1. What is a parlay?
A parlay is a single bet that combines two or more picks (called legs). Every leg has to win. If one leg loses, the whole parlay loses. In return, the payout is much bigger than any single leg because the odds are multiplied together, not added.
On PrizePicks, Underdog, and other DFS apps, this is packaged as a power play, flex, or pick'em — but the underlying math is the same as a sportsbook parlay.
2. The math (this is the important part)
Convert odds to implied probability first
American odds → probability
Negative odds (favorites):
|odds| / (|odds| + 100)Positive odds (underdogs):
100 / (odds + 100)Example: −110 → 110 / 210 = 52.4%. +150 → 100 / 250 = 40.0%.
Two-leg example
Two legs at −110 each (a standard football spread). Each has an implied probability of about 52.4%. Combined:
- 0.524 × 0.524 = 0.274 — roughly a 27% chance to hit.
- Fair payout would be about +265. Books typically pay +265 to +264 — very close to fair on a 2-leg −110/−110 parlay.
Where the house edge shows up
Each −110 leg already contains about 4.5% vig for the book. When you multiply legs, you multiply that vig too. This is called compounding hold.
- 2 legs at −110: house hold ≈ 4.5%
- 4 legs: house hold ≈ 17%
- 6 legs: house hold ≈ 25%
- 8 legs: house hold ≈ 32%
That's why the average parlay bettor loses far more per dollar than a straight-bet bettor — even if their individual pick accuracy is identical.[1]
3. Correlation — the one thing that changes everything
Correlated legs move together
Example of positive correlation (both hit together more often than random):
- Chiefs team total over 27.5 and Mahomes passing yards over 285.5
- Lakers ML and LeBron over 26.5 points
- Ohtani HR and Dodgers team total over
These look tempting because the story ties together. But sportsbooks price same-game parlays (SGPs) using a correlation model thatalready accounts for the linkage — so the payout is usually 15–30% worse than if you multiplied the legs like a normal parlay.[2]
Example of negative correlation (they actively hurt each other):
- Team A ML and game total under — if A blows them out, the total usually goes over.
- Two RBs from the same team both over rushing yards — they share carries.
4. Expected value: when is a parlay actually +EV?
A bet is +EV (positive expected value) when your true probability of winning is higher than the book's implied probability. For a parlay:
- Multiply your true probability of each leg.
- Compare to the book's implied probability of the parlay.
- If yours is higher, the bet has an edge.
Worked example
If instead you'd stacked eight −110 favorites you liked "a little," even a modest 3% edge per leg would still lose to the compounding hold.
5. Common parlay traps
- Chasing the payout number. A +2500 ticket looks like a lottery win. It hits like a lottery ticket too.
- All heavy favorites. Six −250 favorites still parlays to only about +185 — and any single one going down kills the whole ticket.
- Stacking correlated overs on the same offense. The book has already discounted the price. Bet the story once, not five times.
- Ignoring line differences across books. A parlay built at the softest number on each leg (see Best Line) can shift your EV by several percent per leg.
- SGP + straight parlay combos. Compounds both correlation adjustments and vig — usually the worst-priced ticket on the board.
6. The sharp approach
- Keep leg count low. 2–3 legs is where the math is friendliest.
- Look for genuine +EV in every leg first. Never fill a parlay with "I kinda like it."
- Use correlation intentionally. Round-robin uncorrelated legs; only build SGPs when you specifically believe the book's correlation model is wrong.
- Shop lines per leg. Small edges compound the same way vig does — in your favor this time.
- Size stakes small. Variance is enormous. A parlay is a lottery ticket with math, not a bankroll builder.
7. How GameDayLocks helps you build smarter slips
- Every pick shows model projection vs the market line, so you can see the actual edge before adding it to a slip.
- Confidence and Risk labels make it obvious when a leg is "solid" vs "coin-flip payout chase."
- The My Slip page shows the average edge and average confidence of the picks you've saved, so you can see when your slip is drifting into low-quality territory.
- We do not accept wagers or place bets. GameDayLocks is analytics + education only — take the built slip to a licensed operator if you choose to play.
Sources & further reading
- Parlay (gambling) — Wikipedia (definitions, payout tables, house edge math)
- What is a Same Game Parlay? — The Action Network (correlation pricing, SGP mechanics)
- Parlay Odds Calculator & Explainer — Vegas Insider (odds conversion, payout formulas)
- Why Parlays Are a Bad Bet — Pinnacle (compounding vig analysis)
- ESPN Chalk — Parlay ROI studies (long-term parlay ROI data)