Why do I keep losing parlays?
Because every leg you add multiplies the book's vig. One -110 bet carries roughly 4.5% of margin, two legs about 9%, four legs about 17%. You're not unlucky, you're stacking a tax. The exception is when a promo pushes the combined price above the fair price.
Your Saturday slip. Four legs, +850, and one leg lets you down. Again. That’s not a run of bad luck, and it’s not a you problem. It’s the most profitable product the book sells, working exactly as designed.
How parlays actually price
A parlay multiplies the decimal odds of every leg. Two legs at -110 (decimal 1.909) combine to 3.645, which is +264. The payout grows fast, and that’s the part the app shows you in big friendly numbers.
Here’s the part it doesn’t show. Every -110 leg carries the book’s margin, about 4.5% per bet. Multiply legs and you multiply the fee:
A four-leg parlay bettor is paying roughly four times the tax of a straight bettor, on every single ticket. That’s why the losses feel relentless. They’re priced in.
Why the book pushes them so hard
Because the math above is the book’s income statement. A bettor grinding single bets pays around 4.5% margin. The same bettor on four-leg parlays pays around 17%. Parlays are the highest-margin product on the app, dressed up as a lottery ticket, and every “same game parlay of the day” banner exists to move you up that chart.
One extra wrinkle on same-game parlays: the legs are correlated, so books reprice them internally. The multiplication above assumes independent legs. For SGPs the true math is murkier, which never favors the person holding the slip.
The one time a parlay is worth it
When a promo pushes the combined price above the fair price. A big parlay boost can beat the stacked vig, and when it does, the parlay stops being a tax and becomes an edge. The same 5-second test from the boost article applies: compare the boosted price to the fair price, and the price decides.
You can run your exact slip through the parlay calculator. It shows the combined price, the payout and the win probability that price implies, and its page charts what the book holds on -110 legs from one leg to ten. For independent legs, strip the vig from each leg with the no-vig calculator and multiply the fair probabilities: that is an estimate of the ticket’s fair probability, and it does not hold for legs from the same game. If the boosted number beats that estimate, play it. If it doesn’t, the book already won.
Quick answers
Are parlays always -EV?
At standard prices, effectively yes, because the vig compounds per leg. A parlay becomes +EV when a boost or promo pushes the combined payout above the fair no-vig price of the slip, or occasionally when the underlying legs are mispriced in your favor. Either way it's rare without a promo, which is exactly why books hand out parlay boosts.
How much vig is in a parlay?
Roughly 4.5% on one -110 leg, 8.9% on two, 13% on three, and about 17% on four. Every added leg multiplies the margin you're paying.
What is a fair parlay price?
The product of each leg's no-vig win probability, converted back to odds. Two legs that are each a fair 50% make a fair parlay of 25%, which is +300. Anything less than +300 on that ticket is vig.
Do profit boosts fix parlays?
Sometimes. A big enough boost can push a parlay past its fair price, and boosted parlays were a regular part of the boost desk this site started as. The boost has to beat the stacked vig first, so check the math before assuming the badge means value.
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