Odds Calculator
Convert between probability (%) and odds formats.
Calculate odds for/against, decimal odds, and "1 in N" from probability or event counts.
Probability vs Odds — what is the difference?
Probability is the chance something happens, expressed as a fraction of all outcomes.
P = favourable outcomes / total outcomes
Example: rolling a 6 on a die → P = 1/6 ≈ 16.67%
Odds express the ratio of favourable to unfavourable outcomes.
Odds for = favourable : unfavourable
Example: rolling a 6 → odds are 1:5 (one way to win, five ways to lose)
Converting between them:
- Probability → Odds for: P / (1 − P) e.g. 0.25 → 0.25/0.75 = 1:3
- Odds X:Y → Probability: X / (X + Y) e.g. 1:3 → 1/4 = 25%
Odds formats:
- Fractional (UK): 3/1 means you win 3 units per 1 staked
- Decimal (Europe): 4.0 means your total return per 1 staked (includes stake)
- Moneyline (US): +300 means win $300 on a $100 bet (underdog)
- “1 in N”: “1 in 4 chance” — the total number of outcomes needed for one success
Real-world examples:
- A fair coin: P = 50%, odds = 1:1 (evens), decimal = 2.0, 1 in 2
- Drawing an ace from a deck: P = 4/52 ≈ 7.69%, odds = 4:48 = 1:12
- Rolling any number ≤ 3 on a die: P = 50%, odds = 1:1
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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