How American Sports Odds Really Work
American odds are built around a standard $100 baseline. As highlighted in the discussion:
- Minus signs (-) indicate the Favorite: The number reveals what you must risk to win $100 profit (e.g., -210 requires risking $210 to win $100).
- Plus signs (+) indicate the Underdog: The number reveals the pure profit returned on a $100 stake (e.g., +175 wins $175 plus your $100 stake back).
- Odds are disguised probabilities: Converting them reveals how often the book expects each side to win.
Underdog Implied % = 100 / (American Odds + 100)
Favorite Implied % = |American Odds| / (|American Odds| + 100)
Because sportsbooks take a cut (the vig or juice), summing both probabilities always exceeds 100% (typically 104% to 108%). De-vigging isolates the true no-juice probability.
The Math of Long-Term Underdog Value
Betting solely on underdogs does not make you profitable by itself. It merely reshapes variance: rarer wins, larger single payoffs, and longer drawdown streaks.
When is an underdog actually worth betting?
An underdog bet possesses Positive Expected Value (+EV) only when your assessed true win probability is strictly greater than the break-even de-vigged implied probability:
EV = (True Win % × Decimal Payout) - 1.00
How does the Kelly Criterion manage underdog bankroll risk?
Because underdogs hit less frequently than favorites, standard bet sizing often causes catastrophic drawdown runs. The Kelly formula calculates the mathematically optimal fraction of bankroll to wager to maximize compound growth while preventing ruin:
f* = (b × p - q) / b, where b is net payout odds, p is true win rate, and q = 1 - p.
Why do retail bettors lose on underdogs over time?
Retail bettors frequently fall victim to the "longshot bias"—overvaluing extreme underdogs—or fail to realize the bookmaker vig eats away 4% to 7% on every single wager, requiring an underdog win rate far higher than natural sports variance allows without a true model edge.