Key takeaways

• A sportsbook prices both sides with its margin built in. A standard -110/-110 line holds 4.76%, so two sportsbooks have to disagree by more than that before an arb appears.

• A prediction market price with little or no fee sits close to the fair price, which leaves more room for an arb against a single sportsbook price.

• Convert a contract price to decimal odds by dividing 1 by the price. A 47¢ contract is 2.13 in decimal, or +113 in American odds, before fees.

• Fees decide whether the arb survives. In the example below, the same 47¢ price gives a 1.78% arb on Novig's pre-game straights and 0.04% after Kalshi's taker fee.

Why a no-vig leg finds more arbs

Every sportsbook price carries the book's margin. On a -110/-110 line, each side implies 52.38%, and the two add up to 104.76%. That extra 4.76% is the hold, and it's on both sides of every market the book posts.

Two sportsbooks can disagree on a game and still not produce an arb, because both prices have the hold in them. Say FanDuel has Team A at -105 and another book has Team B at -110. Those imply 51.22% and 52.38%, which add up to 103.60%. The books disagree, and there's still no arb.

Put Team B on a prediction market instead. Novig, Kalshi, Polymarket US and ProphetX match traders against each other, so the price is set by orders on both sides rather than a book protecting a margin. If Team B trades at 47¢ there, the total drops to 98.22% and the same FanDuel price becomes a 1.78% arb.

Converting contract prices to odds

Prediction markets quote a price per contract that pays $1 if the outcome happens. The price is the implied probability. A 47¢ contract implies 47%.

• Decimal odds: 1 divided by the price. 1 / 0.47 = 2.13.

• American odds: for a price under 50¢, (1 / price − 1) × 100, so +113. For a price over 50¢, −100 / (1 / price − 1). A 60¢ contract is −150.

• Stake: the number of contracts times the price. $478.52 buys 1,018 contracts at 47¢, which pays $1,018 if it wins.

The odds converter does these conversions both ways, including implied probability.

How each platform's fee changes the price

A fee raises the effective price of a contract, and a higher effective price means worse odds. Add the fee per contract to the price before converting:

On $1,000, the Novig pre-game version stakes $521.48 at FanDuel and $478.52 on Novig, and both outcomes return $1,018.13 for an $18.13 profit. The Kalshi version, sized on the effective price, returns $0.37.

Here's where each fee comes from, per each platform's published schedule:

• Novig charges no fee on pre-game straight trades. Live straights carry 0.03 × price × (1 − price) per contract on most sports and 0.06 on college football, hockey and baseball. Makers pay $0.

• Kalshi charges takers 0.07 × contracts × price × (1 − price), rounded up to the next cent. Maker fees are a quarter of that and apply only in some markets.

• Polymarket US charges takers 0.0695 × contracts × price × (1 − price) on sports and pays makers a rebate.

• ProphetX takes a commission on net winnings rather than a per-contract fee, so the cost only lands on the leg that wins. Check its current rate before sizing, since published reviews in 2026 don't agree on the number.

A maker order avoids most of these fees, but it only works if someone takes the other side of your order before the sportsbook price moves.

What to check before placing both legs

• Liquidity: A contract price is only good for the size resting on the book at that price. A $478 order at 47¢ may fill partly at 47¢ and the rest at 48¢ or 49¢. Check the depth before you commit the sportsbook leg.

• Settlement rules: A sportsbook grades a bet under its house rules. A prediction market settles a contract under the contract's own terms. Before you call it an arb, make sure both legs pay on the same event, including whether overtime counts and what happens if the game is postponed.

• Order of placement: Place the prediction market leg first when liquidity is thin, since that's the leg most likely to fill at a worse price. Place the sportsbook leg first when its price is the one most likely to move.

• Account limits: Only the sportsbook leg carries the risk of a limit. The account limits guide covers what Massachusetts found when it reviewed how books limit winners.

• Availability: Prediction market access is being fought out state by state, and each platform's list of available states changes. Check the platform's current state list before funding an account.

Prediction market arbitrage questions

Is a prediction market leg as safe as a sportsbook leg?

Both are regulated. Sportsbooks are licensed by each state, and Novig, Kalshi, Polymarket US and ProphetX operate as designated contract markets under the CFTC. The risks differ: a sportsbook can limit the account, while a prediction market leg can fill short when the book is thin.

Can both legs be on prediction markets?

Yes, when two markets price the same contract differently. The fees then apply on both legs, which makes the margin harder to clear.

Why isn't every prediction market price an arb against a sportsbook?

The two usually track each other closely. An arb only appears when one side moves and the other hasn't caught up yet, which is why speed matters.

Do I need a separate calculator for contract prices?

No. Convert the effective price to decimal or American odds and use the standard arb split.

Run the numbers

Convert the contract price in the odds converter, add the fee to the price, then size both legs in the arbitrage calculator.