PPI Calculator

Resolution on its own means nothing

"4K" describes a pixel count, not sharpness. The same 3840 × 2160 gives 163 PPI on a 27-inch monitor and 80 PPI on a 55-inch television. Twice the density, the same specification on the box.

PPI = √(width² + height²) ÷ diagonal

The diagonal in pixels, divided by the diagonal in inches. Everything else on the page follows from that.

The one-arcminute rule

A person with normal 20/20 vision resolves detail down to about one arcminute — one sixtieth of a degree. Closer together than that and two points blur into one.

That gives a usable rule: a pixel becomes invisible once it subtends less than an arcminute, which happens at a distance of about 3,438 times the pixel size. On a 300 PPI phone screen a pixel is 0.085 mm, so pixels disappear beyond about 29 cm — roughly where a phone is held, which is not a coincidence.

It is a simplification. Real acuity varies between people, is better than 20/20 for some, and the eye is noticeably better at spotting misaligned lines than isolated dots — which is why text still looks crisper at higher densities than the rule predicts.

Densities in practice

• Under 100 PPI — pixels visible at a desk. Older 1080p monitors at 27 inches sit here, around 82 PPI, and text looks noticeably rough.
• 110–140 PPI — the ordinary desktop range. Fine, not crisp.
• 160–220 PPI — sharp. A 27-inch 4K panel is 163; a 24-inch 4K is 184.
• 220 PPI and above — the density Apple called "retina" for laptops. Text edges are indistinguishable from print.
• 400–500 PPI — modern phones, comfortably past what the eye resolves at typical holding distance.

Why televisions get away with so little

A 55-inch 4K television at 80 PPI sounds poor and looks excellent, because it is viewed from two or three metres rather than sixty centimetres. At that distance the pixels are far below the acuity limit.

It also explains why 8K televisions are a hard sell: at normal seating distance, most people cannot see the difference from 4K on any screen that fits in a living room.

Scaling, and why high-PPI screens need it

A 27-inch 4K display at 163 PPI shows everything at half the physical size of a 1080p screen of the same dimensions. Unscaled, text would be unreadable.

So operating systems scale — typically 150 or 200 % — drawing interface elements at the size they should appear while using the extra pixels for smoothness. That is what a device pixel ratio above 1 means, and it is why the browser reports fewer CSS pixels than the panel has.

Common uses

• Comparing two monitors whose specifications look similar.
• Deciding whether 4K is worth it at a given size.
• Choosing an image resolution for a display.
• Working out physical dimensions from a diagonal.

Questions people ask

How is PPI calculated?

Divide the diagonal in pixels — √(w² + h²) — by the diagonal in inches.

What is a good PPI for a monitor?

Around 110–140 is ordinary, 160–220 is sharp. A 27-inch 4K panel is 163 PPI.

Why does a 4K TV have such low PPI?

Because the same pixels are spread over a much larger panel. A 55-inch 4K screen is about 80 PPI, which is fine at three metres.

At what distance do pixels become invisible?

Roughly 3,438 times the pixel size, from the one-arcminute acuity limit. On a 300 PPI phone that is about 29 cm.

Why does my 4K monitor show fewer pixels to the browser?

Because the operating system is scaling, typically to 150 or 200 %. The extra pixels go into smoothness rather than more space.

Nothing is sent anywhere

The arithmetic runs in the page. Where a tool reads your screen dimensions, it reads what the browser already exposes to every site you visit — and it does not transmit them. The about page lists every request this site makes.