CORTENThe margin is the part of the summed implied probabilities (1 / odds) of all outcomes in a market that exceeds 100%. In the synthetic 2.10 / 3.40 / 3.60 example the total is 104.81%, so the margin is 4.81%. The simplest way to remove it is to divide each raw probability by the total; only use prices recorded at the same time.
The margin (overround) is the part of the total implied probability across all outcomes of a market that goes above 100%. The higher the margin, the further prices move from the raw probabilities. The calculation should only use prices recorded at the same time, for the same market, covering every outcome.
Step-by-step calculation
A synthetic 1X2 example: home 2.10, draw 3.40, away 3.60.
| Outcome | Odds | Raw probability (1/odds) |
|---|---|---|
| 1 | 2.10 | 47.62% |
| X | 3.40 | 29.41% |
| 2 | 3.60 | 27.78% |
| Total | — | 104.81% |
The total is 104.81%, so the margin is about 4.81%. You can repeat this with your own prices in the probability and margin calculator, and convert a single price to a probability (or a probability to fair odds) in the odds ↔ probability converter.
Method 1: basic normalisation
Divide each raw probability by the total: qᵢ = (1/oᵢ) / Σ(1/oⱼ). In the example this gives 1: 45.43% · X: 28.06% · 2: 26.50%, which adds up to 100%. This method spreads the margin across all outcomes in proportion to their raw probabilities.
Method 2: the power method
Here you find the k for which Σ(1/oᵢ)ᵏ = 1; the margin-free probability is then qᵢ = (1/oᵢ)ᵏ. In the example k ≈ 1.0461 and the result is 1: 46.02% · X: 27.80% · 2: 26.18%. The power method takes more of the margin out of long-priced (low-probability) outcomes, based on the observation that those prices tend to carry relatively more margin.
Which one is right?
Both are assumptions; the true split of the margin is unknown. The difference is usually a few tenths of a point for the favourite and the low-probability outcomes, but it can grow in high-margin markets such as nine-outcome HT/FT (see HT/FT explained). What matters is to use the same method whenever you compare, and to state which one you used.
What to watch for
Combining prices recorded at different times in one calculation can make the margin look lower or higher than it really is; for timing, see the opening vs closing odds guide. A margin-free probability is a ratio derived from market prices; it is not a model prediction or a guarantee.
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Published 28 October 2026 · Last updated: 28 October 2026 · Responsible: Cortenscore editorial team · Spotted an error? Report a correction. This page is for statistics education; it is not a prediction or betting advice. Example odds are synthetic.