Arbitrage betting explained, and why price gaps are rarely risk-free

Arbitrage betting means backing every outcome of an event at prices that add up to less than 100%, so that on paper you win whichever way it goes. If Player A is 52¢ on one prediction market and Player B is 45¢ on another, buying one share of each costs 97¢ and one of them will pay $1, a gross gap of 3¢. In practice that gap has to survive trading fees on both sides, rules that settle odd results differently on each platform, orders that don't fill at the price you saw, and limits on who can trade where. Most of the time, at least one of those closes it. This page shows how, with real fee figures.

Nothing here is advice to try it. The aim is to show what an arbitrage really involves, so the word "risk-free" doesn't do your thinking for you.

How an arbitrage works on paper

Every price implies a chance. For two players, the two chances add up to about 100% on a single market and to more than 100% at a bookmaker, whose margin is built in. An arbitrage appears when you take the best price for each player from different places and the total comes to less than 100%.

• On prediction markets, add the two prices: 52¢ + 45¢ = 97¢, a gap of 3¢ per pair of shares.
• With bookmaker odds, convert each to a chance and add them: 2.10 and 2.05 imply 47.6% and 48.8%, a total of 96.4%.

The no-vig calculator does the sum for any two prices: a negative margin means the prices add up to less than 100%. The odds converter turns bookmaker odds into the same cents.

Fees: the first thing to close the gap

Both markets charge a fee for orders that fill straight away. Polymarket's sports taker fee is shares × 5% × price × (1 minus price), which at 52¢ is about 1.25¢ a share. Kalshi's fee schedule page, checked on 1 November 2026, lists taker fees of $0.07 to $1.75 per 100 contracts, up to 1.75¢ a contract depending on the price, and shows you the exact fee before you confirm. Together that can be about 3¢ a pair: the whole gap in the example above.

Patient orders that wait on the book avoid Polymarket's fee, which charges takers only, though Kalshi's ATP match series charges a smaller maker fee as well. Either way, a resting order may never fill, so you can't be sure both sides happen. Polymarket fees explained and what is Kalshi have the full fee details.

Different rules for the same match

An arbitrage across two platforms only works if both settle every possible result the same way. For tennis, Polymarket and Kalshi don't, according to the rules on their live ATP match markets, checked on 31 October and 1 November 2026:

Those are exactly the cases where a cross-platform arbitrage stops being one. Kalshi vs Polymarket for tennis sets out the rest of the differences, and what happens if a player retires covers retirements.

Orders that don't fill

An arbitrage needs two trades, and prices can move between them. Polymarket's order documentation says a market order takes a price from the liquidity on the book at that moment, that any amount that can't fill is cancelled, and that if the book moves before you submit, the order may fill only partly or not at all. If your first side fills and the second doesn't, you're left holding an ordinary one-sided position, which is the opposite of what you set out to do. Each market also has a minimum order size: 5 shares on the Polymarket tennis markets checked on 31 October 2026.

Who can trade where

Arbitrage across two platforms means having an account on both, and the platforms decide who may use them. Polymarket's Terms of Use for polymarket.com list restricted territories where trading isn't permitted, including the United States and the United Kingdom. Kalshi says it is accessible from many countries but that some jurisdictions are restricted under its Member Agreement. Check both platforms' own terms for where you live before anything else.

Bookmakers and account limits

With bookmakers there's one more obstacle. In July 2025 the UK Gambling Commission published data from major operators covering about 14.9 million active accounts in 2024. 643,779 of them, 4.31%, had been restricted in some way. The most common restriction was stake factoring, a cap on the maximum stake below what other customers can bet, affecting 2.68% of active accounts. The Commission said it is not within its remit to mandate how operators manage their commercial liabilities, and that being a successful bettor is not a protected characteristic. So a bookmaker can lower your limits or close your account, which ends a strategy that depends on finding the best price every time.

What to take from this

Price gaps between platforms are real and worth understanding, because they show how prices form and where the costs are hidden. But a gap on the screen isn't money: fees, rule differences, partial fills, access rules and account limits all sit between the two. If you want to see how prices move without any of that risk, the 77 Tennis demo shows live prediction market prices with play money.

Sources

Checked on 31 October and 1 November 2026: Polymarket's fee page; Polymarket's order documentation; the rules and minimum order size of live ATP match markets from Polymarket's and Kalshi's public data; Polymarket's Terms of Use; Kalshi's fee schedule and its help pages on fees and signing up; and the UK Gambling Commission's blog post Commercial restrictions by betting operators of 23 July 2025. Where this page and those sources disagree, the sources win.

FAQ

What is arbitrage betting?

Backing every outcome of an event, usually at different bookmakers or markets, at prices that add up to less than 100%, so that on paper you win whichever way it goes.

Is arbitrage betting risk-free?

No. Fees can cancel the gap, platforms can settle unusual results differently, an order can fail to fill, and bookmakers can limit or close accounts. Any of these can turn a paper gain into a loss.

How do you find an arbitrage?

Compare the best price for each player across places. If the two implied chances add up to less than 100%, there's a gap on paper. The no-vig calculator shows this as a negative margin.

Can you arbitrage Polymarket and Kalshi?

Prices can differ, but both charge fees, and their tennis rules differ on walkovers and postponed matches. You also need to be allowed to use both, which depends on where you live.

Do bookmakers limit arbitrage bettors?

UK Gambling Commission data for 2024 show 4.31% of active accounts at major operators were restricted in some way, most often by capping stakes. The Commission said it is not within its remit to mandate how operators manage their commercial liabilities.