How Polymarket spreads compare to sportsbook vig

Filed 22 Jul 2026 · The Cent Signals desk

Prediction-market prices and sportsbook lines are both read as implied probability, which invites a natural comparison between Polymarket's bid-ask spread and a sportsbook's built-in vig. The two are not the same kind of cost, and the difference comes from how the underlying contracts are constructed rather than from one platform being cheaper than the other. This guide describes the structural difference only.

What vig is, mechanically

A sportsbook typically prices both sides of a line above its own estimate of the true probability, a practice widely known as vig or the overround. A familiar example is a pair of lines both set near -110, which each imply roughly 52.4 percent, summing to about 104.5 percent across the two sides of a single event rather than 100 percent. That extra 4.5 percentage points is not friction from trading activity; it is a markup the book chooses when it sets the line, and it is present on both sides regardless of how much action either side receives.

Why a Polymarket Yes and No pair works differently

A Polymarket Yes share and No share on the same question are not two independently priced products the way two sides of a sportsbook line are. They are complementary claims on one fixed one-dollar payout: hold both together and they can be redeemed for exactly one dollar. That redemption relationship creates a direct arbitrage constraint, since anyone who sees the two sides summing meaningfully above one dollar can acquire both and redeem the pair for a profit, and anyone who sees them summing meaningfully below one dollar can look for the same gap from the other direction. The mechanics of reading that relationship are covered in how to read implied probability on Polymarket. A sportsbook's two sides carry no equivalent redemption link forcing them together, which is structurally why a book is free to price both above their fair share and keep the gap.

What actually costs money on each side

That does not mean trading on Polymarket is free of cost. The bid-ask spread, the gap between the best resting bid and the best resting ask, is a real transaction cost paid by whoever crosses the book to get an immediate fill, described in makers vs takers on Polymarket, and any applicable trading fees are covered in Polymarket fees explained. Both are real frictions. The distinction from vig is structural: a spread is a cost of immediacy that narrows or widens with the depth behind a price, while vig is a fixed markup the book sets regardless of how much liquidity is behind either side of the line.

Why a small gap on Polymarket is not the same thing as vig

A Yes-plus-No sum that lands slightly off one dollar in practice, or a negative risk field that lands slightly off one dollar across all its legs, described in what is negative risk on Polymarket, reflects the spread, any fees, and timing differences between legs, not a deliberately embedded margin. That gap is variable and tends to shrink as liquidity deepens; a sportsbook's vig is a fixed design choice present from the moment the line is posted. Both are real costs of participating; they are simply built into the product in different ways.

Frequently asked questions

What is vig at a sportsbook?

Vig, short for vigorish, is the margin a sportsbook builds into both sides of a line by pricing each side's implied probability slightly above its own estimate of the true probability. A common example is a pair of even-money lines both priced near -110, which imply roughly 52.4 percent each, summing to about 104.5 percent rather than 100 percent. The extra amount is a structural margin embedded in the line itself.

Does Polymarket have an equivalent to vig?

Not structurally in the same way. A Polymarket Yes and No share are complementary claims on the same one-dollar payout, redeemable together for exactly one dollar, so their prices are bound by arbitrage to sum close to one dollar rather than being independently priced above it by design. The costs a Polymarket participant does face are the bid-ask spread and any trading fees, covered in Polymarket fees explained, which are frictions layered on top of the contract rather than a deliberately embedded margin baked into both sides.

Why do Polymarket's Yes and No prices sum close to a dollar while sportsbook lines sum above 100 percent?

Because a Yes share and a No share are the same underlying contract split in two, redeemable together for one dollar at any time, which creates a direct arbitrage constraint keeping their sum near a dollar. Two sides of a sportsbook line are two separately priced products with no equivalent redemption mechanism forcing them together, which leaves the book free to price both sides above their fair share and keep the difference as margin.

Is a gap between one dollar and a market's actual Yes-plus-No sum the same thing as vig?

No. A small gap on Polymarket typically reflects the bid-ask spread, any trading fees, and timing differences between when each side last traded, all of which are transaction frictions rather than a margin deliberately built into the contract design. Vig, by contrast, is a fixed structural markup the sportsbook chooses when it sets the line, present regardless of how tight the market otherwise is.

Does this comparison mean one structure is better for a participant than the other?

This page describes a structural difference in how the two products are built, not a judgment about either one. Fees, spreads, liquidity, and each platform's own terms all vary and change over time, and reading either figure requires checking the platform's own current disclosures rather than treating this comparison as the final word.