Coin Flip: heads or tails, and how to use it well

How the coin flip works

Open the tool, pick the Coin tab and press the flip button. A coin spins on screen and lands on one side. You can also switch to the Dice tab and roll one, two or three six-sided dice instead.

The two sides do not have to be heads and tails. Each side has a name box, so you can type your own options, such as Pizza and Sushi, or Me and You. The result then shows the winning name. Names can be up to twelve characters, and clearing a box brings back the default.

• Choose Coin or Dice.
• Optionally rename the two sides of the coin.
• Press the button once and accept the result.
• Flip again only for a new question, not to get a better answer.

Why the result is fair

The outcome is drawn first, using your browser's cryptographic random number generator, the same kind of source used for security features. A technique called rejection sampling throws away any random value that would make one outcome slightly more likely, so each side of the coin has exactly the same chance, and so does every face of a die.

The spinning animation only reveals a result that already exists. It cannot change it. Nothing is sent to a server for the toss, your side names are not stored, and the running tally on the result screen simply counts what you have thrown since opening the page. Reloading resets it.

When a coin flip is the right tool

A toss works best when the options are roughly equal and the real cost is the time spent arguing. It removes the awkwardness of one person choosing for everyone.

• Deciding who goes first in a board game or card game.
• Choosing between two restaurants when nobody has a preference.
• Splitting a small chore fairly between housemates or siblings.
• Picking sides for a casual sports match or a classroom activity.
• Breaking a tie in a club vote or a group chat poll.

A short history of tossing coins

Settling matters by chance is very old. Romans are said to have played a game of heads or ships, named after the emperor's head on one side of a coin and a ship on the other. Today a coin toss still opens many football and cricket matches to decide who kicks off or who bats first.

Real coins are not perfectly even. A well-known study by the mathematician Persi Diaconis and colleagues found that a physical coin tends to land on the same face it started on slightly more than half the time, about 51 percent. That is far too small to notice in a few flips, and a digital draw does not have this bias at all.

Dice, and tips for deciding with friends

With two dice, a total of seven is the most common result, because six of the thirty-six combinations produce it. A total of two or twelve needs one exact combination each. Three dice make totals of ten and eleven the most frequent. This is why games that add dice give the middle numbers more weight.

A few habits make tossing more useful. Agree on what each side means before you flip, never best-of-three your way to the answer you wanted, and pay attention to your own reaction. If you feel a pang of disappointment when the coin lands, that tells you what you actually prefer. Share the result with friends through the share buttons, and keep the toss for low-stakes choices, not for decisions about health, money or safety.