How this instrument works
Pixels per inch, or PPI, measures how tightly a display's pixels are packed together. It is calculated from the screen's pixel resolution and its physical diagonal size: more pixels squeezed into the same diagonal means a higher PPI and a visually sharper picture, because individual pixels become too small for the eye to pick out at normal viewing distances.
The calculation runs through the pixel diagonal rather than the width or height alone, because resolution is specified in two dimensions. Squaring the width and height in pixels, adding them, and taking the square root gives the diagonal pixel count by the Pythagorean theorem — the same geometry used to size any rectangle's diagonal. Dividing that pixel diagonal by the physical diagonal in inches converts a pixel count into a density figure.
PPI is what manufacturers print on spec sheets to describe sharpness, and it is the same underlying idea behind marketing terms like 'Retina' — a display where PPI is high enough, relative to typical viewing distance, that individual pixels are no longer resolvable by the naked eye. A 5-inch phone screen and a 27-inch monitor can carry the same resolution yet have wildly different PPI, because the same pixel count is stretched across a much larger diagonal on the monitor.
- Enter the screen's pixel resolution into Resolution width (px) and Resolution height (px) — check the device spec sheet or display settings if you don't know it.
- Enter the panel's diagonal size in Screen diagonal (in), the number normally advertised as the screen size, such as 15.6 or 27.
- Read Pixels per inch (PPI) for the resulting density — higher numbers mean a visually sharper display at normal viewing distance.
- To compare two displays, run each through the instrument separately and compare the PPI values directly; a higher PPI at the same viewing distance will look sharper.
Worked example — a 15.6-inch Full HD laptop panel
Enter 1920 into Resolution width (px), 1080 into Resolution height (px), and 15.6 into Screen diagonal (in) — a standard 15.6-inch Full HD laptop display. The pixel diagonal is √(1920² + 1080²) = √4,852,800 ≈ 2202.91 pixels. Dividing by the 15.6-inch physical diagonal gives Pixels per inch (PPI) = 141.21.
That 141.21 PPI sits in the mid-range for budget and mainstream laptops — noticeably sharper than an older 1366x768 panel at the same size (about 100 PPI), but well below a high-density 4K laptop panel, which can exceed 280 PPI on the same 15.6-inch diagonal.
Questions
What counts as a 'good' PPI for a screen?
There's no universal cutoff, because perceived sharpness depends on viewing distance as well as PPI. As a rough guide, phones held close to the face typically use 300+ PPI, laptops viewed at arm's length look crisp from around 220 PPI up, and desktop monitors viewed from further away can look sharp at 100-140 PPI. A 141 PPI laptop panel, like this page's worked example, reads as reasonably sharp precisely because it's viewed closer than a monitor but not as close as a phone.
Why does a phone need a much higher PPI than a monitor to look equally sharp?
Because sharpness is about angular resolution, not raw pixel density — how large each pixel appears in your field of view. A phone is held roughly 10-14 inches from the eyes, so its pixels must be much smaller and more densely packed to stay individually invisible. A monitor viewed from 2-3 feet away can use noticeably larger, less dense pixels and still look equally crisp, which is why a 141 PPI laptop and a 460 PPI phone can look comparably sharp to their respective users.
Does PPI depend on the screen's aspect ratio?
Not directly — the formula only needs total pixel resolution and diagonal size, both of which already account for whatever aspect ratio the panel uses. A 16:9 screen and an unusually shaped ultrawide panel with the same pixel diagonal and the same physical diagonal produce identical PPI, even though their width-to-height proportions differ substantially.
Is PPI the same thing as DPI?
They're closely related but describe different devices. PPI (pixels per inch) describes a digital display's pixel density. DPI (dots per inch) traditionally describes a printer's dot density on physical paper. The two terms get used interchangeably in casual conversation, and this site's pixels-to-print-size instrument uses DPI in exactly that printing sense, but strictly speaking PPI is a screen measurement and DPI a print one.
How is 'Retina display' related to PPI?
Retina is a marketing term, not a fixed technical value — it describes displays whose PPI is high enough, given typical viewing distance for that device class, that the human eye can no longer distinguish individual pixels. Because the threshold shifts with viewing distance, a Retina-labelled phone display and a Retina-labelled laptop display don't share the same PPI number; each is simply dense enough for how closely people tend to use it.