A sportsbook prices a parlay by multiplying the decimal odds of each leg, which multiplies the margin in each leg too. It does not add. Measured on real MLB moneylines from our own archive, the margin runs 3.98 percent on one leg, 7.86 percent on two and 11.61 percent on three. That compounding is the whole reason parlays are promoted.
Every figure here comes from our own capture archive. The MLB parlay board runs the same arithmetic on today's prices.
How does a sportsbook price a parlay?
By multiplication, and by nothing else. Convert each leg to decimal odds, multiply them
together, and that product is the parlay price. Three legs at -110 are 1.9091
each, and 1.9091 cubed is 6.9587, which the book rounds and offers as roughly
+595.
Nothing unusual happens in that step. The book is not adding a parlay charge on top. The reason a parlay is a worse bet than its legs is that the margin multiplies along with the price, and most people compare the payout to what feels big rather than to what the same legs would be worth without the margin.
Formally: if each leg's two prices imply probabilities summing to R (a fair market sums to
exactly 1.00), then an independent n-leg ticket's combined overround is R1 x R2 x ... x Rn,
because the offered decimal is the product of the legs' decimals while the true probability
is the product of the legs' fair probabilities. The margin on the ticket is
1 - 1 / (R1 x ... x Rn).
What that costs, measured on real prices
That is a claim about arithmetic, so we measured it on real tickets rather than on a worked example. Every pair and every triple of different MLB games that the same sportsbook was pricing at the same instant, before first pitch:
| Legs | Median overround | Median margin | Combinations measured |
|---|---|---|---|
| 1 | 104.14 | 3.98% | 49,194 quotes |
| 2 | 108.53 | 7.86% | 314,339 |
| 3 | 113.13 | 11.61% | 1,305,309 |
MLB moneylines, 11 sportsbooks, 2025-02-23 to 2026-08-14, 3,935 book-slates. Cross-game only: no combination takes two legs from the same game, so nothing here relies on correlated outcomes. Pre-game captures only.
Read the last row plainly. On a three-leg parlay the price structure keeps a median 11.61 percent of the payout, against 3.98 percent on a single moneyline. Nearly three times the margin, for a ticket that is harder to win.
The arithmetic checks out against the measurement
If compounding is really what is happening, then raising the one-leg overround to the power of n should land close to the measured multi-leg overround. It does:
| Legs | Predicted from the 1-leg median | Measured on real combinations |
|---|---|---|
| 2 | 108.45 | 108.53 |
| 3 | 112.94 | 113.13 |
Close but not identical, and the difference is not an error: the median of a product is not the product of medians. The agreement is what tells you the mechanism is multiplication rather than a special parlay charge.
Why the odds feel better than they are
The payout on a parlay is large and the margin is invisible, which is a difficult
combination to reason about. Three separate -110 bets and one three-leg parlay
of the same three sides are not the same wager: the parlay needs all three to land and pays
once, and it has been priced through three margins instead of one.
The same-game parlay is a further case, and a genuinely different one. Legs inside one game are correlated, so multiplying their probabilities is simply wrong arithmetic, and a book prices those with its own correlation model. Our own parlay tools refuse same-game combinations entirely rather than publish a number that treats a quarterback's yards and his team's total as independent.
What actually reduces the cost
Nothing on this page is advice about whether to place a parlay. But the arithmetic does say plainly where the cost comes from, and that is worth stating:
- Fewer legs carry less margin. The table above is the whole argument.
- The price you take on each leg is the only lever you control. Each leg priced at the best available book compounds a slightly smaller margin. On the boards we measured, that difference is small and it is real.
- A parlay is placed at one book. Taking the best price on each leg from a different book produces a number nobody can bet. Our MLB parlay board prices every cross-game combination at each book that would take the whole ticket, which is the only kind of price a reader can actually reach.
How this was measured
- Source. Our own MLB odds archive:
odds.linesjoined toodds.events, 2025-02-23 to 2026-08-14, 11 sportsbooks, 4,372 events, 449 capture instants, 145,474 two-way quotes. Produced byseo/f78_parlay_hold_facts.pyover a read-only database session. - Two-way only. Exactly two priced outcomes for one game, book, capture instant, market and line. Any other count is excluded and counted, never averaged in.
- Pre-game only. 66,244 rows captured after first pitch (10.28% of the window) are excluded. In-play margins are far wider and no pre-game bettor can reach them.
- One slate per book per day. The day's earliest capture, so an evening board with most games already started cannot bias the combination counts toward night baseball.
- Cross-game only. No combination takes two legs from the same game.
- Moneyline for the combination measurement. Single-market margins are reported for the moneyline and the total; the pair and triple enumeration runs on the moneyline alone, so the one-leg figure is directly comparable. The compounding law does not depend on which two-way market the legs came from.
- Percentiles use percentile_disc, so every figure is a value that actually occurred. These are pre-game listed prices from a capture archive, not closing lines.
Next steps
Frequently asked questions
How is a parlay payout calculated?
Convert each leg to decimal odds, multiply them together, and multiply by your stake. Three legs at -110 are 1.9091 each, and 1.9091 cubed is 6.9587, so a 10 dollar stake returns about 69.59 dollars including the stake.
Do sportsbooks add extra vig to parlays?
They do not need to. The margin in each leg multiplies with the price, so a three-leg ticket carries roughly three times the margin of one leg without the book adding anything. We measured a median of 3.98 percent on one leg and 11.61 percent on three across real cross-game MLB combinations.
Why is a same-game parlay priced differently?
Because the legs are correlated. Multiplying probabilities is only correct for independent events, and two outcomes inside one game are not independent. Books price those with their own correlation model, and our tools refuse same-game combinations rather than publish arithmetic we know is wrong.
Does taking the best price on each leg fix it?
It reduces the compounded margin, and it is the only part of the price a bettor controls. It does not remove it: on the boards we have measured, almost every posted price already sits below fair value, so a smaller multiplication of margins is still a multiplication of margins.
Is a two-leg parlay better than a three-leg one?
It carries less margin, which is a fact about the price and not a recommendation. Our measured medians are 7.86 percent on two legs against 11.61 percent on three. We publish no picks and nothing here is betting advice.
Related: How to calculate parlay odds · What percentage of parlays win · Why most +EV tools mislead · Hedge betting guide
Every figure on this page is a historical measurement of what already happened, produced by a committed, re-runnable script over our own odds archive. Past frequencies are not forecasts and are not betting advice: Nebula Insights publishes no picks, no selections and no recommended wagers. 18+ only.