Parlay
A parlay multiplies the legs: every one has to land. The combined decimal odds are the product of the leg decimals, and the combined true probability is the product of each leg's de-vigged fair probability. If you also enter your own per-leg win probabilities, we show whether the parlay has a real edge or just stacks the vig. Note: this assumes independent legs. Correlated legs (e.g. same-game) break the math.
Combined decimal = Π(leg decimals) · Payout = stake × combined · Implied = 1 / combined · Your edge = your combined prob − implied
Each leg you add multiplies the vig you pay. Correlated same-game legs are not independent, so the combined probability here will be wrong for them.
Market-neutral utility. It computes numbers you supply, it does not predict outcomes or guarantee profit. Compare lines across books and make the call yourself. 21+ · research, not advice.