What is it?
The Probability Calculator works out the chance of two things happening together, either one happening, or neither. Enter the chance of A and the chance of B, and it also shows the chance over many tries.
Who is it for?
Students, teachers, game players and anyone weighing chances of independent events.
How to use it
Enter chances as % (50 for a coin). Use Repeat A this many times for the chance of A happening at least once in many tries. The answer appears as you type; there is no Calculate button. Tap Copy result to copy it, or Share on WhatsApp to send it.
Worked examples
- Two coin flips both heads: 50% and 50% → 25%.
- Rolling at least one six in 4 dice rolls: chance of a six 16.67%, repeated 4 times → about 51.8%.
- Rain on either of two days, with 30% and 40%: 58%, not 70%.
Independent events only
The formulas assume A doesn’t change B’s chance, like separate coin flips. Drawing cards without putting them back, or the weather on two following days, are dependent: the answer will be off.
Common traps
- Past results don’t change independent future ones: after five heads, the next flip is still 50%.
- Lottery and gambling odds are designed so the house wins over time.
Errors and how to fix them
- “Chances must be between 0 and 1 (or 0% and 100%)”: type 30 or 0.3, not 300.
Open the tool: Probability Calculator
Still stuck?
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Questions, or an error not listed here?
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