Learn how decimal, fractional, and American odds work. Calculate implied probability, potential returns, and understand what odds really mean.
In This Guide
- What Are Betting Odds?
- Decimal Odds (European)
- Fractional Odds (UK)
- American Odds (Moneyline)
- Implied Probability
- The Bookmaker's Margin (Overround)
What Are Betting Odds?
Betting odds represent the probability of an outcome happening and determine how much you can win.
Every set of odds can be converted to an implied probability — the bookmaker's estimate of how likely something is.
Odds also include a built-in margin (the overround) which is how bookmakers make their profit.
Decimal Odds (European)
Decimal odds are the most straightforward format. Your return = stake x odds.
Example: A 10 stake at 2.50 odds returns 25 (10 x 2.50), giving you 15 profit.
Decimal odds of 2.00 represent a 50% implied probability (an even-money bet).
| Decimal Odds | Implied Probability | 10 Stake Returns |
|---|---|---|
| 1.50 | 66.7% | 15.00 |
| 2.00 | 50.0% | 20.00 |
| 3.00 | 33.3% | 30.00 |
| 5.00 | 20.0% | 50.00 |
| 10.00 | 10.0% | 100.00 |
Fractional Odds (UK)
Fractional odds like 5/2 mean you win 5 for every 2 staked, plus your stake back.
Evens (1/1) is the same as 2.00 decimal — a 50% probability.
Odds-on prices like 1/2 mean the outcome is more likely than not.
American Odds (Moneyline)
Positive odds (+150) show profit on a 100 stake. Negative odds (-200) show how much you need to stake to win 100.
+150 means stake 100 to win 150 profit. -200 means stake 200 to win 100 profit.
American odds are standard in the US and on prediction markets like Polymarket.
Implied Probability
Convert decimal odds to probability: Probability = 1 / Decimal Odds.
A price of 3.00 = 33.3% implied probability. If you think the true chance is higher than 33.3%, you have a value bet.
The combined implied probabilities of all outcomes in a market always exceed 100% — the excess is the bookmaker's margin.
Tip: Compare odds across multiple bookmakers using wagerbase odds comparison to find the best available price.
The Bookmaker's Margin (Overround)
If a coin flip were priced fairly, both sides would be 2.00 (50%). But bookmakers price it at 1.91/1.91 (52.4% each), totaling 104.8%.
That 4.8% overround is the bookmaker's edge. Lower overrounds mean better value for bettors.
Sharp bookmakers like Pinnacle have margins of 2-3%. High-street bookmakers often have 5-10%.
Next Steps
Compare odds across 60+ bookmakers. Odds Comparison →
For more guides, tools, and analysis, visit wagerbase.