Coin Flip — Heads or Tails

When you press Flip the coin, the page asks your browser's built-in cryptographic random generator (crypto.getRandomValues) for one 32-bit number. An even number is heads and an odd number is tails. Of the 4,294,967,296 possible values, exactly half are even, so the split is precisely 50/50, not just close to it.

The result is fixed the moment you click. For about 0.7 seconds the panel flickers between HEADS and TAILS and the button is disabled, but the flicker is decoration. It can't change the outcome, and clicking at a particular instant gives you no edge.

No flip looks at earlier results. After five tails in a row, the next flip is still 50/50. The feeling that heads is due is the gambler's fallacy, and an independent random source is exactly where it doesn't hold.

Streaks are also more common than most people expect. Over 10 flips:

• a run of at least three identical results somewhere in the sequence has an 82.62% chance;
• a run of at least four has a 46.48% chance;
• an exact five-and-five split happens only 24.61% of the time (252 of the 1,024 equally likely sequences).

Over 100 flips, the number of heads lands between 40 and 60 about 96.48% of the time, yet exactly 50 heads comes up only 7.96% of the time. A streak in your history is normal, not a sign that something is off.

The large panel shows the latest result. Once you have flipped at least twice, a History line appears under the button with earlier flips, newest first. The tool keeps your last 12 flips, so you see the current one plus up to 11 before it.

The history exists only in the open tab. Reload or close the page and it starts empty, and none of it is sent anywhere. If you are running a probability exercise in class, tally the results on paper as you go.

• Make the call before the flip. Agree out loud who has heads before anyone presses the button. Deciding afterwards invites an argument.
• Best of three is no fairer than one flip. Each side still wins the series exactly half the time. It only takes longer, which can be the point if you want a bit of drama.
• Three people can't share one flip evenly. Flip twice instead: give HH, HT and TH to the three people, and flip both again on TT. Each person then has exactly a one-in-three chance.