Odds to Probability Converter
By Santaji GadepatilinUpdated October 27, 2026
Odds and probability express the same chance in different ways. Betting and medical studies often use odds, while everyday statements use probability. This free odds to probability converter converts instantly in both directions and includes a quick reference table.
How to use the odds to probability converter
Enter the odds as two numbers — for and against (1:4 means 1 for, 4 against). The probability and decimal odds are calculated. Or type a probability to get the odds.
Odds to probability formulas
p = a ÷ (a + b) decimal odds = 1 ÷ p
Examples
| Odds (for:against) | Probability | Decimal |
|---|---|---|
| 1:1 | 50% | 2.00 |
| 1:4 | 20% | 5.00 |
| 3:1 | 75% | 1.33 |
| 1:99 | 1% | 100 |
Odds, probability and decimal odds
| Fractional odds (against) | Probability | Decimal odds |
|---|---|---|
| 1/4 | 80.0% | 1.25 |
| 1/2 | 66.7% | 1.50 |
| 4/5 | 55.6% | 1.80 |
| 1/1 | 50.0% | 2.00 |
| 6/4 | 40.0% | 2.50 |
| 2/1 | 33.3% | 3.00 |
| 3/1 | 25.0% | 4.00 |
| 5/1 | 16.7% | 6.00 |
| 10/1 | 9.1% | 11.00 |
| 100/1 | 1.0% | 101.00 |
American (moneyline) odds
| American | Meaning | Probability |
|---|---|---|
| +200 | Win 200 on a 100 stake | 33.3% |
| +100 | Even money | 50% |
| −150 | Stake 150 to win 100 | 60% |
| −300 | Stake 300 to win 100 | 75% |
Positive: p = 100 ÷ (odds + 100). Negative: p = −odds ÷ (−odds + 100).
The bookmaker’s margin
Add up the implied probabilities for all outcomes in a market. If they total more than 100% — for example 52.4% + 52.4% = 104.8% — the extra 4.8% is the bookmaker’s margin (overround). Fair odds would add to exactly 100%.
Odds in medicine and research
In studies, odds compare people with and without an outcome, and the odds ratio compares two groups. Odds and probability are similar only when the outcome is rare. See the relative risk calculator.
Odds against
Fractional betting odds such as “4/1” usually mean 4 against 1 for — a 20% implied chance.
Frequently asked questions
How do I convert odds to probability?
For odds of a to b in favour, probability = a ÷ (a + b). Odds of 1:4 mean a 20% chance.
What are decimal odds?
The total return per unit staked, including the stake. Decimal odds of 5.0 imply a 20% chance.
Are bookmaker odds true probabilities?
No. Bookmakers build in a margin, so the implied probabilities of all outcomes add up to more than 100%.
What does 3 to 1 odds mean?
Odds of 3 to 1 against mean 1 win for every 3 losses — a 25% probability.
How do I convert probability to odds?
Odds in favour = p ÷ (1 − p). A 20% probability is 0.25, or 1 to 4.