Odds & Implied Probability
Definition
Bookmaker odds encode an implicit probability estimate for each outcome. Converting odds to implied probability reveals what the bookmaker believes the true chance of each outcome is — including their profit margin (overround). Understanding implied probability is essential for identifying value bets.
Example
Converting odds to implied probability: Decimal odds of 2.50 → implied probability = 1 ÷ 2.50 = 40%. Odds of 1.80 → 1 ÷ 1.80 = 55.6%.
If a match is priced Home 2.10 / Draw 3.40 / Away 3.80, the implied probabilities sum to 103.3%. The extra 3.3% is the bookmaker's overround — their built-in profit margin across all outcomes.
How CalibrSports Predicts This
We pull real-time odds from multiple bookmakers, convert each to an implied probability after stripping the overround, and compare them to our ML model's probability estimates. The difference is our edge. We always use the best available odds across bookmakers to maximise expected value.
Key Facts
Formula
Implied Prob = 1 / Decimal Odds
Typical overround
3–8%
Overround effect
Negative EV for all random bettors
Edge to overcome margin
Must exceed overround to profit
Related Terms
Frequently Asked Questions
What is the difference between decimal and fractional odds?
Decimal odds (2.50) include your stake in the return — you profit £1.50 for every £1 wagered. Fractional odds (3/2) show only the profit — £3 profit for every £2 wagered, equivalent to 2.50 decimal. We use decimal odds throughout for simplicity.
How does the overround hurt bettors?
The overround means the average bet has negative expected value. If you bet randomly on all outcomes, you lose 3–8% of your stake to the bookmaker's margin over time. Value betting only works if your probability estimates are accurate enough to identify the subset of bets where your edge exceeds this margin.
What is line shopping?
Comparing odds across multiple bookmakers to find the best available price on a bet. Even small differences in odds (1.85 vs 1.92) compound significantly over hundreds of bets. We always reference the best available odds in our picks.