Parlay calculator

How parlay odds are computed

A parlay pays only when every leg wins, so the combined decimal odds are the product of the leg decimals. Two legs at -110 are each decimal 1.909; multiplied together they make 3.64, so $100 returns $364.46 if both hit. The multiplication is why parlays feel generous: each added leg roughly doubles the quoted payout. The same multiplication is working on the probability, in the other direction.

The probability collapses faster than the payout grows

A -110 line implies 52.4%, but that number contains the bookmaker's cut. In a market priced -110 on both sides, the two implied probabilities sum to 104.8%; strip that 4.76% overround and each side is a fair coin flip. Two fair 50% legs combine to 25%. Five of them combine to 3.1%. The table above shows this collapse leg by leg, and it is the number to watch: the payout column is what you are quoted, the probability column is what you actually hold.

Why the payout is not value

Here is the worked example the calculator reproduces at its defaults. Two legs at -110, $100 staked, pays $364.46. The true chance of winning is 25%, so the fair payout is $100 divided by 0.25, which is $400.00. The gap is the margin: 1 minus 364.46 over 400, or 8.9%. Expected value confirms it, since 0.25 times $364.46 is $91.11, an average loss of $8.89 per $100. A single -110 bet costs you about 4.5% in expectation; the two-leg parlay costs nearly double, because the vig on each leg compounds. Every leg you add multiplies the house edge again, which is precisely why books promote parlays so hard.

What this calculator assumes

Legs are treated as independent and as carrying the same overround, which you can adjust. Same-game parlays break the independence assumption: books price the correlation themselves, rarely in your favor. And if you genuinely hold an edge on individual legs, combining them concentrates your outcomes into one low-probability payoff with brutal variance. The odds calculator prices a single leg against your own estimate, which is the honest place to start.