Arbitrage Betting: Risk-Free Profit

An arb lets you bet on every outcome of a game and still come out ahead no matter who wins. The math is airtight. The catch is that sportsbooks hate it — and that's why the pros mostly do something else.

What an arbitrage bet is

Arbitrage betting means backing every possible result of an event at different sportsbooks, at a combination of prices that guarantees a profit regardless of the outcome. It's not a prediction — you're not guessing who wins. You're exploiting the fact that two books have priced the same game differently enough that their numbers no longer add up to a complete, vig-loaded market.

When Book A is unusually generous on the underdog and Book B is unusually generous on the favorite, backing both sides at those two books can leave a locked-in edge on the table. That gap is the arb.

The math: when does an arb exist?

Convert each side's odds to decimal, then take the inverse (1 ÷ decimal odds) of each and add them together. That sum is the total "cost" of buying the whole market:

If the inverses sum to less than 1, you can cover every outcome for less than you'll get back — a guaranteed profit. If they sum to more than 1 (which is nearly always the case, thanks to the vig), no arb exists.

Worked example

Say a two-way market splits across books like this:

Outcome	Best odds	Decimal	Inverse (1/odds)
Team A (Book 1)	+120	2.20	0.4545
Team B (Book 2)	−105	1.952	0.5123
Total	—	—	0.9668

The inverses sum to 0.9668, which is under 1 — so this is an arb. The guaranteed return is:

Splitting the stakes

To guarantee the profit no matter who wins, you have to size each bet so both outcomes pay back the same total. Split your total stake in proportion to each side's inverse. On a $1,000 bankroll for this event:

If this wins…	Payout	Total staked	Profit
Team A (+120)	$470.13 × 2.20 = $1,034.30	$1,000	+$34.30
Team B (−105)	$529.87 × 1.952 = $1,034.30	$1,000	+$34.30

The catch — and it's a big one

If arbitrage were free money, everyone would do it forever. Here's why they don't:

• Books limit and ban you. Sportsbooks are very good at spotting arbers — the mirrored, oddly-sized bets across accounts are a fingerprint. Get flagged and your limits get cut to pennies, or your account is closed. Arbing is the fastest way to get restricted.
• The margins are thin. Real arbs are usually 1–3%, not 3.4%. To make meaningful money you need large stakes on tiny edges — the exact behavior that gets you banned.
• Lines go stale. One of your two prices can move or vanish in the seconds between placing the first leg and the second, leaving you with a one-sided bet you never wanted.
• Capital is tied up. You need money spread across many books simultaneously, sitting idle, just to chase fractions of a percent.

Arbitrage vs. +EV betting

Arbitrage and +EV betting both come from the same source — books disagreeing on a price — but they play out very differently. An arb takes a guaranteed small profit today. A +EV bet takes a bigger expected profit that only shows up over many bets, because you're betting a single side at a price that beats its true value.

Serious bettors overwhelmingly favor +EV for longevity. Betting one side, at a normal-looking size, on genuinely mispriced numbers looks like ordinary sharp play — not the tell-tale hedged pattern of an arber. Accounts survive far longer, and the long-run ceiling is much higher. Arbitrage caps your upside at the arb percentage; +EV compounds.

Many bettors use both: arbs when a fat one appears, +EV as the day-to-day engine. The key is knowing which is which the instant a line moves.

Frequently asked questions

What is arbitrage betting?

Arbitrage betting (an 'arb') is backing every possible outcome of an event at different sportsbooks, at prices that together guarantee a profit no matter what happens. It becomes possible when two books disagree enough that their combined implied probabilities sum to less than 100 percent. Worked example: one book hangs an underdog at +120 while another still has the favorite at −105. Those prices imply roughly 45.5% and 51.2% — a total of 96.7% — so staking the right amounts on each side returns a profit whichever team wins. Arbs exist because books move independently and at different speeds; the window is usually brief, and the profit is small — typically a low single-digit percentage of total stakes.

How do you know if a bet is an arbitrage opportunity?

Convert each side's odds to decimal, take the inverse of each (1 divided by the decimal odds), and add the inverses together. If the sum is less than 1, an arbitrage exists, and your guaranteed return is 1 divided by that sum, minus 1. Example: +120 is decimal 2.20 and −105 is about 1.95; the inverses are 0.455 and 0.512, summing to 0.967 — under 1, so it's an arb paying about 3.4% on total stakes. To split your money, stake each side in proportion to its inverse — divide each side's inverse by the sum and bet that share of your total. Both outcomes then pay the same amount, locking the profit.

Is arbitrage betting worth it compared to +EV betting?

Arbitrage locks in small guaranteed profits, and that certainty is its entire appeal — but it carries heavy costs. You need large stakes to make tiny percentages meaningful, your bankroll gets scattered across many books, and the betting pattern is unmistakable, so recreational books identify and restrict arbers quickly. There's also execution risk: a line moves or a leg gets voided after you've placed one side, leaving a position you never wanted. Most serious bettors graduate to +EV betting instead — it carries short-term variance, but the same price discrepancies power it, the long-run ceiling is far higher, and +EV action blends into ordinary betting longer before the books react.

What happens if one leg of an arbitrage bet is voided or the line moves?

This is the real risk in arbitrage: the profit is only guaranteed once every leg is confirmed. If a book voids one side — for an obvious pricing error, a rules difference, or a postponed event — you're left holding a single ordinary bet you never wanted, at whatever the market now says it's worth. The same happens when the line moves between your first and second legs. Sharp practice: place the leg at the slower or stricter book first, so failures happen before you're committed; read each book's void and settlement rules; and before hedging out at a loss, check whether the surviving leg is +EV on its own — sometimes the leftover position is worth keeping.

How quickly do sportsbooks limit arbitrage bettors?

Faster than almost any other kind of winning customer. Arbitrage leaves a fingerprint books are explicitly trained to detect: precise, odd-looking stake sizes calculated to equalize returns, bets placed exclusively on off-market prices moments after a line moves, and action clustered wherever a price is stale. There's no fixed timeline, but the more your account resembles arbitrage software output, the sooner the limits arrive. You can slow the clock by rounding stakes, mixing in ordinary bets, and skipping obvious error prices. Note where the pressure comes from: sharp books that welcome winners tolerate the action — it's the slower recreational books, whose lagging lines create the arbs in the first place, that shut the door.

Iron Marker is an analytics tool, not a sportsbook, and this guide is educational — not betting or financial advice. Odds shown are illustrative. Must be 21+. Problem gambling? Call 1-800-GAMBLER.