Why Keep Tor Browser at Default Window Size

Tor Browser recommends keeping its default window size to protect users against browser fingerprinting, a technique used by websites to track individuals across the internet without relying on cookies. When you resize or maximize the Tor Browser window, you expose your monitor’s unique resolution and viewport dimensions. By keeping the browser at its default startup dimensions, your traffic appears identical to thousands of other Tor users, preserving your online anonymity.

Understanding Browser Fingerprinting

Browser fingerprinting collects specific configuration details about your device and software. When you visit a website, scripts can easily query several metrics, including:

• Your screen’s total resolution (width and height in pixels)
• The available screen space (excluding OS taskbars and docks)
• The exact viewport size of the browser window
• The device pixel ratio

While one metric alone might not identify you, combining your exact window dimensions with other hardware and software characteristics creates a unique “fingerprint.” Trackers can use this fingerprint to recognize and follow your browsing activity across different sessions and websites.

The Principle of Uniformity

The primary defense mechanism of the Tor network is uniformity. The goal is to make every Tor user look completely identical to external observers and websites.

If millions of people use the exact same browser version, user-agent string, and window dimensions, a website cannot distinguish User A from User B based on their browser profile. When you alter the window size—especially by maximizing it on a high-resolution or uncommon monitor—you separate yourself from the general pool of Tor users and create a distinctive profile.

Letterboxing as an Extra Layer of Defense

To mitigate the risk for users who accidentally or intentionally resize their windows, newer versions of Tor Browser include a feature called Letterboxing.

Letterboxing automatically adds margins (gray borders) around the webpage to snap the viewport dimensions to standard multiples of 200px by 100px. While Letterboxing significantly reduces the uniqueness of custom window sizes, maintaining the initial default window size remains the most effective way to blend into the largest possible crowd of Tor users.