What sports betting odds actually are
Odds are a sportsbook's price tag on a future outcome. They do two jobs at once: they quote a payout (how much you win per dollar risked) and they encode an implied probability (the break-even win rate that payout demands). Every notation - American, decimal, fractional - describes the same underlying number in a different alphabet.
The price you see is never the sportsbook's true estimate of the outcome. It is the true estimate plus a margin called the vig, juice, or hold. That margin is the reason implied probabilities on a two-sided market sum to more than 100%.
American odds (moneyline odds)
American odds are quoted as a positive or negative number anchored to $100. A minus price (for example -150) tells you how much you must risk to win $100 in profit. A plus price (for example +200) tells you how much profit a $100 stake returns.
At -150 you risk $150 to win $100, for a total return of $250. At +200 you risk $100 to win $200, for a total return of $300. The same underlying probability can be written as either a minus or plus price depending on which side of the market you take - before vig, -150 and +150 are mirror images.
- Implied probability of a negative price: |odds| / (|odds| + 100). -150 → 150/250 = 60%.
- Implied probability of a positive price: 100 / (odds + 100). +200 → 100/300 ≈ 33.3%.
- Even money in American format is +100 (equivalent to 2.00 decimal, 1/1 fractional, 50% implied).
Decimal odds (European format)
Decimal odds (1.67, 2.50, 3.00) are the dominant format outside the US and the cleanest format for math. The number is a payout multiplier that includes your stake: a $10 bet at 3.00 returns $30 total - $20 profit plus the original $10 back.
Implied probability is simply 1 divided by the decimal odds. 2.00 → 50%. 1.50 → 66.7%. 4.00 → 25%. Because of this, decimal is the format used by virtually every quantitative tool, including ours.
Fractional odds (UK / horse racing format)
Fractional odds (5/2, 7/1, 1/4) come from the British racing tradition and still dominate UK sportsbooks. They express profit relative to stake: 5/2 means $5 profit for every $2 risked, so a $10 bet returns $35 total ($25 profit + $10 stake).
Convert to decimal by dividing the fraction and adding 1: 5/2 → 2.5 + 1 = 3.50. Convert to implied probability by adding the numerator and denominator and dividing the denominator by the sum: 5/2 → 2 / (5 + 2) ≈ 28.6%.
Conversion cheat sheet
These six identities are all you need to move between any two formats. American (A), decimal (D), implied probability (p):
- A < 0 → D = 1 + (100 / |A|). Example: -150 → 1 + 100/150 ≈ 1.667.
- A > 0 → D = 1 + (A / 100). Example: +200 → 1 + 200/100 = 3.00.
- D ≥ 2.00 → A = (D − 1) × 100. Example: 2.50 → +150.
- D < 2.00 → A = −100 / (D − 1). Example: 1.50 → -200.
- p = 1 / D. Example: D = 1.91 → p ≈ 52.4%.
- Fractional N/M → D = (N / M) + 1, p = M / (N + M).
Implied probability vs true probability
Implied probability is the break-even win rate the price assumes. True probability is whatever the real-world chance of the event is. The gap between them is where edge and vig live.
On a standard -110 / -110 NFL spread, both sides imply about 52.38%, summing to 104.76%. The 4.76% above 100% is the sportsbook's hold. A no-vig calculation splits that margin proportionally to estimate fair lines roughly equivalent to 50% / 50%.
Why -110 is the standard US price
Point spreads, totals, and most prop markets in the US are priced near -110 on both sides. That gives the book a 4.5%–4.8% theoretical hold per market. Some books offer reduced juice at -105 (about 2.4% hold) or, in baseball, dime lines where favorite and underdog are within 10 cents of each other.
Lower juice means smaller break-even win rates, which means more bets you place are actually profitable in the long run. Shopping for the best price across multiple books is the single highest-ROI habit a recreational bettor can adopt.
Frequently asked questions
What do +200 odds mean? You profit $200 on a winning $100 bet. The price implies roughly a 33.3% chance of winning.
What do -200 odds mean? You must risk $200 to profit $100. The price implies roughly a 66.7% chance of winning.
Are American, decimal, and fractional odds different bets? No. They describe identical payouts in different notation. Most sportsbook apps let you toggle the display format in settings.
Which odds format is best for math? Decimal. The break-even probability is just 1 / odds, and the payout is just stake × odds.