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).

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).
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). Making your details look like many other people's shrinks the effect. See how to reduce fingerprinting and Firefox's protection.

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