What is PPI (pixels per inch) and how do you calculate it?

PPI — pixels per inch — is the number that tells you how sharp a screen really is. Resolution and size each tell half the story; PPI combines them into a single figure you can actually compare across devices.

The formula

PPI uses the diagonal in pixels (from the Pythagorean theorem) divided by the diagonal in inches:

diagonal pixels = √(width² + height²)
PPI = diagonal pixels ÷ screen size (inches)

For a 27″ 2560 × 1440 monitor:

√(2560² + 1440²) = 2937 diagonal pixels
2937 ÷ 27 ≈ 109 PPI

You don’t have to do this by hand — the PPI calculator does it instantly from any resolution and size.

What counts as “sharp”?

There’s no single magic number, because sharpness depends on how far away you sit. Roughly:

PPI	Looks like
Under 90	Individual pixels visible up close; text edges look coarse
90–120	The comfortable desktop-monitor range at arm’s length
120–160	Very sharp; the low end of “retina” territory
200+	Phone/high-DPI laptop density; pixels invisible

Apple’s original “Retina” claim was that at a normal viewing distance, you can’t resolve individual pixels. The threshold is distance-dependent — a phone held 12″ away needs far more PPI than a TV across the room.

Why the same resolution looks different everywhere

PPI is why a 1080p phone looks flawless while a 1080p 32″ TV shows visible pixels from your couch. Identical pixel counts, hugely different physical sizes — so hugely different densities.

It also explains device pixel ratio: operating systems use high PPI to render crisp interfaces by packing multiple physical pixels into each logical one. If that part is new to you, read what is device pixel ratio.

The practical takeaway

When comparing two displays, convert both to PPI before you decide — it’s the only apples-to-apples sharpness number. Plug your size and resolution into the PPI calculator, or detect your current resolution first if you’re not sure what you’re running.