Calculate the probability of an event using P(A) = favorable outcomes / total outcomes — find P(A), P(not A), and odds.
Enter a known probability P(A) as a fraction (e.g. 3/10) or decimal (e.g. 0.3), and the calculator will find P(not A) = 1 − P(A).
P(A) = (# favorable outcomes) / (# total outcomes)
All outcomes must be equally likely for classical probability to apply — such as rolling a fair die, flipping a fair coin, or drawing a card from a well-shuffled deck.
The result is always a number between 0 and 1 inclusive:
• P(A) = 0 → the event is impossible
• P(A) = 0.5 → the event is equally likely as its complement
• P(A) = 1 → the event is certain
P(not A) = 1 − P(A) = (total − favorable) / total
Complement rule: The event A and its complement (not A) always sum to exactly 1, because together they cover every possible outcome.
Odds in favor = favorable : unfavorable
Odds against = unfavorable : favorable
Odds are a ratio, not a probability. "Odds of 1:5" means 1 favorable outcome for every 5 unfavorable — which is a probability of 1/6, not 1/5.
One-on-one Algebra 1 tutoring makes probability, outcomes, and complement click — so you can tackle any problem with confidence.