PPI Calculator

How screen PPI is calculated

PPI stands for pixels per inch: how many pixels a display packs into one linear inch. Manufacturers publish the resolution and the diagonal, never the density, so you work it out with Pythagoras. Take the diagonal pixel count, the square root of width squared plus height squared, then divide by the physical diagonal in inches. A 2560 x 1440 panel has a diagonal of 2937 pixels; across 27 inches that lands at 108.8 PPI.

Two numbers fall straight out of that. Dot pitch is 25.4 divided by PPI, giving the width of one pixel in millimetres — 0.2335 mm for the panel above, which is the figure spec sheets quote for older monitors. Total pixel count is width times height, 3.69 megapixels here, and that is what your graphics card has to redraw sixty or more times per second.

Worked comparison: 27-inch 1440p versus 27-inch 4K

This is the single most searched monitor decision, so run both. The 1440p panel gives 108.8 PPI and a dot pitch of 0.2335 mm. The 4K panel at the same size has a 4406-pixel diagonal, so 163.2 PPI and a 0.1557 mm pitch — exactly 50% more pixels per inch and 8.29 megapixels instead of 3.69, more than double the work per frame.

What that means in practice: at 100% scaling, 4K text on a 27-inch screen is around two-thirds the size of 1440p text, which most people find too small to read comfortably. You end up at 125% or 150% scaling, which recovers the readable size and spends the extra pixels on smoother letterforms rather than more desktop space. If you specifically want more room for windows, 1440p at 100% already gives it, and 4K only matches it if you can tolerate tiny text. If you want crisper text at the same layout, 4K plus 150% scaling is the answer.

The viewing distance maths

A normal eye resolves detail down to roughly one arcminute, a sixtieth of a degree. Convert that to a simple rule and the distance at which pixels blur together is about 3438 divided by PPI, in inches. The 27-inch 1440p monitor hits that at 31.6 inches, or 80 cm — further than most people sit, which is why you can still see pixel edges on it. The 4K version blurs out at 21.1 inches, comfortably inside normal desk distance, so the grid genuinely disappears.

The same rule explains why phones need such extreme numbers. A 6.1-inch handset at 2556 x 1179 runs 461 PPI and vanishes at 7.4 inches, because you hold it at arm's bend rather than desk distance. It also explains why a 55-inch 4K television at 80 PPI looks flawless: its threshold is 43 inches, and almost nobody sits closer than that to a TV. Density is meaningless without the distance it is judged from.

Scaling on Windows and macOS

Windows exposes this in Settings, Display, Scale — with 100%, 125%, 150% and 175% as the usual steps. It recommends a setting based on the panel, and the recommendation is normally right. Older applications that were never made scaling-aware can still come out blurry, which is the practical cost of a high-PPI monitor on Windows.

macOS takes a different route: it renders at double resolution and downsamples, which is why Apple ships panels near 220 PPI. A 13.3-inch 2560 x 1600 MacBook display is 227 PPI, almost exactly two times the 110 PPI of a classic desktop monitor, so everything lands on a clean pixel boundary. Plugging a 27-inch 1440p monitor into a Mac often disappoints for that reason — it is too dense for crisp 1x rendering and far too sparse for 2x.

What density to buy for what

Under 100 PPI, pixels are visible at a desk and text edges look ragged; a 24-inch 1080p panel at 91.8 PPI is the common example. Between 100 and 150 is standard territory — 1440p at 27 inches, 4K at 32 inches (137.7 PPI) — sharp enough for daily work with no scaling required. Above 150 you get crisp text without tricks, and past roughly 220 you are in retina-class territory where the grid is invisible at normal distance.

For office work and coding, 1440p at 27 inches remains the value sweet spot. For photo and print work, higher density genuinely helps because you judge fine detail. For gaming, remember that pixel count drives frame rate: 8.29 megapixels is a serious load, and a lower-density panel with a faster refresh rate is often the better trade.

Limitations of a PPI number

Density says nothing about panel quality. Contrast, colour accuracy, response time, coating and subpixel layout all affect perceived sharpness — a matte anti-glare finish visibly softens text, and non-standard subpixel arrangements can make text look fringed regardless of the PPI figure. Manufacturers also round diagonals, so a 27-inch monitor is often 27.0 or 26.9 inches of actual glass, which shifts the result by a fraction of a percent. Treat the number as a solid comparison tool between panels, not as a quality verdict on any one of them.