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Polymarket and sportsbook arbitrage
Arbs research desk · Updated 2026-11-01 · Reviewed 2026-11-01 · 2 min read
The short answer: An arb is backing every outcome at prices that add up to less than 100% after costs, so you profit whichever side wins. Between Polymarket and a book that means the Polymarket price plus its taker fee, plus the book’s implied probability, under $1.
The test
Write each leg as its cost per $1 of payout. For Polymarket that is the price plus the fee; for a book it is one over the decimal odds.
Polymarket cost = p + r × p × (1 − p); book cost = 1 ÷ d
If the costs add to less than $1, every outcome is covered for less than it pays:
arb when the sum of costs is under 1; return = 1 ÷ sum − 1
Splitting the stake
To win the same amount whichever side wins, stake each leg in proportion to its cost. With a total stake S:
payout P = S ÷ sum of costs; stake on each leg = P × its cost
Worked example: Polymarket 52¢ on one side, +115 at a book on the other, $1,000 total
- Polymarket cost: 0.52 + 0.05 × 0.52 × 0.48 = 0.5325
- Book cost: 1 ÷ 2.15 = 0.4651
- Sum: 0.9976, under 1, so it is an arb
- Payout: $1,000 ÷ 0.9976 = $1,002.41
- Stakes: $533.76 on Polymarket (1,002 shares) and $466.24 at the book
$2.41 profit on $1,000, a 0.24% return. Without the fee it would have been 1.5%.
The arbitrage calculator runs the test and splits your stake.
Why gaps appear
Books move on their own customers’ money and their traders’ calls; Polymarket moves on its order book. After an injury report or a lineup change one side often updates before the other. Most of the time the gap is a point or two, which the fee and spread eat.
Settlement traps
An arb is only an arb if both legs settle on the same event. Check each of these on both venues before you place anything:
- Overtime and extra time: included, or regulation only
- Tennis retirements and walkovers: void, or graded on who advanced
- Postponed or relocated games: void after how long
- Player props when the player does not start
- Two-way against three-way: a soccer moneyline with and without the draw
Practical limits
- Prices move between your two orders; place the harder-to-fill leg first
- Polymarket depth at the quoted price may be smaller than your stake, which raises your average price
- Books may limit accounts that only bet arbs
- Money sits on two venues until both settle
Try it: Arbitrage calculator
Frequently asked questions
Is Polymarket arbitrage legal?
Betting both sides is not itself prohibited, but you must be allowed to use each venue where you are. See the eligibility guide.
How big are Polymarket arbs?
Usually well under 1% after fees, and they close quickly. Larger gaps tend to come with a settlement difference.
Do I need a lot of money?
No, but small arbs on small stakes earn very little, and fees and spreads take a bigger share.