 300 PPI at phone distance — the density where a typical eye stops resolving individual pixels. It was marketing, but honest marketing: past that density, at that distance, sharpness gains become invisible. Viewing distance is the whole trick. A 100-PPI desktop monitor at arm's length looks as smooth as a 300-PPI phone at 30 cm, and a highway billboard gets away with pixels the size of golf balls.

Where the diminishing returns kick in

Phone panels above ~450 PPI are past what eyes verify — battery and GPU pay for pixels nobody perceives. For desktop monitors, the meaningful jump is from ~90-110 PPI (standard) to ~180-220 PPI (4K at 27 inches, rendered at 2×): text goes from 'fine' to print-like, and it's the single best screen upgrade for anyone who reads or writes all day. My take: for productivity, a 27-inch 4K at 2× scaling beats any ultrawide gimmick.

The checker above gives megapixels and aspect ratio for any resolution you're comparing; pair it with the panel's physical size and you can compute PPI yourself — resolution diagonal divided by inch diagonal. Or just remember the rule: if you can't see pixels at your normal distance, more of them buys nothing.