About the Rock Paper Scissors
Play rock paper scissors against the computer whenever there is nobody around to throw hands with — or use it to settle a decision fairly when two people cannot agree. Choose rock, paper or scissors (or let the tool pick for you) and the computer throws back a move chosen by your browser’s secure random number generator. It cannot see your choice, has no pattern to exploit and never “reads” you, so every game is a genuine one-in-three chance to win, lose or tie.
Switch to Rock Paper Scissors Lizard Spock, the five-move version invented by Sam Kass and Karen Bryla and made famous by the TV series The Big Bang Theory. Each move beats two others and loses to two, so ties drop to one in five and decisive games become more common.
Set the number of rounds to play a best-of series or run a quick experiment: play rock 300 times in a row and you will see that against a random opponent no strategy wins more than a third of the time on average. The results show your win rate, the full round-by-round history and how your record compares with the expected odds.
How to use the rock paper scissors
- 1Choose the classic game or Rock Paper Scissors Lizard Spock.
- 2Pick your move, or let the tool choose a random one each round.
- 3Leave rounds at 1 for a single game, or raise it for a best-of series.
- 4Read the result and the computer’s throw.
- 5Change any input or the seed to play again.
Formula and method
The computer picks each move uniformly at random from the available moves, independently of your choice and of previous rounds. In the classic game each move beats exactly one other move and loses to one, so whatever you throw you have a 1/3 chance to win, lose or tie.
In Rock Paper Scissors Lizard Spock there are five moves and each one beats two and loses to two, so a round is won 2/5 of the time, lost 2/5 and tied 1/5. The expected number of wins over several rounds is simply rounds × P(win). Against a uniformly random opponent no strategy — always rock, cycling, or random — changes these odds.
- P(win)
- Probability of winning a single round
- E[wins]
- Expected (average) number of wins
Worked examples
Rock against a seeded computer
With the seed “friday lunch” the computer’s first throw is always paper, and paper covers rock, so this replay is a loss. On any fresh, unseeded game your chance of winning is exactly one in three.
Spock vaporizes rock
In the five-move game the seed “spock time” makes the computer throw rock, which Spock vaporizes. Spock also beats scissors and loses to paper and lizard, so it wins 2 of 5 match-ups: 40%, with a 20% chance of a tie.
Always throw rock for 300 rounds
Against a random opponent, sticking to rock wins one round in three on average, so 300 rounds produce about 300 × 1/3 = 100 wins, 100 losses and 100 ties. Your actual record will vary around those numbers from run to run.
Frequently asked questions
What beats what in rock paper scissors?+
Rock crushes scissors, scissors cuts paper and paper covers rock. If both players throw the same move it is a tie and you play again.
What are the rules of Rock Paper Scissors Lizard Spock?+
Scissors cuts paper, paper covers rock, rock crushes lizard, lizard poisons Spock, Spock smashes scissors, scissors decapitates lizard, lizard eats paper, paper disproves Spock, Spock vaporizes rock, and rock crushes scissors. Every move beats two and loses to two.
Is there a winning strategy for rock paper scissors?+
Not against a truly random opponent like this computer — every move wins one third of the time. Against people, patterns help: many beginners open with rock and tend to avoid repeating a move after losing, which a thoughtful player can exploit.
Can the computer see my move or cheat?+
No. The computer’s throw is drawn from a secure random number generator independently of your choice, so it cannot react to what you picked. Use a seed only if you want to replay a specific game.
What is the most common first move?+
Rock is widely reported as the most common opening throw among casual players, and scissors as the least common, though the figures vary between surveys. That is why “paper first” is popular advice against human opponents, though it does nothing against a random computer.
How do you play best of three?+
Set rounds to 3 and play your move, or play single rounds and keep score until someone has two wins. Ties do not count, so a best-of-three can take more than three throws in real life.