1. Probability is an estimate

A probability between 0% and 100% describes the estimated likelihood of an outcome under a defined model or judgment. Different models can produce different estimates because they use different data, assumptions and weighting.

The quality of a probability is evaluated across many comparable predictions. A calibrated group of 60% predictions should succeed close to 60% of the time over a sufficiently large and relevant sample.

2. Decimal odds can be converted

The direct implied probability of decimal odds is calculated as 100 divided by the odds. Decimal odds of 2.00 imply 50%. Odds of 1.50 imply 66.67%, while odds of 3.00 imply 33.33%.

This conversion is useful, but it is only the first step because bookmaker prices normally include a margin.

  • 1.50 → 66.67%
  • 1.85 → 54.05%
  • 2.00 → 50.00%
  • 3.00 → 33.33%

3. Fair odds reverse the formula

If you already have a probability estimate, divide 100 by that percentage to calculate the corresponding fair decimal odds. A 60% estimate corresponds to fair odds of approximately 1.67.

Fair odds are a mathematical translation of an estimate. They do not prove that the estimate itself is accurate.

4. Margin changes the complete market

In a two- or three-way market, convert every available price to implied probability and add the results. The total often exceeds 100%. That excess is the overround or margin.

Comparing one model probability directly with one unadjusted market probability can still be useful for orientation, but a deeper evaluation should consider the full market and normalize its probabilities.

5. A difference is not a guarantee

Suppose odds of 1.85 imply 54.05%, while a model estimates 60%. The difference is 5.95 percentage points. This signals disagreement between two estimates — not certain value, profit or success.

Before drawing a conclusion, consider data freshness, market movement, model calibration, sample size and the possibility that the market knows something the model does not.