How Parlay Probability Works
A parlay only wins if every leg wins. Under the assumption that the legs are independent, the combined probability is the product of each leg's probability.
The short answer
Two legs at 50% each: 0.5 × 0.5 = 25%. Four legs at 50% each: 6.25%. Add legs and the win chance collapses quickly, even when every individual leg is a coin flip. That is why parlay payouts are large — they have to be.
The independence assumption
The multiply-together math is only valid when legs are independent — when one outcome tells you nothing about another. Legs from different games are usually close enough to independent for estimation. Legs from the same game often are not, and in those cases the combined probability is an approximation that can be materially off.
A worked example
Try the math yourself
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