The label is not the width

A 55-inch screen is measured diagonally across its image area, not horizontally across the television. Multiplying 55 by 2.54 gives a 139.7 cm diagonal, not the cabinet width you need. Treat the advertised size as a starting point: the actual viewable diagonal may differ slightly from the nominal size, and the surrounding frame is extra.

Calculate the image area first

For a flat rectangular screen with diagonal d and aspect ratio a:b, width = d × a / √(a² + b²) and height = d × b / √(a² + b²). Keep all lengths in the same unit. A nominal 55-inch 16:9 image is about 121.76 × 68.49 cm; a 27-inch 16:9 image is about 59.77 × 33.62 cm. These are calculated image dimensions, not guaranteed product dimensions.

A different ratio changes the fit

At the same diagonal, a 21:9 rectangle is wider and shorter than a 16:9 rectangle. Choosing a wider format does not simply add space on either side of an unchanged screen. Use the model’s actual ratio where available; ultrawide labels can be approximate. For curved displays, rely on the manufacturer’s external dimensions for desk clearance.

PPI answers a different question

Pixel density describes how closely pixels are packed, not how much desk space the monitor occupies. PPI = √(horizontal pixels² + vertical pixels²) / diagonal in inches. For a nominal 27-inch display at 2560 × 1440, this gives about 108.79 PPI. Resolution alone cannot tell you the physical width; viewing distance and interface scaling also affect the reading experience.

Measure the complete installation

Measure the available width, height and depth. Then check the exact model’s dimensions with and without its stand, the feet’s spacing and the base’s depth. Allow the ventilation clearances specified in the manual, room for connectors and cable bends, and access for your hands. Check nearby doors and shelves. Mark the complete outline with removable tape or cardboard, and try your normal seated position. Verify support capacity and mounting compatibility separately: fitting inside an opening does not establish safe installation.