What Is My Screen Size?

Reading your display's real numbers

A screen resolution checker ought to answer more than one question, because "how big is my screen" has several different answers depending on why you are asking. The figure most people mean is the pixel grid the operating system reports: 1920×1080, 2560×1440, and so on. A browser sees that through a second layer called CSS pixels. A CSS pixel is a layout unit, and on any scaled display it stops mapping one to one onto hardware. That is why this page reports both, along with the device pixel ratio that ties them together.

The distinction has practical consequences:

• Checking whether a wallpaper will look sharp? You want the native device-pixel figure.
• Testing a web page or writing a media query? You want the CSS pixel figure.
• Buying a monitor? You want the physical diagonal in inches.

Three different numbers, and most tools hand you only one of them.

Why physical size needs calibration

Here is the limitation that most "what is my screen size" pages quietly skip past. The browser has no access to your display's physical dimensions. It knows how many pixels there are. It does not know how large they are. Any site that shows you a diagonal in inches without asking you a single question is guessing, usually by matching your resolution against a table of common devices, and that fails the moment you use display scaling, an external monitor or an unusual panel.

This tool takes the other road. Calibrate once by holding a bank card against the screen and matching an on-screen outline to it. Every payment card worldwide is made to the same ISO/IEC 7810 ID-1 standard at 85.6 mm wide, so that one match reveals your display's true pixel density. Physical width, height and diagonal are then simple arithmetic, and the answer belongs to the screen in front of you rather than to a lookup table. The calibration is shared with the online ruler, so it is a one-off per device.

Aspect ratio, colour depth and scaling

The aspect ratio row reduces your resolution to its simplest whole-number form and, where that sits close to a standard panel ratio, names it as well. This is more useful than it sounds, because plenty of common resolutions do not reduce neatly. A 1366×768 laptop panel is exactly 683:384, which is near 16:9 without being it. Colour depth tells you how many bits per pixel the display pipeline carries, usually 24-bit or 30-bit.

One thing to watch: browser zoom changes the device pixel ratio, and with it several values on this page. If a number looks wrong, reset zoom to 100% with Ctrl+0 or Cmd+0 and it will settle. Dragging the window to a second monitor of different density changes the readings too, and invalidates a calibration made on the first screen. Everything updates live as you move or resize, so you can watch it happen.

Screen, window and viewport are three different things

They get used interchangeably and they are not the same, which is why two tools can disagree about your screen and both be right.

• Screen is the whole panel, whether or not a browser is open. It is what your operating system means by resolution.
• Window is the browser itself, including its tab bar, address bar and any borders the system draws round it.
• Viewport is the area a page actually gets to paint in, so it shrinks when you open the developer tools or a sidebar, and it is the number a CSS media query tests.

A responsive layout that misbehaves at a particular width is nearly always a viewport question rather than a screen one. This page reports all three, which makes the difference easy to see: drag the window narrower and watch which figures move.

Finding your screen size without any tool at all

If you only want the diagonal in inches, three routes are quicker than measuring:

• The model number. It is printed on a label on the back of a monitor, or under the foot of a laptop, and the size is usually in the name. Dell's U2723QE is a 27-inch panel; the 23 that follows is not a second size.
• System settings. On Windows, Settings then System then Display then Advanced display names the panel. On macOS, hold Option and choose System Information from the Apple menu, then Graphics/Displays.
• A tape measure. Corner to corner across the glass, diagonally, ignoring the bezel. Screens have always been sold on the diagonal, which is why a 27-inch monitor is only about 60 cm wide.

Any of those three is enough to fill in the estimate on this page, and none of them is as good as matching a bank card, because a quoted diagonal is a nominal marketing figure and takes no account of the scaling your operating system is applying.

What the density figure is actually telling you

Pixels per inch decides how much detail fits into a given area of glass, and there is a band where it stops mattering. Below roughly 90 PPI at desk distance, individual pixels become visible on text edges. Between about 90 and 140 the pixels disappear for most people, which is where a 24-inch 1080p or a 27-inch 1440p panel sits. Above about 200, as on phones and high-density laptops, the extra resolution goes into smoothness you can no longer resolve, which is why those screens scale their interface up rather than showing you more of it.

This is also why the same 4K panel is a different proposition at 27 inches and at 43 inches. The pixel count is identical; the density is not, and density is what your eye reacts to. Viewing distance counts for as much as size, which is why a television across a room can get away with a figure that would look coarse on a desk.

For the maths behind PPI, worked through with a formula and a comparison table, see pixel density explained. For what 1080p, 1440p and 4K mean beyond the marketing names, see screen resolution explained.