CalcVind

Probability Calculator

Combine the chances of two independent events

Your Events

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Enter probabilities as percentages: a 1 in 4 chance is 25%.

Probabilities

0.2 (20%)A and B
0.7 (70%)A or B (or both)
0.5 (50%)
Exactly one
0.3 (30%)
Neither
0.5 (50%)
Not A
0.6 (60%)
Not B
0.3 (30%)
A but not B

Working

  1. P(A and B) = P(A) × P(B) = 0.5 × 0.4 = 0.2
  2. P(A or B) = P(A) + P(B) − P(A and B) = 0.7
  3. P(neither) = 1 − P(A or B) = 0.3
  1. 1
    Choose a calculation
    Two events, or repeated tries
  2. 2
    Enter the chances
    As percentages
  3. 3
    See every combination
    And, or, neither, at least once

Probability rules

For independent events (one doesn't affect the other):

P(A and B) = P(A) × P(B)

P(A or B) = P(A) + P(B) − P(A and B)

P(not A) = 1 − P(A)

P(at least once in n tries) = 1 − (1 − p)ⁿ

Independent events

Two coin tosses, two dice, or rain in two different cities are independent: knowing one result tells you nothing about the other. These formulas don't apply when events affect each other, like drawing two cards without putting the first back.

At least once

A 1 in 6 chance (16.7%) feels small, but over 10 tries the chance of it happening at least once is 1 − (5/6)¹⁰ ≈ 84%.

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Frequently asked questions

What is the probability of getting two heads in a row?

½ × ½ = ¼, or 25%. Each toss is independent, so you multiply the chances.

How do I convert odds to a probability?

Odds of 1 to 4 (1:4) mean 1 success for every 4 failures, so the probability is 1 ÷ (1 + 4) = 20%.

What does a probability of 0 or 1 mean?

0 (0%) means impossible; 1 (100%) means certain. Every other probability lies between them.

Last reviewed: October 2026