Every number on a sportsbook screen represents a probability disguised as a payout. New bettors often see +150 and think "free money," or see -110 and assume it's a coin flip. Neither interpretation is correct.
This is the foundational lesson in understanding what you're truly betting on. The math behind the line matters far more than which team you prefer.
Favorites vs. Underdogs
The minus sign (-) denotes the favorite. The plus sign (+) denotes the underdog. The number tells you how much you need to risk, or how much you stand to win, relative to a $100 bet.
For example, a -110 line means you must bet $110 to win $100. The sportsbook asks for more than you get back because it believes that outcome is more likely.
A +150 line means a $100 bet wins $150. The larger payout reflects a less probable outcome, according to the sportsbook.
The key takeaway: the odds don't predict which team will win. They reflect the market's assessment of likelihood, adjusted for the sportsbook's built-in margin.
Implied Probability
Every set of odds carries an implied probability -- the percentage chance the market assigns to that result. Calculating it removes the dollar figures and reveals the underlying math.
For a favorite at -110:
Implied Probability = 110 / (110 + 100) = 110 / 210 ≈ 52.4%
For an underdog at +150:
Implied Probability = 100 / (150 + 100) = 100 / 250 = 40.0%
Applying those numbers to real payouts:
A $100 bet at -110 returns approximately $190.91 total -- your $100 stake plus about $90.91 profit. The implied probability of winning is roughly 52.4%. The sportsbook prices this outcome as slightly more likely than a coin flip.
A $100 bet at +150 returns $250 total -- your $100 stake plus $150 profit. The implied probability is 40.0%. The sportsbook estimates this outcome occurs about four times out of ten.
Notice these two implied probabilities add to 92.4%, not 100%. The missing 7.6% represents the sportsbook's built-in edge -- the vigorish or "vig."
The Vig (or Juice)
The vig is how sportsbooks profit. It's the commission embedded in every line.
In a fair market with evenly matched teams, odds would be +100 on both sides -- bet $100 to win $100 -- translating to a 50% implied probability each, totaling 100%.
Sportsbooks never offer +100 on both sides of a fair game. Instead, they typically offer -110 on both sides. That difference represents the vig.
With a typical -110/-110 spread, each side implies a probability of about 52.4%, totaling 104.8%. The extra 4.8% is the sportsbook's guaranteed margin. Over many bets, this ensures profit regardless of the event's outcome.
The vig isn't hidden or deceptive; it's a normal cost of doing business in sports betting. The bettor's task is to find bets where their own assessment of probability exceeds the line's implied probability, overcoming the vig.
Decimal Odds
While American odds (+150, -110) are standard in the U.S., decimal odds are common in Europe, Canada, and Australia and are easier to calculate.
Decimal odds represent the total return on a $1 bet, including your stake. To convert American odds to decimal:
- For a favorite at -110: 1 + (100 / 110) = 1.91
- For an underdog at +150: 1 + (150 / 100) = 2.50
A $100 bet with decimal odds of 1.91 returns $191 total; with 2.50, $250 total. The math is the same; only the presentation differs.
To calculate implied probability from decimal odds: divide 1 by the decimal odds.
For 1.91 decimal: 1 / 1.91 ≈ 52.4%
For 2.50 decimal: 1 / 2.50 = 40.0%
This offers a quicker way to estimate probabilities.
What This Means for You
Every bet has a break-even percentage determined by the odds. At -110, you must win about 52.4% of the time to break even. At -200, that threshold rises to 66.7%.
Because of the vig, betting long-term is usually a losing proposition unless you consistently spot bets where your assessed chance of winning is greater than the implied probability--i.e., where the line is mispriced.
You beat the sportsbook not by just picking winners, but by betting on favorable odds that offer you an edge above the vig.
The One Rule
Sports betting involves risk; any bet can lose. Only wager what you can afford to lose. Must be 21+ (varies by jurisdiction). Call 1-800-GAMBLER for help.
Put This Into Practice
Reading about a number and pricing one yourself are different skills. Both of ours are free and neither costs you a cent.
- Try the PFNN Sportsbook Trainer — play the real markets with points instead of money, and find out whether your read holds up before it ever costs you anything.
- Play PFNN Pick'em — pick the winners straight up every week, free, with no wagering and no barrier.
- Play Pick'em Against the Spread — the same slate priced with the spread, which is the skill this series is actually teaching.