Every market is two things at once: an estimate of probability and a charge for making it. Separating them lets you see the estimate on its own, compare it with your own, and compare it between bookmakers on equal terms. The arithmetic takes a minute.
The simple method
- Convert each price. Home 1.91 → 52.36%. Away 1.99 → 50.25%.
- Add them. 102.61%. The margin is 2.61 points.
- Divide each by the total. 52.36 ÷ 102.61 = 51.03%. 50.25 ÷ 102.61 = 48.97%. They now add to 100.
- Convert back. 1 ÷ 0.5103 = 1.96. 1 ÷ 0.4897 = 2.04. Those are the fair prices.
The method assumes the margin is spread in proportion to each outcome’s probability. That is approximately true on tight two-way markets at sharp operators, and less true everywhere else.
Where it breaks
Bookmakers do not apply their margin evenly. Long shots are shaded harder than favourites — the favourite–longshot bias — because casual money prefers big prices. On a three-way football market the draw and the away side typically carry more of the margin than the home favourite. Proportional removal therefore slightly understates the favourite’s true probability and overstates the outsider’s.
A better approximation
Remove the margin in proportion to the odds rather than the probabilities — the “power” or “Shin” methods in the literature. For everyday use, take the vig-free line from the sharpest operator you can see, which will already have less bias baked in than any correction you apply to a wide one.
What to do with it
Use the vig-free price from Kestrel or NovaBet — the two sharpest in our guide — as the market’s probability, and compare your own estimate with that rather than with any bookmaker’s shaded price. Then find the best price actually available, anywhere, and check it against the same fair line. The gap between those two numbers, in probability points, is your expected edge on the bet; if it is smaller than zero, the market is offering you nothing, however strongly you feel.
