How sports contracts differ from sportsbook bets

Sports contracts attract bettors because they look familiar: a team, a price, a game tonight. The mechanics differ in ways that matter to the price, and the biggest difference is that on an exchange you are trading against other people, not against a bookmaker who has built a margin into every line.

No vig, but a spread

A sportsbook moneyline of -150 and +130 contains the book's margin. Convert both to implied probabilities and they sum to more than 100; the excess is the vig. On an exchange, YES and NO sum to about 100 and the cost of trading is the bid-ask spread and any fee. For a popular game the spread is a point or two. That means a fair comparison between a book and an exchange requires removing the vig from the book's line first. Our issues do exactly that when they report "the books make Seattle 62%; Kalshi shows 62%."

The question is not always "who wins"

Read the rule. Sports contracts come in several shapes:

• Game winner. Usually includes overtime, but some rules settle on regulation time for sports with ties. A contract that says "regulation" on a hockey game is a different contract from the moneyline.
• Spread contracts. Some markets ask whether a team wins by more than a number. That is a point-spread bet in exchange form and is not comparable to a winner contract.
• Totals. Over or under a combined score.
• Season markets. Championship, division, win totals, awards. These are the durable contracts and the ones our trackers follow.

Postponements and cancellations

A book usually voids a bet if a game is postponed beyond a set window. An exchange contract has its own rule: some settle on the eventual result whenever the game is played, some void and refund, some settle NO if the game is not played by the close date. Weather-delayed and cancelled games are where the difference between a book and an exchange shows up as real money.

Comparing the same outcome

To compare a book to an exchange, match four things: the event, the outcome (winner, not spread), the treatment of overtime, and the treatment of postponement. Only then does the difference in price mean anything. Where they do not match, the comparison is "related, not equivalent," and a gap between them is not an edge.

A worked example

Nine sportsbooks price the Seahawks at -160 at home. Remove the vig across the two sides and the consensus win probability is about 62%. Kalshi's Seahawks contract trades at 62. There is no gap. The game is worth reading about; it is not worth a trade on price. If Kalshi instead showed 56, the four-point gap between the books' consensus and the exchange would be a documented edge, and the sentence that describes it is "the books make Seattle 62%, Kalshi sells it at 56." That is the standard our coverage is held to.

Beginner mistakes

• Comparing a book's line with the vig still in it.
• Treating a spread contract as a winner contract.
• Assuming overtime and postponement rules match the book's.
• Following heavy volume as if it were information; a lot of money on a favorite is not evidence the price is wrong.

What to take away

• Exchanges have a spread, not a vig; remove the vig before comparing.
• The contract's shape and rule define the outcome; the title does not.
• A gap between books and an exchange is only an edge when the questions match.
• Season markets are the ones with a story worth following over time.