Screen Size Calculator - Width, Height & PPI From Diagonal

About the Screen Size Calculator

Monitors, televisions, laptops and phones are all sold by a single number: the length of the screen's diagonal in inches. That tells you little on its own about how wide or tall the screen actually is, whether it will fit on a desk or in a cabinet, or how sharp the picture will look. Two screens with the same diagonal can have quite different shapes, and the same size can look crisp or grainy depending on the resolution packed into it.

This screen size calculator turns the diagonal and aspect ratio into the real width and height of the visible screen, in inches and centimetres. It gives the screen area, and when you add the resolution it works out the pixel density in pixels per inch (PPI), the size of each pixel and a note on how sharp the screen will look at a normal distance.

How to Use the Screen Size Calculator

Enter the diagonal in inches, as advertised.

Enter the aspect ratio as a width and a height, such as 16 and 9 for most monitors and televisions, 16 and 10 for many laptops, 21 and 9 for ultrawide monitors, or 19.5 and 9 for many phones.

Optionally, enter the resolution in pixels, such as 2560 × 1440, to see the pixel density.

How Screen Size Is Worked Out

The diagonal, width and height form a right-angled triangle, so Pythagoras' theorem links them.

  ratio diagonal  =  √(ratio width² + ratio height²)
  width           =  diagonal × ratio width ÷ ratio diagonal
  height          =  diagonal × ratio height ÷ ratio diagonal
  PPI             =  √(pixel width² + pixel height²) ÷ diagonal

For 16:9 the ratio diagonal is √(256 + 81), about 18.36, so the width is the diagonal × 0.8716 and the height is the diagonal × 0.4903.

Step-by-Step Example

A 27-inch 16:9 monitor at 2560 × 1440.

  Width:     27 × 16 ÷ 18.36 = 23.5 in   (59.8 cm)
  Height:    27 × 9 ÷ 18.36 = 13.2 in    (33.6 cm)
  Area:      312 sq in
  Diagonal pixels:  √(2560² + 1440²) = 2,937
  PPI:       2,937 ÷ 27 = 109 pixels per inch

The screen is 23.5 × 13.2 inches, and at 109 PPI each pixel is about 0.23 mm across — sharp at a normal desk distance.

A 24-inch 1080p monitor, 1920 × 1080, is 20.9 × 11.8 inches with 91.8 PPI, so text looks noticeably less crisp than on the 27-inch 1440p screen. A 6.1-inch phone at 2532 × 1170 packs in about 457 PPI, far beyond what the eye can resolve at arm's length.

Why Aspect Ratio Matters

Because the diagonal is measured corner to corner, wider shapes get more of it as width and less as height. A 34-inch 21:9 ultrawide is 31.3 × 13.4 inches — about the same height as a 27-inch 16:9 monitor, but nearly 8 inches wider. When comparing screens of different shapes, compare the width, height or area rather than the diagonal.

For the same diagonal, a squarer screen has more area. A 16:10 laptop screen is a little taller than a 16:9 one of the same size, which many people prefer for documents and web pages.

Pixel Density and Viewing Distance

How sharp a screen looks depends on its PPI and how far away you sit. At a desk, about 60 to 70 cm away, 90 to 110 PPI looks sharp with standard scaling, and 140 or more lets text look very crisp with display scaling turned on. Laptops, closer to the eyes, usually have 130 to 220 PPI. Phones, held 25 to 30 cm away, typically have 400 to 500.

Televisions are watched from much further away, so a lower PPI is fine. A 65-inch 4K television, 3840 × 2160, has about 67.8 PPI, which would look grainy on a desk but is seamless from a sofa two or three metres away.

Planning Space for a Screen

The calculator gives the size of the visible picture only. The frame, or bezel, adds a little on each side — from a few millimetres on modern monitors to a couple of centimetres on some televisions — and a stand adds depth and height. For a wall or cabinet, allow an extra inch or so all round and check the maker's specifications for the exact outer size.

Resolution Names

Screens are often described by a short name rather than their pixel count. Full HD or 1080p is 1920 × 1080, QHD or 1440p is 2560 × 1440, and 4K or UHD is 3840 × 2160, all 16:9. Ultrawide monitors commonly use 3440 × 1440 at 21:9. The name tells you the number of pixels, but not the size they are spread over, which is why the same 4K resolution gives about 163 PPI on a 27-inch monitor and only about 68 PPI on a 65-inch television. Enter both numbers to see how sharp a particular screen will be.

Understanding Your Result

The headline is the visible width × height in inches.

The centimetre line gives the width, height and diagonal in metric units.

The area is width × height, useful for comparing screens of different shapes.

The pixel density gives the PPI and the size of each pixel.

The sharpness note describes how the screen will look at its usual viewing distance.

When Should You Use This Calculator?

Use it when choosing a monitor or television, to see how big it really is and whether it will fit the space.

Use it to compare screens of different shapes, such as 16:9 and ultrawide monitors.

Use it to check how sharp a screen will be before buying.

Use it when designing for a device, to understand how large content will appear.

Common Mistakes

Comparing diagonals across shapes. A larger diagonal on a wider screen does not always mean a taller picture.

Forgetting the bezel. The diagonal covers only the visible area, not the frame.

Judging sharpness by resolution alone. 4K on a 65-inch television is less dense than 1440p on a 27-inch monitor; divide by the size.

Using the wrong ratio. Many laptops are 16:10 or 3:2, and phones vary widely.

Ignoring viewing distance. A lower PPI is fine for screens watched from further away.