Why is the diagonal measurement standard instead of width or height?

The diagonal is the longest dimension and a single number that uniquely identifies screen size given a fixed aspect ratio. It became industry convention decades ago and has persisted.

My monitor says 27″ but it looks smaller than my 27″ TV. Why?

The TV may have a different aspect ratio, or the bezel may be smaller, making the viewable area feel larger. Use this calculator to compare actual screen areas numerically.

What aspect ratios are supported?

16:9 for most modern monitors and TVs, 4:3 for older displays, 21:9 for ultrawide, 16:10 for many laptops, and Custom for any ratio you enter.

guide

How to Calculate Screen Size and Dimensions — Free (2026)

By Rui Barreira · Last updated: 18 June 2026

Screen sizes are quoted as diagonal measurements, but the dimensions that matter for desk space, wall mounts, and field-of-view calculations are width and height. Given the diagonal and aspect ratio, the exact width and height follow from basic Pythagorean geometry.

How to Use

Enter the diagonal screen size in inches (the number printed on product listings).
Select the aspect ratio (16:9 for most modern monitors and TVs, 4:3 for older displays, 21:9 for ultrawide, 16:10 for many laptops).
For unusual screens, choose Custom and enter your own width and height ratio.
Click Calculate to see width, height, and area in both inches and centimetres.

How It Works

For a screen with diagonal d and aspect ratio w:h, the width is d × w / √(w² + h²) and the height is d × h / √(w² + h²). This is derived from the Pythagorean theorem applied to the right triangle formed by the screen's width, height, and diagonal. Centimetre values multiply inches by 2.54.

Common Aspect Ratios

16:9 is the standard for most televisions, gaming monitors, and consumer laptops since around 2010. 16:10 appears on many productivity laptops and older MacBooks — it offers slightly more vertical space than 16:9. 4:3 was standard before widescreen became dominant and is still found in older projectors and commercial displays. 21:9 ultrawide monitors are popular for immersive gaming and multi-window productivity work.

Frequently Asked Questions

Why is the diagonal measurement standard instead of width or height?: The diagonal is the longest dimension and a single number that uniquely identifies screen size given a fixed aspect ratio. It became industry convention decades ago and has persisted.
My monitor says 27″ but it looks smaller than my 27″ TV. Why?: Because the TV may have a different aspect ratio (often 16:9 at a slightly larger pixel pitch), or because the bezel is smaller, making the viewable area feel larger. Use this calculator to compare actual screen areas numerically.
Is this free?: Yes, entirely free with no signup required.

Frequently Asked Questions

Why is the diagonal measurement standard instead of width or height?: The diagonal is the longest dimension and a single number that uniquely identifies screen size given a fixed aspect ratio. It became industry convention decades ago and has persisted.
My monitor says 27″ but it looks smaller than my 27″ TV. Why?: The TV may have a different aspect ratio, or the bezel may be smaller, making the viewable area feel larger. Use this calculator to compare actual screen areas numerically.
What aspect ratios are supported?: 16:9 for most modern monitors and TVs, 4:3 for older displays, 21:9 for ultrawide, 16:10 for many laptops, and Custom for any ratio you enter.