🖥️ Displays

Screen resolution and PPI: what the numbers mean

"Resolution" gets used for at least three different things, and "Retina" is a marketing claim about eyesight rather than a specification. Here is the arithmetic that separates a genuinely sharp display from a big one, and how to size an image for print without guessing.

Written and reviewed by SM Arijul Islam · Last reviewed 20 October 2026 · Editorial standards · Report an error

Three things called resolution

The pixel dimensions of a screen — 1920 × 1080, for example — describe how many pixels it has, not how sharp it looks. The pixel dimensions of an image describe how much detail it contains. Spatial resolution in physics describes the smallest detail an instrument can distinguish. Only the first two are commonly called resolution, and confusing them is the root of most display arguments.

What people actually perceive as sharpness is pixel density: how many pixels fit into an inch of the thing you are looking at. That is what PPI measures, and it depends on the physical size of the screen as well as the pixel count.

The formula

Take the diagonal in pixels, divide by the diagonal in inches:

PPI = √(width² + height²) ÷ diagonal inches

A 27-inch monitor at 2560 × 1440 has √(2560² + 1440²) = 2937.2 pixels on the diagonal, so 2937.2 ÷ 27 = 108.8 PPI. The same panel at 3840 × 2160 gives 4406.9 ÷ 27 = 163.2 PPI. The PPI calculator does this for any combination, including whether a given resolution fits a laptop lid you are considering.

Real devices, measured

Pixel density of common screens
Device	Pixels	Diagonal	PPI
Budget 24" monitor	1920 × 1080	24 in	91.8
27" QHD monitor	2560 × 1440	27 in	108.8
34" ultrawide	3440 × 1440	34 in	109.7
15.6" laptop	1920 × 1080	15.6 in	141.2
27" 4K monitor	3840 × 2160	27 in	163.2
13.3" laptop	2560 × 1600	13.3 in	227.0
5.5" phone	1920 × 1080	5.5 in	400.5
6.1" phone	2532 × 1170	6.1 in	457.3

The pattern is stark. A phone and a monitor can carry the same 1920 × 1080 pixel count and differ by a factor of four in density. This is why a 1080p phone from a decade ago looked sharp while a 1080p 27-inch monitor still looks slightly soft.

Why "Retina" is a distance, not a number

Human visual acuity is usually quoted as one arcminute — the ability to separate two lines about 1/60 of a degree apart, which corresponds to 20/20 vision. Whether you can see individual pixels depends on the angle each pixel subtends, and that depends on how far away you sit.

At 12 inches, one arcminute spans about 0.0035 inches, which is roughly one pixel on a 287 PPI screen. At 20 inches the same acuity maps to about 172 PPI. That is the whole idea behind Retina: Apple's claim was that a screen exceeds the eye's resolving power at a specified viewing distance, not that it has any particular PPI. A 218 PPI laptop qualifies because you sit 18 inches away; a 4K television at 110 PPI also qualifies, because you sit eight feet away.

For a concrete limit, about 60 pixels per degree of viewing angle is the point beyond which further density adds nothing perceptible. Work out the viewing angle your screen subtends, multiply, and you have the useful pixel budget.

What extra pixels do buy

Beyond sharpness, more pixels buy space. A 4K monitor at 100 percent scaling shows four times as much content as a 1080p one. Most operating systems scale the interface up on dense panels so that text is physically larger, which gives smoother text and photographs but not more room to work.

Scaling also explains a common complaint: running a 4K laptop at 200 percent is effectively a 1080p desktop, and it looks better than a native 1080p panel of the same size because each logical pixel is drawn from four physical ones. That is not wasted resolution — it is antialiasing with real pixels.

Upscaling does not add detail

If a 1280 × 720 video is displayed on a 3840 × 2160 screen, the player invents the missing pixels by interpolation. The result is smoother than nearest-neighbour scaling and it is still not 4K detail. The same applies to images: enlarging a 2000-pixel-wide photo to 6000 pixels does not create information, it creates a bigger file. Good upscalers — algorithmic or machine-learned — make a convincing guess, not a measurement.

PPI for print, and the 72 DPI myth

Printing inverts the question. Here you have a fixed number of pixels and you choose the physical size, so the density follows: PPI = pixels ÷ inches. A 6000 × 4000 photograph printed 20 inches wide has 300 PPI. Printed 40 inches wide it has 150 PPI, which is perfectly acceptable at poster viewing distance.

The old rule that screens are 72 DPI and print is 300 DPI is a relic of the 1984 Macintosh, whose 512 × 342 display was about 72 pixels per inch. Modern CSS defines a reference pixel as 1/96 inch, and browsers scale that to real device pixels according to the display's density. Nothing in a JPEG has an inherent DPI; the number in the file's metadata is a hint for layout software, not a property of the image.

Practical print targets depend on how close the reader stands. Below about 100 PPI, edges and text start to look soft even at a distance.

Print resolution by output and viewing distance
Output	Target PPI	Typical viewing distance
Photograph held in the hand	300	Arm’s length
Magazine page	200	Reading distance
Poster	150	About a metre
Banner	100	Several metres

Frequently asked questions

Is 72 DPI the standard for screens?

No. It is a leftover from the first Macintosh display. CSS defines a reference pixel as 1/96 inch, and real screens range from roughly 90 to over 500 pixels per inch.

Does a higher PPI always look better?

Only up to the limit of your eyes at your viewing distance. Past roughly 300 PPI at arm's length, extra density is invisible and simply costs battery life and graphics performance.

How do I calculate PPI?

Square the width and height in pixels, add them, take the square root, then divide by the diagonal in inches. The result is pixels per inch along the diagonal, which is the figure manufacturers quote.

How large can I print at 300 PPI?

Divide the pixel dimensions by 300. A 6000 × 4000 photo is 20 × 13.3 inches at 300 PPI. Halve the density to 150 PPI and you can print twice as wide before the eye notices.

Why does my monitor look less sharp than my phone?

Because density, not pixel count, drives apparent sharpness. The same pixels spread across 24 inches instead of six are four times further apart, and you also sit closer to the phone.

Sources

W3C CSS Values and Units Module Level 3 — definition of the CSS reference pixel.
ISO 9241-3 and its successors — ergonomic requirements for visual displays, including character height and viewing distance.
Manufacturer specifications for the device dimensions quoted above; PPI values calculated from those numbers.