Screen Size, Aspect Ratio and PPI Made Simple

The diagonal is the distance between opposite corners. To get the actual width and height you also need the aspect ratio — the proportion of width to height, such as 16:9 or 21:9. With both known, the width and height become the two legs of a right triangle whose hypotenuse is the diagonal, so they follow the Pythagorean theorem.

This is why a 27-inch 16:9 monitor and a 27-inch 21:9 ultrawide have different widths, heights and — crucially — different total areas. A wider aspect ratio at the same diagonal is shorter and usually covers less area, even though on the shelf both are sold as '27-inch'.

Screen area — width multiplied by height — is a far better measure of 'how much screen' you get than the diagonal alone. Because area grows with the square of size, small diagonal differences translate into large area differences. A 32-inch 16:9 display has roughly 40% more area than a 27-inch 16:9 one, even though its diagonal is only about 19% larger.

When you are deciding between two monitors of similar shape, comparing area is much more honest than comparing the number on the box. Two inches of extra diagonal can mean a surprisingly large jump in desk presence and usable working space.

Pixels per inch (PPI) measures how densely pixels are packed, and it is what makes text look crisp or blocky. It is the diagonal measured in pixels divided by the diagonal measured in inches. A 27-inch 1440p monitor has about 109 PPI, while a 27-inch 4K monitor has about 163 PPI — the same physical size, but a noticeably sharper image. Phones push much higher, often above 400 PPI, because you hold them close to your eyes.

Remember that higher resolution on a bigger screen does not automatically mean sharper. A 32-inch 4K panel is actually less dense than a 27-inch 4K one, because the same number of pixels is spread over more inches. Sharpness depends on resolution and physical size together, never on resolution alone.

The diagonal and the aspect ratio together give you everything. For a ratio of w:h, the width is d × w / √(w² + h²) and the height is d × h / √(w² + h²). A 27-inch 16:9 panel therefore measures 23.53 by 13.24 inches, and a 34-inch 21:9 ultrawide measures 31.36 by 13.44 — barely taller than the 27-inch, despite seven more inches of diagonal.

Area scales with the square of the diagonal when the ratio is held constant, which makes upgrades less dramatic than they sound. Moving from a 24-inch to a 27-inch 16:9 monitor multiplies the area by (27/24)², about 1.27 — a quarter more glass, not a half. Going from 27 to 32 inches is another 1.40×, and that compounding is why the jump from 24 to 32 inches feels like a different category of desk.

PPI is pixel density on the panel: √(w_px² + h_px²) divided by the diagonal. A 24-inch 1080p screen comes to about 92 PPI, a 27-inch 1440p to 109, a 27-inch 4K to 163 and a 32-inch 4K to 138. Those numbers look decisive until you notice that a phone sits at 460 PPI and nobody calls a 27-inch monitor blurry by comparison.

What the eye actually resolves is angular size. Normal vision separates detail down to roughly one arcminute, which works out at about 60 pixels per degree of view. A phone held at 30 cm and a monitor at 70 cm subtend very different angles, so the monitor needs far less density to look equally sharp. Compare screens at the distance you will really use them, and treat PPI as one input rather than the verdict.