Screen size and resolution calculator

The advertised diagonal is the panel, not the product. A television or monitor is wider and taller than these figures by the bezel, and a stand adds depth and height that the specification often buries. When measuring for an alcove or a wall mount, the panel dimensions here are the floor and the manufacturer product dimensions are what actually has to fit. Ultrawide monitors are the case where the difference is most striking. A 34-inch 21:9 is barely taller than a 27-inch 16:9 but nearly 20 cm wider.

Density is only meaningful against a distance

Pixels per inch is the diagonal pixel count divided by the diagonal in inches, and on its own the number says very little. What decides whether pixels are visible is angular resolution: the eye resolves about one arcminute, so the density a display needs falls linearly with how far away it sits. Around 110 PPI is the comfortable zone for a desktop monitor at arm length, which is why 27-inch 1440p at 109 sits right in it, while 24-inch 1080p at 92 and 27-inch 1080p at 82 look progressively softer. A phone needs roughly 300 PPI at 30 cm, a laptop about 220, and a billboard is perfectly sharp at 10.

Phone densities have run well past that threshold, and the reason is not sharpness. Very high density leaves room for subpixel arrangements such as PenTile that trade real subpixels for panel efficiency, and it allows clean integer scaling of interface layouts across a range of physical sizes. A 460 PPI phone is not visibly sharper than a 380 PPI one at arm length; it is easier to build a consistent interface on. On the desktop the practical warning is the opposite one: above about 150 PPI an operating system generally needs fractional scaling, and how well that is handled still varies by platform, so a density landing near a clean 100% or 200% is worth more than a higher number.

Advertised ratios are often approximations

The reduced ratio of a resolution occasionally disagrees with the box. "21:9" ultrawide is really 43:18 at 3440×1440 and 64:27 at 2560×1080, neither of which is 21:9, since that would be 2.333 against 2.389. DCI 4K at 4096×2160 is sold as 17:9 and is not 16:9 — it reduces to 256:135, which the aspect tab prints — and that is why cinema 4K and television 4K are different resolutions sharing a name. The ratio shown here is the mathematical one. Pixel counts are less slippery: 4K really is four times 1080p, since twice the width and twice the height gives four times the pixels, and 16:10 survives on some laptops because the extra vertical space suits documents and code.

DPI is a tag, not detail

Print size is pixels divided by DPI, so a 3000×2400 image prints 10 × 8 inches at 300 DPI. What trips people up is that DPI is not stored in the pixels; it is a number attached to the file, and changing it in software without resampling only declares that the image should print smaller or larger. The two honest ways to gain print size are to capture at a higher resolution or to accept a lower DPI, and the second is usually fine: 300 is the standard for anything held at reading distance, 150 is ample for a poster, and large-format work routinely goes far lower.