Online Ruler Tools
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What Is My Screen Size?

Your resolution, pixel density, aspect ratio and physical screen size, all detected in your browser. Nothing is uploaded.

Your screen

Screen resolution

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Physical screen size

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Not calibrated

Detected display properties
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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.

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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.

Frequently Asked Questions

What is my screen resolution?
The card above reads it from your browser. Two figures matter. The CSS pixel resolution is what web pages are laid out in. The native resolution counts real device pixels. On a scaled display, which covers most laptops and every modern phone, the native number is larger, because several device pixels go into drawing each CSS pixel.
How do I find my screen size in inches?
A browser can read your pixel dimensions but not your monitor's physical size, so no website can honestly give you a diagonal unaided. Calibrate once with a bank card and this page works it out, dividing the pixel dimensions by the measured pixel density to get true width, height and diagonal.
What is device pixel ratio?
The number of device pixels used for each CSS pixel. A ratio of 1 means they match. A typical phone runs 2 to 3, and a Retina Mac is 2. Zooming the page changes it too, which is why this tool re-reads the value whenever the window changes.
Why is my resolution reported lower than my monitor's?
Display scaling. If Windows is set to 150%, or macOS is using a scaled resolution, the browser reports the smaller logical size. A 3840×2160 monitor at 150% comes back as 2560×1440 CSS pixels. The native row above multiplies it out again using the device pixel ratio.
What is PPI or DPI, and how is it measured here?
Pixels per inch describes how tightly your display packs its pixels. Software cannot detect it, so the figure here comes from your calibration: you match a bank card of known width, 85.6 mm, against the screen, and the pixel width of that match gives the density.
Does this work on a phone or tablet?
Yes, and the readings tend to be more interesting there, because phones run very high pixel ratios. Rotate the device and the values update on their own.
Is any of this information sent anywhere?
No. Every value is read in your browser and stays there. No account, no logging, no upload, and your calibration is stored only on your own device.

Calibrate your screen

Hold a real object against the screen and drag the slider until the outline matches it exactly. The result is stored on your device only, and you do it once.

Screen density: 96 DPI

No card to hand? Estimate from your screen size

Tell us your screen's advertised size, meaning the number printed on the box, measured corner to corner. From that and the resolution your browser reports we can work out the density. It handles display scaling and external monitors correctly, which guessing your device from a list does not. It is still an estimate, because an advertised diagonal is a nominal figure, so matching a card stays the only route to a verified reading.

Enter or pick a size to see the estimate.