Odds are just a probability with the margin added
Every set of odds carries an implied probability. Decimal odds are the cleanest to read: the probability is one divided by the decimal price. American and fractional odds are the same idea in a different skin.
Converting each odds format
Get everything to a decimal price first, then apply the one formula above. These are the exact conversions.
- Decimal. Already there. 2.50 → 1 ÷ 2.50 = 40%.
- American, negative (favorite). decimal = 100 ÷ |line| + 1. So -110 → 1.909 → 52.4%.
- American, positive (underdog). decimal = line ÷ 100 + 1. So +200 → 3.00 → 33.3%.
- Fractional. decimal = numerator ÷ denominator + 1. So 6/5 → 2.20 → 45.5%.
Common odds and what they imply
| Decimal | American | Fractional | Implied probability |
|---|---|---|---|
| 1.50 | -200 | 1/2 | 66.7% |
| 1.91 | -110 | 10/11 | 52.4% |
| 2.00 | +100 | 1/1 | 50.0% |
| 2.50 | +150 | 3/2 | 40.0% |
| 3.00 | +200 | 2/1 | 33.3% |
| 5.00 | +400 | 4/1 | 20.0% |
| 10.00 | +900 | 9/1 | 10.0% |
-110 on both sides of a two-way market is the standard "vig-inclusive coin flip": each side reads 52.4%, and the two add to 104.8%.
Why both sides add up to more than 100%
A fair two-way market would have the two probabilities sum to exactly 100%. Books price them higher so they profit whichever side wins. That excess is the vig, and stripping it out gives the market's honest estimate.
The extra 4.8% is the bookmaker's built-in margin. Divide each side by 104.8% to recover the fair probability it hides.
If your own estimate of an outcome is higher than the no-vig implied probability, that is a value bet.
Common questions
What is implied probability?
It is the chance of an outcome baked into the odds a bookmaker is offering. Decimal odds of 2.00 imply a 50% chance; 1.50 implies 66.7%. It is the price expressed as a probability, and it includes the bookmaker margin, so it is slightly higher than the true chance.
How do I turn American odds into a probability?
For a negative line, divide the number by itself plus 100: -150 gives 150 / 250 = 60%. For a positive line, divide 100 by the number plus 100: +200 gives 100 / 300 = 33.3%.
What is the vig, and how do I remove it?
The vig is the bookmaker margin. Add the implied probabilities of both sides: if they total 105%, the vig is about 5% and the book is overround by 5%. To get the fair (no-vig) probability, divide each side by that total, so 52.4 / 105 gives about 49.9%.
