A parlay combines multiple selections into one bet, multiplying the odds to create a much larger potential payout than any single wager. Understanding the math behind parlays — and specifically how the sportsbook's margin compounds — helps bettors make informed decisions about when parlays are worth the risk.
The math behind parlay odds
Parlay odds are calculated by multiplying the decimal odds of each leg. To convert American odds to decimal: for positive odds (+150), divide by 100 and add 1 to get 2.50; for negative odds (-110), divide 100 by the absolute value and add 1 to get 1.909. Once all legs are in decimal form, multiply them together.
A 3-leg parlay at -110 / -110 / +200 converts to 1.909 × 1.909 × 3.000 = 10.94 decimal odds. On a $100 bet, total payout is $1,094 and profit is $994. The payout grows exponentially as legs are added — each additional leg multiplies the existing product rather than adding to it.
How vig compounds across legs
Every -110 line has an implied probability of 52.38%, but the true probability of a fair coin flip is 50%. The extra 2.38% is the sportsbook's cut (vig). For a single -110 bet, the house edge is about 4.55%. For a 3-leg parlay of -110 lines, the house edge rises to roughly 13.5% because the vig compounds multiplicatively.
This is why sportsbooks offer parlay promotions and odds boosts — parlays generate more margin per dollar wagered than singles. The larger the parlay, the more the house edge grows. A 6-leg parlay at -110 carries a house edge above 25%.
Parlay vs. singles — when does each make sense?
With single bets, each game is independent — one loss does not affect others. With a parlay, one loss voids the entire ticket. Singles spread risk across separate bets; parlays concentrate it into one high-variance outcome.
If your goal is bankroll preservation and steady expected value, singles are better in almost every scenario. If your goal is entertainment or a large payout from a small stake, parlays deliver that at the cost of a worse expected value. Correlation matters too: if two legs are correlated (e.g., the same team to win and cover), some books may void the parlay or exclude correlated legs. The vs Singles tab in this calculator shows the exact numbers side-by-side for any combination you enter.
Reading the Leg Breakdown tab
The Leg Breakdown table shows each leg's individual decimal odds, implied probability, and cumulative multiplier. The cumulative multiplier column shows how the combined product grows as each leg is added. If the multiplier grows rapidly from one leg to the next, that leg has high odds (either a big underdog or a high-payout line). The win probability column shows what the sportsbook implicitly says each leg's probability is — your own probability estimate should inform whether the leg has positive value before adding it to a parlay.