Arbitrage calculator
Enter the two prices you can get on opposite sides of the same bet, name the venue on each side, and this works out the stake split and what the position pays once both venues have been paid.
Enter both legs
The split
| Side | Implied | Fee | Net cost | Stake | Returns |
|---|---|---|---|---|---|
| Side A | — | — | — | — | — |
| Side B | — | — | — | — | — |
- Both sides, posted
- —
- Both sides, net of fees
- —
- Margin, posted
- —
- Margin, net of fees
- —
Profit on the total stake
—
The calculation runs in your browser. Nothing you type is sent anywhere.
Two prices are not a position until both are filled. An arbitrage is a snapshot of two prices that were live moments ago, and it decays in minutes — often in seconds once either venue notices. Confirm BOTH prices at BOTH venues before staking either leg. A position with one leg filled and the other gone is not an arbitrage; it is a one-sided bet you did not intend to make. Venues also limit, void or refuse stakes at their discretion, so the profit is guaranteed by the arithmetic and by nothing else.
Why the fee field changes the answer
A standard arbitrage calculator takes two prices, converts them to implied probability, and reports whatever is left under 100%. That is the right arithmetic for two sportsbooks, because a book's margin is already inside the number it posted and there is nothing further to subtract. It is the wrong arithmetic for a prediction market, where the quote is the price two traders agreed on and the venue's cut is charged separately on top.
The gap is not academic on the positions people actually find. A two-sided position is usually worth a fraction of a percent to low single digits of the stake, and Kalshi's per-contract fee is largest exactly where those positions sit — on contracts priced near a coin flip. A calculator that ignores it will report a position on a pair that costs money to hold.
Pick the venue on each side above and this applies that venue's published schedule: Kalshi (Kalshi taker fee schedule, Jul 2026), Polymarket (Polymarket sports theta, raised 0.03 -> 0.05 Jul 2026), Novig (Novig fee schedule: pregame straights zero-fee, live takers 3%), and ProphetX (ProphetX: 2% commission on net winnings only). A sportsbook is the empty option, which means "no separate fee" and not "free" — the margin is in the price you already typed.
American odds and cents, on the same position
Kalshi and Polymarket sell contracts that settle at $1 or $0 and quote them in cents, and since August 2026 they are barred from showing American odds at all. A sportsbook shows nothing but American odds. A position across the two is therefore typed off two screens speaking two languages, which is how a leg gets entered wrong.
Each side above has its own unit, so you enter what is on the screen in front of you. A 42¢ contract is a 42% implied probability; a −110 price is 52.38%. Under the hood both become the same number, and the stake split is computed on that.
One thing the calculator cannot check for you: the two legs must be the same bet on the same line. Over 4.5 at one venue against under 5.5 at another is a middle, not a two-sided position — both legs can win, and both can lose.
Arbitrage calculator FAQ
How does an arbitrage calculator work?
It converts both prices to implied probability and adds them together. If the total is below 100%, the shortfall is the margin, and each leg's stake is its own implied probability divided by the total so both legs return the same amount whichever side wins.
How do I split my stake between two venues?
In proportion to what each side costs. A side implying 47% of a certainty takes 47 parts of the stake and a side implying 51% takes 51, so the two returns come out equal. This calculator does that split net of each venue's fee, which shifts it slightly toward the venue that charges more.
Does this arbitrage calculator include Kalshi fees?
Yes. Selecting Kalshi on either side applies its published per-contract trading fee, which is a curve that peaks near 50 cents and shrinks toward both extremes, including Kalshi's rounding of the fee up to the whole cent.
How do I enter a Kalshi or Polymarket price?
Switch that side's unit to cents and enter the contract price, for example 42 for a 42-cent contract. Prediction markets are barred from displaying American odds, so cents is what their screens show.
What is a good arbitrage margin?
On public venues, a fraction of a percent to low single digits of the total staked. A figure above roughly eight percent almost always means the two legs are not the same bet, one price is stale, or a market is settling a different outcome under a similar name.
Can an arbitrage lose money?
Yes, in three ways this calculator can show you and one it cannot. It can be negative once fees are applied; the stake split can be wrong; and the two legs can be different lines. The one it cannot see is a leg that fails to fill, which leaves a one-sided bet rather than a position.