Probability Calculator
Turn outcomes into probability. Enter how many outcomes count as success and how many are possible in total — you'll get the probability as a fraction, decimal and percentage, plus the odds and the chance of it not happening.
How to use this calculator
- Count the favorable outcomes — the ways your event can happen.
- Count the total possible outcomes — all equally likely results.
- Optionally set repeats to see the chance of success at least once over several tries.
- Press Calculate for the probability, its complement, and the odds.
Formula used
This is classical probability, valid when all outcomes are equally likely. Odds are a ratio of favorable to unfavorable outcomes (13:39 = 1:3), which is different from probability (13/52 = 25%). The at-least-once formula assumes independent tries.
Example calculation
Drawing a heart from a deck: 13 favorable / 52 total = 1/4 = 25%. Odds in favor: 13:39 = 1:3.
Drawing at least one heart in 4 draws (with replacement): 1 − 0.75⁴ = 1 − 0.3164 = 68.36% — not 100%, as intuition often suggests.
Probability vs odds — and a common trap
Probability is favorable outcomes divided by all outcomes, running from 0 (impossible) to 1 (certain). Odds compare favorable to unfavorable: a 25% probability is 1:3 odds, not 1:4 — the most common confusion between the two notations.
The other trap is repeated tries: a 25% chance tried four times is not 100%. Failures compound multiplicatively — the chance of missing every time is 0.75⁴ ≈ 32%, so success at least once is only about 68%. That formula, 1 − (1−p)ⁿ, is the honest answer to "if I keep trying, what are my chances?"
Why use this calculator?
- Convert between fraction, percentage and odds without mixing them up.
- See the at-least-once probability over repeated tries — where intuition fails most.
- Check game, lottery and dice reasoning with the exact classical formula.
Frequently asked questions
What is the difference between probability and odds?
Probability = favorable ÷ total (13/52 = 25%). Odds = favorable : unfavorable (13:39 = 1:3). Bookmakers and gamblers use odds; scientists use probability. Converting: odds of a:b means probability a/(a+b).
If something has a 1 in 10 chance, will it happen in 10 tries?
Not guaranteed — the probability of at least one success in 10 independent tries at p = 0.1 is 1 − 0.9¹⁰ ≈ 65%. There's a 35% chance of ten straight failures. Independent events never "owe" you a success.
How do I calculate the probability of two events both happening?
If they're independent, multiply: P(A and B) = P(A) × P(B). Two coin heads in a row: 0.5 × 0.5 = 25%. If the events affect each other (drawing cards without replacement), the second probability must be updated first.