Playing heads or tails over the internet
Invite a friend to play and you hold one face of the coin; whoever opens your link holds the other. When you’re both ready the coin is tossed once, and both screens show the same spin and the same landing within about a tenth of a second of each other.
Just want a quick answer for yourself? Open the page and tap the coin. Same coin, no second player needed.
Is an online coin toss really 50/50?
A real coin is very slightly biased toward the side that started facing up, and a practiced thumb can do worse than that. Here the face is read from the first hex digit of an HMAC-SHA256 digest: sixteen equally likely digits, eight for heads and eight for tails.
You don’t have to take our word for it. Before the toss the room shows a fingerprint of the server’s secret seed, each player’s browser adds a seed of its own, and after the match the secret is revealed. Anyone can recompute the digest and confirm the coin landed the way the math says it had to.
When heads or tails is the right tool
Who serves, who picks the map, who pays for pizza. Anything with exactly two outcomes and two people who’d rather not argue about it.
If the two outcomes aren’t really “heads” and “tails”, rename the sides before you flip, and the result shows your own words instead.
Questions
Who gets heads and who gets tails?
Whoever sends the invite picks Heads or Tails, or leaves it to chance and the sides are drawn when a friend joins. Want the other face? Ask for a swap before round 1. It goes through only if your opponent accepts.
Do both of us see the same result?
Yes. It’s a single toss decided by the server and pushed to every device, not a separate random flip on each screen.
Is it free?
Completely. No signup, no ads, no limit on how many rooms you open.