Screen and viewport fingerprinting
A website can measure the exact shape of your window.
Screen size, visible page area, zoom, orientation, and touch support can reveal the kind of device in use. A rare—or repeatedly identical—window can become a strong recognition signal after a single page load—and one layer of alarger browser fingerprint.
One page load is enough
The screen and the page are different sizes.
The screen describes the display environment. The viewport describes the part of the page available inside the browser. Comparing the two exposes scaling, browser chrome, window state, and the way a person tends to arrange their workspace.
Resize this window and watch the viewport values change. The full screen usually stays the same while the page-visible rectangle moves through a much larger range of possibilities.
screen.width × screen.height—window.innerWidth × window.innerHeight—window.outerWidth × window.outerHeight—window.devicePixelRatio—The difference between the outer window and page viewport can reflect tabs, toolbars, borders, and window state.
From dimensions to device class
Phones and tablets often expose a recognizable geometry.
Mobile screens are sold in a limited set of physical sizes and resolutions. Browsers then expose CSS dimensions, pixel density, orientation, and touch capability. Matching that combination against known hardware can suggest a model family—even when no value names the device.
390 × 844
- Density
- 3×
- Touch
- 5 points
This combination resembles a family of modern 6.1-inch phones. It does not prove one exact model. Size varies slightly but consistently by specific model.
820 × 1180
- Density
- 2×
- Touch
- 5 points
A large portrait viewport, high density, and touch support can narrow the device class quickly.
1536 × 864
- Density
- 1.25×
- Touch
- 0 points
Screen scaling, browser chrome, and a repeated window size can distinguish one setup from similar ones.
Human error
A damaged screen can create a repeatable window habit.
A website cannot see a physical crack in the glass. But if someone consistently sizes a browser to avoid that crack, the resulting viewport may recur at the same unusual dimensions. The damage becomes visible indirectly through behavior.
That repeated rectangle can survive cookie clearing, private browsing, and IP changes. Combined with other signals, it can help an observer recognize the same browser again.
- Monday1217 × 731same shape
- Thursday1217 × 731same shape
- Next month1217 × 731same shape
Why privacy browsers add empty space
Letterboxing trades exact dimensions for a shared bucket.
Instead of reporting every possible window size, a browser can constrain the page to a smaller standardized rectangle. Many different outer windows then expose the same content size.
Move either control. The exact window changes continuously; the page-visible bucket changes only after a threshold.
Each slightly different rectangle can separate one browser from another.
Browsers in the same bucket become harder to distinguish by viewport alone.
What changes the measurement?
Display changes matter. Storage and network resets usually do not.
| Action | What changes | Recognition effect |
|---|---|---|
| Resize the browser window | Changes the viewport immediately | Can create a new size or repeat a familiar one |
| Show or hide browser UI | Bookmarks bars, sidebars, and other browser chrome change the viewport | A repeated UI layout can become part of the recognizable pattern |
| Rotate a phone or tablet | Swaps the viewport orientation | Still describes the same device family |
| Change page zoom or display scaling | Can change CSS dimensions or pixel ratio | May alter several related measurements |
| Connect a different display | Changes screen, available area, or density | Can produce a different display profile |
| Open a private window | Usually preserves display characteristics | Does not create an anonymity set |
| Clear cookies or change VPN | Does not change the local display | The measurement can be reconstructed |
Meaningful defenses
The goal is to make your screen size less unique.
Changing one value at random can make a browser more unusual. Strong defenses coordinate the display signal with the rest of the browser profile.
Standardize
Constrain content to a limited set of viewport sizes so many users report the same geometry.
Tor Browser · Mullvad BrowserReduce precision
Round, clamp, or otherwise limit dimensions and related hardware values exposed to scripts.
Useful when applied consistentlyCoordinate defenses
Align screen behavior with fonts, graphics, locale, hardware hints, and other high-entropy surfaces.
A system property—not one toggle