# How unique is your browser? The arithmetic

> Why a few common browser details add up to a rare combination, with a calculator multiplying the share of browsers with each one. An illustration, not a test.

- Canonical URL: https://antitrackingsoftware.com/how-unique-is-your-browser/
- Source: [EFF, What Is Fingerprinting?](https://ssd.eff.org/module/what-fingerprinting)
- Checked: 2026-10-22

This page includes a free interactive tool that runs in the browser and sends no data. The figures and rules it uses are the ones stated on this page, with their sources.

## In short
- **Each detail narrows the crowd.** If 20% of browsers share your screen size and 10% share your time zone, only 2% share both (if the two are unrelated).
- **Details multiply.** Five common details can leave a very small group.
- **The EFF describes the same idea:** sites combine small details, such as screen resolution, time zone and operating system version, into a profile ([EFF](https://ssd.eff.org/module/what-fingerprinting)).

## Worked example
With the calculator's starting numbers (20%, 10%, 8%, 15% and 5%), the share that matches all five is 0.0012%, about 1 in 83,000 browsers. In a group of one million visitors, that is about 12 browsers like yours. The group shrinks quickly as you add details.

## Bits
Information is counted in bits: each halving of the crowd is one bit. A 1-in-83,000 mix is about 16 bits. The calculator shows this number so you can compare mixes.

## Why the real number differs
- **The details are not independent.** A language list and a time zone often go together, so multiplying overstates how rare you are.
- **Your numbers are guesses.** The calculator does not know the real shares. The EFF's Cover Your Tracks runs a real test ([EFF](https://coveryourtracks.eff.org/)).
- **Sites use more than five details.** The EFF page lists others.

## What helps
A fingerprint is not stored on your device, so you cannot delete it ([EFF](https://ssd.eff.org/module/what-fingerprinting)). Making your details look like many other people's shrinks the effect. See [how to reduce fingerprinting](https://antitrackingsoftware.com/how-to-reduce-fingerprinting/) and [Firefox's protection](https://antitrackingsoftware.com/firefox-fingerprinting-protection/).

*Outside this page:* We do not give real percentages for any detail. The ones in the calculator are examples that we chose.

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