Screen Size in Inches: What It Really Measures
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Screen inches describe the diagonal
An advertised screen size generally measures the display area's diagonal, corner to corner. One inch is exactly 2.54 centimeters. A 55-inch screen therefore has a display diagonal of 139.7 cm.
That is not the width of the television. Converting the diagonal alone cannot tell you how much space is needed on a shelf. Width and height also depend on the display's aspect ratio.
The same diagonal can have different dimensions
For a 16:9 display, multiply the diagonal by 16 and divide by the square root of 337 to find the width. Use 9 in the numerator for the height. A 55-inch, 16:9 display area is about 121.8 cm wide and 68.5 cm high.
An ultrawide display with the same diagonal has different dimensions. These measurements also exclude the frame, stand, connectors, and required ventilation space. Check the manufacturer's complete dimensions before deciding whether a screen will fit.
Round after calculating
The conversion factor of 2.54 cm per inch is exact. Aspect-ratio calculations can produce more decimal places. Keep that precision during the calculation and round the final measurement to a practical level.
Use the full conversion factor for other units too, including kilograms and pounds. Temperature is different: converting between Celsius and Fahrenheit involves an offset as well as a multiplier.