Smarter Than Free calculator
Odds Converter
Convert fractional, decimal and American odds plus implied probability. Optional book overround.
Calculator
How do you convert fractional odds to decimal?
Decimal = fractional + 1. So 5/2 → 3.5. Implied probability = 1 ÷ decimal.
What is overround?
The sum of implied probabilities across outcomes minus 100%. Positive overround is the book’s built-in edge.
What does evens mean in betting?
Fractional 1/1, decimal 2.0, American +100, implied probability 50%. You win the same as you staked, so £10 returns £20 including your stake.
How do you read negative American odds?
Negative shows what you stake to win 100. At −150, staking £150 wins £100, which is decimal 1.67. Positive shows what a 100 stake wins, so +200 is decimal 3.00.
How do you take the margin out of implied probabilities?
The simple way is to divide each one by the book total. At 1.90 and 1.90, each implied 52.6% over a 105.3% book gives 50%. It is an estimate, because bookmakers do not always spread margin evenly.
Common odds in every format
Implied probability includes any bookmaker margin. It is the price, not the true chance.
| Fractional | Decimal | American | Implied |
|---|---|---|---|
| 1/4 | 1.25 | −400 | 80.0% |
| 1/2 | 1.50 | −200 | 66.7% |
| 4/5 | 1.80 | −125 | 55.6% |
| 10/11 | 1.91 | −110 | 52.4% |
| Evens (1/1) | 2.00 | +100 | 50.0% |
| 11/10 | 2.10 | +110 | 47.6% |
| 5/4 | 2.25 | +125 | 44.4% |
| 6/4 | 2.50 | +150 | 40.0% |
| 2/1 | 3.00 | +200 | 33.3% |
| 5/2 | 3.50 | +250 | 28.6% |
| 3/1 | 4.00 | +300 | 25.0% |
| 4/1 | 5.00 | +400 | 20.0% |
| 5/1 | 6.00 | +500 | 16.7% |
| 10/1 | 11.00 | +1000 | 9.1% |
| 20/1 | 21.00 | +2000 | 4.8% |
| 100/1 | 101.00 | +10000 | 1.0% |
The formula
Decimal = fractional + 1 (e.g. 5/2 -> 3.5) Implied probability = 1 ÷ decimal American positive: decimal = 1 + American/100 American negative: decimal = 1 + 100/|American| Overround = sum of implied probabilities − 1 (multi-outcome book).
Worked example
Odds 5/2: decimal 3.5, implied probability 28.57%, American +250.
Two-way book 1.90 / 1.90: implied 52.6% + 52.6% = 105.3%. Overround ≈ 5.3%.
Methodology
How it calculates
- Keeps fractional, decimal, American and implied probability in sync from whichever field you edit.
- Optional multi-outcome overround is the sum of implied probabilities minus 100%.
Formula
Decimal = fractional + 1 (e.g. 5/2 -> 3.5) Implied probability = 1 ÷ decimal American positive: decimal = 1 + American/100 American negative: decimal = 1 + 100/|American| Overround = sum of implied probabilities − 1 (multi-outcome book).
Assumptions
- Implied probability is the book price as a percent, with margin baked in. Not a true chance.
Edge cases
- Decimal = fractional + 1 (so 5/2 → 3.5, not 2.5).
Sources
None. Pure maths; no external rate or spec feed.
FAQ
Is implied probability the true chance?
No. It is the book price turned into a percent, with margin baked in.
Popular variants
Same maths, aimed at a specific search. Pre-filled where it helps.
Not financial advice. This calculator is for education only. Double-check results before you make money decisions. Past moves do not predict future ones.