What arbitrage (a "surebet") is

Arbitrage — also called a surebet, sure bet or arbing — means backing every possible outcome of the same game, but at different bookmakers, at prices that (added together) guarantee a small profit no matter who wins. This lesson explains the idea and the maths. The next one covers how FinalSkore shows it and why "guaranteed" needs an asterisk.

The one idea behind it

Every price carries an implied probability: 1 ÷ decimal odds. Odds of 2.00 imply 50%, odds of 4.00 imply 25%. Normally a bookmaker prices a market so the two (or three) sides add up to more than 100% — that extra slice is their margin (the vig), and it's why betting is a losing game on average.

Arbitrage flips that. When two bookmakers disagree enough about a price, you can sometimes take the best odds of each outcome from different books and have them add up to less than 100%. That gap below 100% is the locked profit.

A worked two-way example

Take a basketball money line — a clean two-way market (one side must win).

• Book A prices the home team at 2.10.
• Book B prices the away team at 2.10.

Add the implied probabilities: 1 ÷ 2.10 + 1 ÷ 2.10 ≈ 47.6% + 47.6% = 95.2%. That total is below 100%, so an arbitrage exists. The profit margin is roughly (1 ÷ 0.952) − 1 ≈ 5%** of everything you stake.

Splitting the stake so every result pays the same

The trick is to divide your money so that whichever outcome happens, you get back the same amount. You stake in proportion to each outcome's implied probability:

• stake on an outcome = total × (1 ÷ its odds) ÷ (sum of 1 ÷ odds)

With 100 units at 2.10 / 2.10 that's ~50 on each side. Each side returns 50 × 2.10 = 105 units — a locked +5 no matter who wins.

When the odds are lopsided the split shifts, but the principle holds. Say 2.50 and 1.75:

• 1 ÷ 2.50 + 1 ÷ 1.75 = 0.400 + 0.571 = 0.971 (a ~3% edge).
• Stake ~41.2% on the 2.50 side and ~58.8% on the 1.75 side. Both then return the same ≈ 103 units on a 100 stake.

Three-way markets work the same

For a football 1X2 (home / draw / away) you take the best price of each of the three outcomes, possibly from three different books, and check whether 1÷home + 1÷draw + 1÷away falls below 1. If it does, the same proportional split locks a return across all three.

Why real arbs are rare and small

Books usually price with margin, so most markets add up to 104–110%, not below 100. A genuine arb only appears when books briefly disagree — one is slow to move after news or heavy betting. As a result real edges are typically just 1–3% and they vanish in seconds as prices correct. That scarcity, and what happens when you actually try to take one, is the subject of the next lesson.

Arbitrage is a maths fact, not a money machine. The number on the screen is real; capturing it reliably is a different story — read on.