Beginner Guide
How betting odds work — and what the book isn't telling you
Every odds format, what the numbers mean, the implied-probability math behind them, and how to spot when a price is rigged against you.
The three odds formats
American odds use plus and minus signs around a base of $100. Negative odds (-110, -150, -300) mark favorites: the number is how much you must risk to win $100. Positive odds (+110, +180, +500) mark underdogs: the number is how much you win on a $100 risk.
Decimal odds (2.10, 1.91, 3.50) are a single multiplier of your stake including the original wager. A $100 bet at 2.10 decimal returns $210 total — $110 profit. Used by Pinnacle, Betfair, most non-US books, and increasingly by sharp US bettors because they're easier to multiply for parlays.
Fractional odds (5/2, 1/2, 10/1) are mostly British and show profit-to-stake. 5/2 means $5 profit on $2 risked. Convert to decimal by adding the fraction to 1: 5/2 = 2.5 + 1 = 3.50 decimal = +250 American.
Implied probability — the only number that matters
Behind every price is an implied probability — the book's stated likelihood of the outcome. Implied probability = 1 / decimal odds.
A -110 spread (1.91 decimal) implies 52.38%. A +200 underdog (3.00 decimal) implies 33.3%. A +500 longshot (6.00 decimal) implies 16.7%.
Your job as a bettor is comparing the book's implied probability to your own estimate of the true probability. If you think the +200 dog wins 40% of the time (not 33%), it's a +EV bet.
Where the vig hides
Sum the implied probabilities of both sides of any market. A fair market would sum to exactly 100%. A typical US spread market (-110/-110) sums to 104.76% — the extra 4.76% is the book's vig.
That vig is why betting -110 favorites with a coin flip's edge loses money. You need to win 52.4% of -110 bets just to break even — and most casual bettors don't.
Sharp shops like Pinnacle and Circa often run 2–3% vig on majors instead of 4.5–5%. That difference compounds enormously over thousands of bets.
Devigging — finding the 'fair' price
To estimate the true probability of an outcome from a market, you 'devig' the price. The simplest method: divide each side's implied probability by the total (over 100%).
Example: -110/-110 → 52.38% / 104.76% = 50.00% fair on each side.
Sharper shops give better starting points for this calculation, which is why pros anchor their fair-value estimates to Pinnacle, Circa, or Betfair Exchange — and why Parlae devigs sharp lines automatically to surface +EV bets across every other US book.
Reading common American odds quickly
-110 → bet $110 to win $100 (52.4% implied). Standard juiced spread.
-150 → bet $150 to win $100 (60.0% implied). Moderate favorite.
+150 → bet $100 to win $150 (40.0% implied). Moderate underdog.
-300 → bet $300 to win $100 (75.0% implied). Heavy favorite.
+500 → bet $100 to win $500 (16.7% implied). Longshot.
Frequently asked questions
What does -110 mean in betting?
-110 means you must risk $110 to win $100. It implies a 52.38% win probability and is the standard juiced price US books charge on point spreads and totals.
What does +200 mean?
+200 means a $100 wager pays $200 in profit if it wins. The implied probability is 33.3% (1 / 3.00 decimal). +200 underdogs win less than half the time but pay 2-to-1.
How do I convert American odds to decimal?
Positive odds: decimal = (american / 100) + 1. So +150 → 2.50. Negative odds: decimal = (100 / |american|) + 1. So -150 → 1.667.
How do I calculate implied probability?
Implied probability = 1 / decimal odds. -110 (1.91 decimal) → 1/1.91 = 52.38%. Add both sides of a market — anything over 100% is the book's vig.
Why do both sides of a bet add up to more than 100%?
Because of vig — the book's structural margin. -110/-110 implies 52.38% on each side, summing to 104.76%. That 4.76% is what the book keeps long-term.
What's the difference between odds and probability?
Odds are the price you're being offered. Probability is how often something actually happens. The gap between the book's implied probability and the true probability is where +EV bets live.