Surebet Calculator — Arbitrage Betting Math

How an arbitrage opportunity exists at all

Every book prices an event so the implied probabilities sum to more than 100% — that overround is the margin. But books price independently. When Book A goes 2.10 on one side and Book B goes 2.05 on the other, the combined implied probability is 47.6% + 48.8% = 96.4% — under 100%. Split your stake in proportion to the inverse odds and every outcome pays the same amount, which is more than you staked. That gap is the arb.

In the US, arbs appear most often in three windows: openers (books post at different times and disagree before the market consolidates), line moves (one book adjusts, another lags for minutes), and state-specific pricing (a book heavy on local action shades the home team, an out-of-state book doesn’t). The multi-state patchwork — 30+ legal jurisdictions, each with its own books — creates more of these windows than a single national market would.

The four problems nobody puts in the Instagram post

Straight talk from the other side of the counter: arbing is not a career. Margins run 1–3%, stakes get capped, and the accounts that do it are on borrowed time. As a way to squeeze a bonus conversion or lock occasional opener disagreements, it works. As a business model, the books wrote the terms to make sure it doesn’t.

FAQ — surebets & arbitrage

Is arbitrage betting legal in the US?

Yes — placing bets at multiple licensed sportsbooks is legal in every regulated state. What it violates is the sportsbooks’ terms of use, and their remedy is limiting or closing your account, not prosecution.

How much can you make on a surebet?

Typical arbs return 1–3% of total stake. On $500 split across two books that’s $5–15 per opportunity, before the time cost of finding it and the risk of a voided leg. The calculator above shows the exact locked figure for any prices you find.

Overround or hold — which one is this?

The figure the calculator returns is the overround: the implied probabilities summed, minus one. A pair of prices at 1.95 / 1.95 sums to 102.56%, so the overround is 2.56%. The hold — the share of total stakes the book actually keeps if it balances the action — is that same excess divided by the total: 2.56 / 102.56 = 2.50%. Hold is always the slightly smaller number, and it is the one a trading desk quotes. We show overround because it is what you compare across books when hunting a gap.

Why does my arb show a negative margin?

Because most pairs of prices don’t form an arb — the combined implied probability stays above 100% and the book margin covers both sides. Arbs require genuinely divergent prices, usually across different books or during line moves.

What’s the difference between a surebet and dutching?

A surebet covers every outcome for a locked result before stake limits, voided legs and line movement. Dutching covers only the outcomes you choose — equal profit if one of your selections wins, full loss if none does. Same stake-splitting math, different risk profile.