Why parlays cost more than the same bets straight
A parlay pays more because every leg has to win. The catch is that the bookmaker's margin rides on every leg too, and multiplying the legs together multiplies the margin.
One straight bet
A standard prop at -110 / -110 pays $90.91 profit on a $100 stake if it wins. Each side implies 52.4%, and the fair probability (without the vig) is 50%. At a true 50%, the expected result of $100 at -110 is a loss of about $4.55: a 4.5% cost.
Two legs
Two -110 legs parlay at about +264 (1.909 × 1.909 = 3.645 in decimal odds). If each leg is a true 50%, the parlay wins 25% of the time. Expected result on $100: 0.25 × $264.5 − 0.75 × $100 = −$8.88. The cost has roughly doubled, to about 8.9%.
Four legs
Four -110 legs parlay at about +1228 (decimal 13.28). At 50% per leg, the parlay wins 6.25% of the time. Expected result on $100: 0.0625 × $1,228 − 0.9375 × $100 = −$16.98, a cost of about 17%.
| Legs at -110 | Payout | Win chance at 50% each | Expected cost |
|---|---|---|---|
| 1 (straight) | -110 | 50% | 4.5% |
| 2 | +264 | 25% | 8.9% |
| 3 | +596 | 12.5% | 13.0% |
| 4 | +1228 | 6.25% | 17.0% |
"But my legs are better than 50%"
If each leg genuinely has an edge, a parlay compounds that too. The problem is that the edge per leg has to beat the margin per leg and survive the extra variance, and any leg that's slightly worse than you think drags the whole ticket. The same legs bet straight let a good leg pay you even when another leg loses.
Same-game parlays
Legs in the same game are often correlated (a quarterback's passing yards and his receiver's receiving yards rise together). Books know this and price same-game parlays with their own correlation model, usually with extra margin added. The payout shown is set by the book, not by multiplying fair odds, and it is rarely in the bettor's favour.
Why the board publishes singles
OpenSlate publishes one position per player as a single line, not parlays. On the lab's own data, parlays priced far worse than the same legs bet straight, which is what the arithmetic above predicts. If you combine picks yourself, you are adding the margin of every leg.
Boosts and promotions
Parlay boosts and "profit boost" tokens change the arithmetic, which is why books use them as marketing. A boost raises the payout, but the legs offered for boosts are chosen by the book, often with extra margin built in first. Run the same calculation: convert each leg to a fair probability, multiply them, and compare the boosted payout with the fair payout. Sometimes a boost does make the ticket fair or better; often it only makes it less bad.
A quick way to check any parlay
Multiply the decimal odds of each leg to get the payout the book offers. Then multiply each leg's no-vig probability to get the ticket's real chance. Payout times chance, minus one, is your expected return per dollar. If that number is negative, which it usually is, you are paying for the excitement of a bigger number.
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