Implied probability and the bookmaker margin

Every set of odds carries a hidden probability — and a hidden fee. Once you can see both, prices stop looking arbitrary.

Turning odds into a probability

Decimal odds convert to an implied probability with one division: 1 ÷ decimal odds.

• Odds of 2.00 → 1 ÷ 2.00 = 50%.
• Odds of 1.50 → 1 ÷ 1.50 ≈ 67%.
• Odds of 4.00 → 1 ÷ 4.00 = 25%.

That is the chance the price implies the outcome has. Short odds = high implied probability (a favourite); long odds = low implied probability (an underdog).

The margin (the "vig" or "juice")

Add up the implied probabilities on both sides of a bet and you'll get more than 100%. For a coin-flip game a book might offer 1.91 on each side — that's 52.4% + 52.4% = 104.8%. That extra ~4.8% is the bookmaker margin, also called the vig, juice or overround. It's how the book makes money regardless of who wins.

Why it matters

The margin is the cost of betting. It means the "true" probabilities are a little lower than the raw implied numbers, and that over time the odds are tilted against you. Comparing prices across books — exactly what FinalSkore's odds comparison helps with — finds the lines with the smallest margin, so you keep more of any edge you have.