PPI calculator — pixel density of any screen from resolution and size

How to use the PPI calculator

1. Enter the Horizontal resolution in pixels, for example 2560.
2. Enter the Vertical resolution in pixels, for example 1440.
3. Enter the Diagonal size in inches, as sold (27 for a 27-inch monitor).
4. Read Pixel density, Dot pitch, Total pixels, Physical size and Aspect ratio.

What it does and when to use it

Two monitors can share a resolution yet look very different. A 1920 × 1080 picture on a 24-inch screen is noticeably sharper than on a 32-inch one, because the same pixels are spread over a larger area. Pixel density captures that in one number. Use it to:

• compare monitors, laptops, tablets or phones before buying,
• decide whether a bigger screen at the same resolution will look soft,
• plan interface or icon sizes for a particular display,
• check the real width and height of a screen from its diagonal.

The aspect ratio calculator breaks down the screen’s shape in more detail and scales sizes to match. For print work, where DPI really matters, the DPI checker reports an image’s print resolution. If you are choosing a display for a desk, the electricity cost calculator can estimate its running cost from its wattage.

How it works

The diagonal in pixels comes from Pythagoras’ theorem:

diagonal in pixels = √(width² + height²)

Then:

• PPI = diagonal in pixels ÷ diagonal in inches
• Dot pitch (mm) = 25.4 ÷ PPI
• Total pixels = width × height
• Physical width = width ÷ diagonal in pixels × diagonal in inches (and the same for height), also shown in centimetres

The aspect ratio is the resolution reduced to its simplest form. The calculation assumes square pixels and that the diagonal you enter is the visible screen area.

Worked examples

These figures come from the calculator’s own function.

A 24-inch Full HD monitor. 1920 × 1080 at 24 inches gives 91.79 PPI, a dot pitch of 0.2767 mm and 2,073,600 pixels. The screen is 20.92 × 11.77 in (53.1 × 29.9 cm).

A 27-inch 1440p monitor. 2560 × 1440 at 27 inches gives 108.79 PPI and a dot pitch of 0.2335 mm. Physical size 23.53 × 13.24 in (59.8 × 33.6 cm).

A 27-inch 4K monitor. 3840 × 2160 at 27 inches gives 163.18 PPI and a dot pitch of 0.1557 mm, with 8,294,400 pixels. The physical size is the same as the 1440p model, but each inch holds half as many pixels again.

Limits and tips

• Advertised sizes are rounded. A “27-inch” screen may be slightly smaller in visible area, so treat the second decimal with care.
• Curved screens use the diagonal along the curve, which changes the result very little.
• Not for printers. Printer DPI counts ink dots, which is a different measurement.
• Calculated entirely in your browser.