Arbitrage calculator
Do these prices lock a profit whichever side wins? Booksum, return and the stake for each side, for two- and three-way markets, with exchange fees and rounded stakes.
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updates as you typeRun this math on every line, automatically
The Arbitrage screen finds these across your books at net prices after fees, splits the stakes, and hides any arb with a stale leg.
How it works
An arbitrage exists when the best prices on every outcome, taken at different books, add up to less than 100% implied probability:
S = Σ 1/di < 1 → locked return = 1/S − 1
stake on outcome i = T × (1/di) ÷ S, so every outcome returns T ÷ S
Pick a venue per leg and exchange or prediction-market prices are converted to your net price after fees before the booksum is computed; an arb that only exists before fees is not an arb. Rounding stakes to whole dollars changes the result slightly, and the table shows the profit for each outcome after rounding.
Worked example
Book A offers +110 on one side and book B offers −102 on the other.
Booksum S = 1/2.1000 + 1/1.9804 = 0.4762 + 0.5050 = 0.9811 (below 1: an arb)
Return = 1/S − 1 = +1.92%
On $100: stake $48.53 at +110 and $51.47 at −102; either side returns $101.92, profit $1.92
Now suppose the −102 side is a Kalshi contract. After Kalshi's taker fee on a $100 order its net price is −107.3, the booksum becomes 0.9937, and the return falls to +0.63%. That is why fees go in before the arb check.
Things to watch
- Prices move. Place the harder leg (the one most likely to move or be limited) first.
- Limits and voids. A book may cap your stake or void a bet as a palpable error, leaving you one-sided.
- Different rules. Both legs must settle on the same event and rules (overtime, retirements, push rules).
- Thin exchange depth. The quoted price may only be available for a few dollars.