Add any number of legs with American odds. Returns combined parlay odds, payout on your stake, implied probability of hitting, and the effective parlay vig — the metric that explains why parlays rarely pay sharps.
A parlay is a single bet that requires every leg to win. The combined odds are the product of the decimal odds, which is why parlay payouts grow fast as you add legs — and why parlay implied probability collapses just as fast. A 3-leg parlay of standard -110 favorites implies about a 12% chance of hitting; a 5-leg parlay drops to 4%.
The "effective vig" line above is the more useful sharp-bettor number. It's the percentage of fair-value payout that the sportsbook keeps on a parlay relative to the same legs bet straight. A 2-leg parlay at -110/-110 has an effective vig of about 9% — more than double the 4.5% you'd pay on a single bet. Each leg compounds the house's edge.
If every leg of your parlay carries a real positive edge against the close, the parlay can be a positive-EV bet — multiplicatively. The math doesn't care whether you bet legs separately or combine them; what matters is whether each leg has edge. The catch: most public parlays don't have edge on every leg, and any single negative-edge leg destroys the entire parlay's EV.
Correlation matters too. If two legs are correlated (same game or related player props), books often refuse correlated parlays or price them at de-vigged rates. Same-game parlays from major books carry significantly higher vig than the math above implies.