How is screen diagonal calculated from width and height?

The diagonal is the hypotenuse of a right triangle formed by the width and height, so it equals the square root of (width squared plus height squared), per the Pythagorean theorem.

How do I get width and height from a diagonal and aspect ratio?

Divide the diagonal by the square root of (ratio_w squared plus ratio_h squared) to get a scale factor, then multiply that factor by ratio_w for the width and by ratio_h for the height.

Is the advertised screen size always the exact diagonal?

Manufacturers usually round to the nearest common size (24", 27", 32"), and some measurements include the bezel, so a measured diagonal can be a few millimeters off from the labeled size.

What is a typical aspect ratio for monitors and TVs?

Most modern monitors and TVs use 16:9. Ultrawide monitors commonly use 21:9, older displays used 4:3, and some ultrawide gaming monitors use 32:9.

Does this tool work in centimeters?

Yes, switch the unit toggle to centimeters. The math is identical since the diagonal is a ratio of the same length unit as the width and height.

Screen Diagonal Calculator

Name: Screen Diagonal Calculator
Availability: InStock
Author: Nham Vu

Enter a screen width and height to get the diagonal size and aspect ratio, or enter a diagonal and aspect ratio to get the width and height.

Unit

Width Height

Enter values on the left to see the result

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Summary

Enter a screen width and height to get the diagonal size and aspect ratio, or enter a diagonal and aspect ratio to get the width and height.

How it works

Choose a mode: calculate the diagonal from width and height, or calculate width and height from a diagonal and aspect ratio.
Pick a unit, inches or centimeters.
Enter the known values.
Read the diagonal, the simplified aspect ratio, and the area, updated as you type.

Use cases

Find a monitor or TV diagonal size from its physical width and height.
Check whether a screen's advertised diagonal matches its measured dimensions.
Work out the width and height a panel would have at a given diagonal and aspect ratio.
Compare screen area between two displays with different aspect ratios.
Convert a diagonal measurement between inches and centimeters.
Verify the aspect ratio label (16:9, 21:9, 4:3) implied by a set of dimensions.

Frequently Asked Questions

Last updated: 2026-07-21 · Reviewed by Nham Vu