Parlays and Correlation

WeBall3 min readUpdated Aug 15, 2026Reviewed Aug 15, 2026

A parlay combines several selections into one bet where all must win. The odds multiply and so does the bookmaker's margin, which is why parlay hold compounds brutally with each leg. The only sound reason to build one is correlation the book has underpriced.

Parlays are the most heavily promoted and most expensive product a sportsbook sells. Understanding exactly why is worth more than any individual parlay you will ever place.

The compounding problem

Each leg's odds multiply together. Three legs at −110 pay about +596.

The trouble is that each leg's vig multiplies too. A fair three-leg parlay of those same selections — using no-vig probabilities — would pay around +700. The gap between +596 and +700 is the book's cut, and it is roughly three times what you would pay betting the three legs separately.

+596

What a 3-leg parlay pays

Three legs at −110

+700

What it should pay

Same legs, no vig

~12.5%

How often it wins

Roughly one in eight

The parlay calculator shows both numbers side by side for any set of legs, which is the fastest way to see what you are actually being charged.

Correlation: the one real reason

Independent legs multiply cleanly, and there is no way to beat the compounding vig on them. Correlated legs are different.

Two outcomes are positively correlated when one happening makes the other more likely. A team covering a large spread and the game going over are correlated, because blowouts usually involve one offence scoring freely. A quarterback throwing for 350 yards and his top receiver going over on yardage are obviously correlated.

If two legs are genuinely linked and the book prices them as independent, the parlay is worth more than its parts. That is the only mathematically sound case for building one.

Same-game parlays

Because SGP legs come from one game, they cannot be independent, so books use a correlation model to price them. That model is also where the margin hides: SGP hold routinely runs two to four times a straight bet's.

Correlation in same-game parlays covers which relationships are strong, which are traps, and the measured coefficients WeBall's engine actually carries.

Teasers

A teaser is a parlay where you move each spread in your favour in exchange for a much shorter payout. Almost all of them are bad. The exception — the Wong teaser — is defined entirely by key numbers, and teaser math works through when it holds up.

The honest summary

If you enjoy parlays, bet them small and treat them as entertainment rather than strategy. That is a completely legitimate reason to place one.

What is not legitimate is convincing yourself a six-leg SGP is a value play because each leg looked good individually. Six legs that each looked good individually is exactly what the pricing model expects you to build, and it is priced accordingly.

Every guide in this section

Correlation in Same-Game ParlaysWhich same-game legs actually move together, with the measured coefficients — the positive stacks, the negative traps, and where books have already priced it.Advanced4 min Teaser Math and Wong TeasersWhy most teasers lose money, the specific two-team NFL teaser that historically did not, and how to check whether it still clears the bar.Advanced3 min

Common questions

Are parlays always bad bets?

Not always, but the default assumption should be yes. The hold compounds per leg, so a three-leg parlay of standard lines costs roughly three times what a single bet does. Correlation is the only thing that can overcome that, and books price the obvious correlations already.

Why do books promote parlays so heavily?

Because they are the most profitable product on the menu. A same-game parlay can hold over 15% against 4–5% on a main line, and it pays out rarely enough to be memorable when it does.