Probability Calculator
The basic calculation, and what happens to the chance when you try repeatedly.
Probability
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Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
As a decimal —
Probability of not happening —
Odds in favour —
At least once over the tries —
Never over the tries —
Expected number of times —
Reading this —
The formula
P = outcomes you want ÷ all outcomes P(at least once in n) = 1 − (1 − P)^n
The chance of never happening shrinks with each try, so it is easier to work out “at least once” from that.
What to watch out for
Repeating a try does not simply multiply the chance. Ten tries at 1 in 6 do not give 10 in 6.
Frequently Asked Questions
What is the chance of rolling a six in ten tries?
83.85%.
What is the probability of two events both happening?
That depends on how they are related. The two-event probability calculator covers it.