Arbitrage betting
calculator.

What an arbitrage bet is

Two venues will sometimes price the same game so differently that the two sides add up to less than 100%. Back both sides in the right proportion and one of them must win, so you collect more than you staked no matter the result. That gap is the arbitrage, and it usually lasts minutes: it closes as soon as either venue moves its line.

The classic version is a sportsbook against another sportsbook. The newer version, and the one with the most gaps right now, is a prediction market like Kalshi or Polymarket against a sportsbook, because exchanges are priced by traders while books are priced by a trading desk, and the two often disagree.

How the calculator works

Every price converts to an implied probability. +130 at a sportsbook is 100 ÷ (130 + 100) = 43.5%. A 53¢ Kalshi contract is simply 53%. Add the two sides together: 96.5% in that example. Anything under 100% is an arb, and the shortfall (3.5% here) is the margin.

To lock the profit you stake in proportion to each side's implied probability, so both outcomes return the same amount. With $500 across those two prices you would put about $274 on the 53¢ side and $226 on the +130 side; whichever wins returns about $518, a locked profit of roughly $18, or 3.6%. The calculator does that arithmetic for any pair of prices, and the "Already placed leg A" mode works backwards from a stake you have already made to the exact hedge on the other side.

Odds formats it accepts

American odds are what US sportsbooks show: +130 means a $100 stake wins $130, -150 means you stake $150 to win $100. Decimal odds, used by most books in the UK, Europe and Australia, are the total return per $1 staked, so 2.30 returns $2.30 including the stake. Cents are how Kalshi and Polymarket quote contracts: a YES contract at 53¢ pays $1 if it settles YES, which is the same as a 53% implied probability. Mix formats freely; the calculator converts each leg independently.

Why the real profit is a little smaller

The calculator shows the clean number. In practice, exchange fees (Kalshi charges a small fee on winning contracts, Polymarket on some markets), minimum stake sizes and books rounding your stake all shave a fraction off. Bigger risks are practical: the price on the second leg moving before you place it, a book voiding a bet, a market resolving on a technicality you did not read, or a book limiting your account after a run of arbs. None of that changes the math; it changes how you execute it, which is what the playbook inside OddsHedge is for.