How a parlay is calculated
If the selections are independent, the parlay's odds are the product of the odds and its true probability the product of the true probabilities. Edge = probability × odds − 1, as for a single bet. Three bets at 1.80 whose true chance is 53% each: odds 5.83, probability 14.9%, fair odds 6.72, edge −13%.
That is the central mechanism: margins don't add up, they multiply. A parlay only has positive expectation if every leg is itself a value bet; a single leg at −3% is often enough to make the ticket a loser, and legs on the same match are correlated, which distorts the calculation.
Example
Two selections on different matches: 2.10 with a 52% chance (+9%) and 1.90 with 56% (+6%). Parlay: odds 3.99, probability 29.1%, edge +16%. Replace the second with a leg at 1.90 with a 50% chance (−5%): the ticket's edge drops to +4%, and a third mediocre leg turns it negative.
Frequently asked questions
Why do parlays make the bookmaker money?
Because the margin of each leg multiplies with the others: a parlay of four ordinary bets carries four times the margin of a single bet. Bookmakers promote them for precisely that reason, and parlay “boosts” give back only a fraction of that compounded margin.
Can a parlay have value?
Yes, on three conditions: every leg is a value bet, the legs are on different matches, and the stake is reduced because the probability of winning is low. Proba Bet's parlay optimizer only builds those tickets, from grade A bets at the same bookmaker.
Where do the chances of winning to enter come from?
From the market's reference odds, margin removed — not from your gut. The odds checker gives the true chance of each selection on the matches of the next four days; copy it here for an honest calculation.