Probability calculator

And, or, not and at least once.

P(A)

0.5 (50%)

A and B are treated as independent events

P(not A)
0.5 (50%)
P(A and B)
0.15 (15%)
P(A or B)
0.65 (65%)
P(exactly one)
0.5 (50%)
P(neither)
0.35 (35%)
P(not B)
0.7 (70%)
P(A at least once in 3 tries)
0.875 (87.5%)
P(A every time in 3 tries)
0.125 (12.5%)

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How to combine probabilities

  1. Enter the probability of A as a decimal (0.3), a percent (30%) or a fraction (3/10).
  2. Optionally enter the probability of B, and a number of tries.
  3. Read P(A and B), P(A or B), the chance of neither, and the chance of A at least once.

Probability, odds and percent

A probability of 0.25 is a 25% chance, or 1 in 4. Odds are written differently: 1 to 3 against means 1 win for every 3 losses, which is the same 25% chance. To turn odds of a to b into a probability, divide a by a + b.

Common mistakes

  • Adding when you should multiply. Two 50% events don’t give 100%: both happening is 25%, either happening is 75%.
  • The gambler’s fallacy. After five heads in a row, the next fair flip is still 50/50.
  • Assuming independence. Weather on consecutive days, or cards drawn from one deck, affect each other.