What probability is a price actually implying?
A price is a probability statement with a business model attached. Divide one by the decimal price and you get the chance the market appears to be assigning to that outcome, expressed as a proportion. It is the single most useful calculation in betting, and also the one most often misread, because the number it produces is not the market's honest estimate of the world. It is that estimate plus a slice of margin, and telling the two apart is what the rest of this explainer is for.
A useful number with a known bias
Convert every outcome in a market this way and add the results together. If the market were a clean probability distribution, the total would come to one. It never does; it comes to more, and the excess is the margin. That means each individual implied probability is inflated, and inflated unevenly, since operators do not spread margin equally across outcomes. Use implied probability to compare your own view against the market, not to read the market's true opinion straight off the screen.
The calculation
Implied probability equals one divided by the decimal price. A price of four implies twenty-five per cent, a price of two implies fifty per cent, a price of one and a quarter implies eighty per cent. Working in percentages rather than fractions makes it far easier to compare selections across different sports and different markets.
Why the total exceeds one hundred
The excess is the operator margin, often called the overround. It is the structural reason a bettor who backs every outcome at the quoted prices loses money regardless of the result. That excess is the price of the service and it applies to every book, though the amount differs by market and by sport.
Comparing your view to the market
The honest use of implied probability is as a benchmark. If you think an outcome is likelier than the market implies, you have a disagreement worth examining; you do not have a guaranteed profit, because your estimate can be as wrong as anybody's. Outcomes remain unpredictable however carefully the sums are done.
What it cannot tell you
It cannot tell you what will happen, cannot be turned into an expected return without a probability estimate of your own, and cannot be compared meaningfully across markets with very different margins. A twenty-five per cent implied probability in a low-margin market is not the same claim as twenty-five per cent in a high-margin one.
Questions readers send us
Is implied probability the same as the true probability?
No. It is the true probability as the market sees it, inflated by margin, and the market's view is an opinion rather than a measurement. Two operators pricing the same match will produce different implied probabilities from the same underlying reality.
How do I strip the margin out?
The rough method is to divide each implied probability by the total of all of them, which rescales the set to one hundred per cent. It is only an approximation, because margin is rarely applied evenly, but it is close enough to be useful when comparing markets with different overrounds.
