The Minus (-): The Favorite and the Cost of High Probability
A minus sign (-) precedes the odds for the favored team or outcome. This number indicates the amount a person must risk to win $100. Because the favorite is statistically more likely to win, the potential reward is smaller than the amount risked.
Example: If the Kansas City Chiefs have odds of -150, you must wager $150 to secure a $100 profit. Your total return upon winning would be $250 ($150 stake + $100 profit).
This structure demands a higher upfront investment for a lower proportional return, reflecting the higher probability of the outcome. The University of Nevada, Las Vegas’s Center for Gaming Research provides foundational materials on the mathematics of bookmaking that confirm this risk-reward structure.
The Plus (+): The Underdog and the Reward for Assumed Risk
A plus sign (+) precedes the odds for the underdog. This number indicates the amount of profit you will win for every $100 you risk. Betting on the underdog carries a higher potential reward to compensate for the lower probability of winning.
Example: If the Detroit Lions have odds of +130, a $100 wager will secure a $130 profit. Your total return upon winning would be $230 ($100 stake + $130 profit).
This model offers a larger payout than the original stake, rewarding bettors for taking on a statistically less likely outcome. The inverse relationship between risk and reward is a fundamental principle in financial markets, as detailed in publications like the Exploiting risk–reward structures in decision making under uncertainty.
The Bookmaker’s Advantage: Calculating the Vigorish (Vig)
Neither plus nor minus bets exist in a vacuum. Sportsbooks build a mathematical advantage, known as the “vigorish” or “vig” (also called juice), directly into the odds. This is their fee for facilitating the wager, and it guarantees them a long-term profit regardless of which side wins. Acknowledging this built-in cost is critical for any serious analysis.
How to Spot the Hidden Cost in All Bets
The vig is visible by calculating the implied probability of both sides of a bet. If the probabilities add up to more than 100%, the excess represents the bookmaker’s margin. A market with no vig would have implied probabilities that sum to exactly 100%.
A Practical Example: Finding the Vig in a Moneyline Market
Consider a matchup between two teams:
- Team A (Favorite): -120
- Team B (Underdog): +100
A bettor might assume this is a balanced market, but the vig is present. A $120 bet on Team A profits $100. A $100 bet on Team B profits $100. If equal money comes in on both sides (e.g., $120 on Team A, $100 on Team B), the bookmaker takes in $220.
- If Team A wins, the bookmaker pays out $220 ($120 stake + $100 profit), breaking even.
- If Team B wins, the bookmaker pays out $200 ($100 stake + $100 profit), keeping $20.
The bookmaker’s goal is to balance the action to guarantee a profit from this vig. This structural advantage is the primary reason sustained profitability in sports betting is so difficult, a fact often discussed in statistical analyses of gambling outcomes.
Beyond the Payout: Translating Odds into Implied Probability
To make an evidence-based decision, you must convert odds into a more useful metric: implied probability. This percentage represents the bookmaker’s assessment of an outcome’s likelihood, including their vig. A successful bettor finds discrepancies between this implied probability and their own more accurate assessment of the event.
The Formula for Favorites (-)
For minus odds, the calculation is:
Implied Probability = Odds / (Odds + 100)
- Example (-150): 150/(150+100)=150/250=0.60, or a 60% implied probability.
The Formula for Underdogs (+)
For plus odds, the calculation is:
Implied Probability = 100 / (Odds + 100)
- Example (+130): 100/(130+100)=100/230≈0.435, or a 43.5% implied probability.
Notice that 60% + 43.5% = 103.5%. That extra 3.5% is the bookmaker’s vig in this market. Removing this vig is a more advanced step, but simply acknowledging its existence is a major step toward professional analysis.
The Core Question: Which Bet Type Offers Superior Value?
The intelligent professional does not ask whether “plus” or “minus” is “better.” Instead, they ask which specific bet offers value. Value exists when you believe the true probability of an outcome is higher than the implied probability offered by the bookmaker.
Why “Better” is a Flawed Metric
Choosing a side based solely on the plus or minus sign is a losing strategy. A -500 favorite that you assess has a 90% chance to win is a high-value bet. A +200 underdog that you believe has only a 25% chance to win is a poor-value bet despite the attractive payout. The sign is irrelevant; the relationship between price and probability is everything.
A Framework for Identifying Value Bets
- Calculate Implied Probability: Convert the American odds for both sides of a bet into percentages using the formulas above.
- Conduct Independent Analysis: Use your own research, data models, or qualitative assessments to determine your own probability for the outcome. This is where expertise provides an edge.
- Compare and Decide: If your personal probability is significantly higher than the bookmaker’s implied probability, you have identified a potential value bet.
This process transforms betting from a coin flip into a practice similar to value investing, where one seeks to buy assets (in this case, probabilities) for less than their intrinsic worth. See the infographic above for a quick reference on how to convert American odds into implied probability, and how to spot the bookmaker’s hidden margin in any market.
Practical Application: A Case Study in Line Movement
Imagine the Green Bay Packers open at -140 against the Chicago Bears at +120.
- Initial Implied Probability (Packers): 140/(140+100)=58.3%
- Initial Implied Probability (Bears): 100/(120+100)=45.5% (Vig is present)
You conduct your analysis and conclude the Packers have a 62% chance to win, making the -140 a solid value bet. However, you wait. Heavy public betting on the Packers pushes the line to -160.
- New Implied Probability (Packers): 160/(160+100)=61.5%
The line now almost perfectly matches your assessment. The value has decreased. Waiting for a line to move can either increase or evaporate the value of a potential wager. Professionals monitor these movements to find the optimal entry point for their bets.
Conclusion: A Professional’s Approach to Odds Selection
The plus and minus signs are merely risk and reward indicators, not signposts for “good” or “bad” bets. The professional decision-maker discards the notion of a universally “better” sign. Instead, they adopt a market-based approach focused on one objective: identifying pricing errors.
The core of a sophisticated betting strategy is developing an analytical model, whether quantitative or qualitative, that can more accurately predict outcomes than the market average reflected in the odds.
Your task is not to guess winners. Your task is to purchase probability at a discount. A bet at -200 (66.7% implied probability) on an outcome you are certain will occur 75% of the time is a far superior financial decision than a bet on a +200 underdog (33.3% implied probability) that you assess has only a 20% chance of success. The plus or minus is simply part of the price tag; your job is to determine if the asset is worth the price.
