RandJoy
Probability simulator
See how observed results compare with the chances you set.
Latest result
Ready when you are
See how observed results compare with the chances you set.How it works
- Choose a coin, a fair six-sided die or custom weighted outcomes. For a coin, set the chance of Heads.
- Choose the number of trials and generate. Compare each observed count with its expected count.
- Try another seed with the same settings. Export the trial sequence or the report for a lesson or worksheet.
Expected does not mean guaranteed
Expected counts multiply each outcome probability by the number of trials. A fair coin has an expected 50 Heads in 100 trials, but a particular experiment may produce more or fewer. Trials are independent in the specified model; a run of Tails does not increase the next chance of Heads. The chart compares observations with expectation rather than scoring a draw as fair or unfair.
Your inputs stay with you
Your inputs and generated results stay in this browser. Settings and five recent runs use local storage; clearing browser data removes them. Export a copy to keep it. Hosting may record standard page-request logs, but generated rows and participant names are not sent to the server.
Is this the same as the coin-flip tool?
The coin-flip tool makes a quick everyday choice with cryptographic randomness. This simulator uses a visible repeatable seed and records a complete experiment so you can compare models, explain variation and reproduce a classroom example.
Repeatable test data
The research tools use the versioned xoshiro128** pseudorandom generator with named seed streams. They are intended for small examples and testing, not secrets or certified draws. A recipe records parameters and generator version. Floating-point transforms are rounded to six decimal places; keep exported files when exact cross-platform bytes matter.
Seeds are repeatable. Use the password generator for credentials.