A parlay ties several bets into one. Every leg has to win, and in return the payout is much bigger than any single leg on its own. Books show you the final number, but it helps to know how they get there, because the math explains both why parlays pay so well and why they cost more than they look.
The method: decimal odds, multiplied
American odds don't multiply cleanly. Decimal odds do. So the process has three steps:
- Convert each leg to decimal odds.
- Multiply the decimals together.
- Convert the result back to American odds if you want it in that form.
The conversions:
- Positive American to decimal: 1 + (odds ÷ 100). So +150 is 2.50.
- Negative American to decimal: 1 + (100 ÷ |odds|). So -110 is 1 + 100/110 = 1.9091.
- Decimal back to American: if the decimal is 2.00 or more, (decimal − 1) × 100 with a plus sign. If it's under 2.00, −100 ÷ (decimal − 1).
The odds converter handles any of these if you'd rather not do them by hand.
A worked three-leg example
Three player props, each at -110.
Each leg is 1.9091 in decimal. Multiply them:
1.9091 × 1.9091 × 1.9091 = 6.958
Back to American: (6.958 − 1) × 100 = +596.
A $10 parlay returns $69.58 in total if all three win: your $10 back plus $59.58 profit.
The implied probability of the parlay is 1 ÷ 6.958 = 14.4%. That's the win rate you'd need for this bet to break even. The parlay calculator does all of this for any number of legs.
How the margin stacks
Here's the catch. A -110 single needs a 52.38% win rate to break even. If each of those three props is a genuine coin flip, the true chance of all three hitting is:
0.5 × 0.5 × 0.5 = 12.5%
A fair price for a 12.5% shot is 8.00 in decimal, or +700. The book is paying +596. On a $10 bet, that's $69.58 instead of the $80 a fair price would return.
In EV terms:
- A single -110 bet at a true 50%: −4.5% expected return.
- The three-leg parlay of those same bets: (0.125 × 6.958) − 1 = −13.0%.
Each leg carries the book's margin, and multiplying the legs multiplies the margin too. The more legs you add, the bigger the gap between the true odds and the price you're paid. Two -110 legs pay +264 and carry about −8.9% at true coin flips. Three legs, −13.0%. It keeps climbing from there.
That doesn't mean parlays are always bad. If every leg is +EV on its own, the parlay compounds the edge the same way it compounds the margin. But a parlay of break-even legs is a worse bet than the singles.
Why one better price raises the whole payout
Because the legs multiply, improving any one of them lifts the entire ticket by the same proportion.
Take the same three-leg parlay, but find one of the props at +100 instead of -110:
1.9091 × 1.9091 × 2.00 = 7.289, or +629.
The $10 bet now returns $72.89 instead of $69.58. That one better leg made the whole parlay pay about 4.8% more. Even a smaller improvement counts: moving one leg from -110 to -105 takes the parlay to +612, returning $71.16.
Since the margin compounds across legs, line shopping each leg matters more in a parlay than it does on a single. The catch is that most books only let you parlay legs at their own prices, so you're looking for the book whose prices are best across the legs you want, not each leg's best price separately.
What happens when a leg pushes
If a leg lands exactly on the line, it pushes. At most sportsbooks, a pushed leg is simply removed and the parlay is recalculated without it.
Using the example above: if one of the three -110 legs pushes, the parlay becomes a two-leg parlay at 1.9091 × 1.9091 = 3.645, or +264. Your $10 would return $36.45 if the other two win.
Rules vary, though. Some books treat a push differently on certain bet types or sports, a two-leg parlay with a push usually settles as a single at the remaining leg's odds, and DFS pick'em apps have their own rules that often adjust the payout tier rather than recalculating odds. Voided legs (a player who doesn't play, for example) are handled under separate rules again. Check the house rules at the book you're using before you assume.
Correlated legs
The multiplication only works if the legs are independent. When legs are related, such as a quarterback's passing yards and his top receiver's receiving yards, the true chance of both hitting is higher than the simple product. Books know this, and many either block correlated legs, price them differently, or route them through a same-game parlay product with its own pricing. Don't assume the standard math applies to a same-game parlay.
The practical takeaway
To price a parlay: convert each leg to decimal, multiply, and convert back. To judge one: compare the parlay's implied probability to your honest estimate of all legs hitting. And before you place it, check whether a better price on any single leg is available, because that improvement carries through the whole ticket.
The parlay calculator gives you the payout and implied probability for any combination of legs, and the odds converter handles the conversions in between.