Screen Size Calculator
The Number on the Box Is a Diagonal, Not a Width
"24-inch monitor" and "55-inch TV" both refer to the diagonal measurement across the screen, a convention that dates back to CRT tubes and has stuck ever since. That single number doesn't tell you the actual width and height, or how sharp the image will look, unless you also know the aspect ratio and resolution. This calculator moves between all three: diagonal size, pixel resolution, and aspect ratio, solving for whichever one you don't already have.
The Formula
Every mode reduces to the Pythagorean relationship between a screen's width, height, and diagonal:
PPI = Diagonal (px) / Diagonal (in)
Width = Diagonal × (RatioW / √(RatioW² + RatioH²))
Height = Diagonal × (RatioH / √(RatioW² + RatioH²))
Given resolution and pixel density, the diagonal in pixels divided by PPI gives the physical diagonal in inches. Given only a target diagonal and an aspect ratio, the ratio's own hypotenuse scales the diagonal down into a width and height.
Where This Calculation Matters
- Buying a display — comparing whether a 27-inch 1440p monitor will actually look sharper than a 32-inch 4K one at typical viewing distance.
- Furniture and desk planning — converting a diagonal spec into actual width and height before confirming a monitor or TV will physically fit.
- Video and photo output — working backward from a target print or display diagonal to the pixel dimensions needed at a given ratio.
- Multi-monitor setups — comparing the true footprint of displays with different aspect ratios but the same diagonal size.
Diagonal Size vs. Physical Width and Height (by ratio)
| Aspect Ratio | Diagonal | Width | Height |
|---|---|---|---|
| 4:3 | 24 in | 19.2 in | 14.4 in |
| 16:9 | 24 in | 20.92 in | 11.77 in |
| 16:10 | 24 in | 20.35 in | 12.72 in |
| 21:9 | 34 in | 31.25 in | 13.39 in |
A 16:9 and a 4:3 screen with the same diagonal have noticeably different widths — the diagonal alone never fully describes a screen's footprint.
How to Use This Calculator
- Choose a calculation type: Diagonal from Resolution & PPI, Diagonal from Width & Height, or Width/Height from Diagonal & Ratio.
- For the first mode, enter Width (px), Height (px), and Pixel Density (PPI).
- For the second mode, enter Width and Height in inches or centimeters.
- For the third mode, enter the Diagonal plus Ratio Width and Ratio Height (for example, 16 and 9).
- Select Calculate to get the result for the mode you chose.
Related Calculations
Once you know the resolution and diagonal, check sharpness with the Pixel Density Calculator, or simplify a custom width/height into a clean ratio with the Aspect Ratio Calculator.