Odds express a price and an implied probability. They do not predict an outcome with certainty.
Decimal odds
The reciprocal of decimal odds gives an implied probability before adjustments for an operator’s margin. Decimal odds of 2.00 imply 50%; odds of 4.00 imply 25%.
American odds
Positive and negative numbers use different conventions to express the same underlying relationship between price and probability.
Why probabilities may add to more than 100%
An operator’s margin can be built into the offered prices. This makes quoted probabilities different from a fair estimate of an event’s chances.
Uncertainty remains
Even a likely outcome can fail. No odds format, past result, or system guarantees a profit.