Implied Probability: A Working Cheatsheet
American odds to implied probability in your head, in under 10 seconds.
If you cannot convert odds to implied probability on the fly, you cannot tell whether a market is offering you a bet or taking one.
American odds, simplified: - +100 ≈ 50%. The line everyone calibrates around. - +200 ≈ 33%. Two-to-one underdogs. - +300 ≈ 25%. Three-to-one underdogs. - -110 ≈ 52%. Standard vig. - -200 ≈ 67%. Two-to-one favorites. - -300 ≈ 75%. Three-to-one favorites.
The formula: for positive odds, implied = 100 / (odds + 100). For negative odds, implied = |odds| / (|odds| + 100). Memorize the half-dozen anchor points and interpolate.
Key insight: every sportsbook line has 4-6% of vig built in. A 'fair' coin flip is offered at -110/-110, which implies the book thinks it is closer to 52.4% / 52.4%. The vig is the cost of access. Beating it consistently is harder than beating the underlying probability.