The Three Odds Formats
Betting odds are the same underlying price expressed in three different notational conventions. The format used depends on the region and the platform, but all three can be converted to one another and to implied probability. Most professional betting platforms allow you to switch formats in your account settings; Betfair and Pinnacle default to decimal.
| Format | Example | Profit on €100 stake | Total return | Implied probability |
|---|---|---|---|---|
| Decimal | 2.50 | €150 | €250 | 40.0% |
| Fractional | 3/2 | €150 | €250 | 40.0% |
| American (positive) | +150 | €150 | €250 | 40.0% |
| Decimal | 1.50 | €50 | €150 | 66.7% |
| Fractional | 1/2 | €50 | €150 | 66.7% |
| American (negative) | −200 | €50 | €150 | 66.7% |
Converting Between Formats
All three formats represent the same price. The conversion formulas are mechanical:
Decimal → Fractional
Subtract 1, express as a fraction. 2.50 − 1 = 1.50 = 3/2. 4.00 − 1 = 3.00 = 3/1. For non-integer results: 2.25 − 1 = 1.25 = 5/4.
Decimal → American
If decimal ≥ 2.00: American = (decimal − 1) × 100. Decimal 2.50 → +150. If decimal < 2.00: American = −100 ÷ (decimal − 1). Decimal 1.50 → −200.
Decimal → Implied probability
Implied probability = 1 ÷ decimal odds. Odds of 2.50 → 1 ÷ 2.50 = 0.40 = 40%. Odds of 1.33 → 1 ÷ 1.33 = 0.75 = 75%.
Fractional → Decimal
(Numerator ÷ denominator) + 1. Fraction 5/2 → (5 ÷ 2) + 1 = 3.50. Fraction 1/4 → (1 ÷ 4) + 1 = 1.25.
The Overround: The Bookmaker's Built-In Margin
Every bookmaker market contains a built-in margin called the overround (also: vig, juice, margin, or take). It is the amount by which the sum of all implied probabilities in a market exceeds 100%. In a fair market, all outcomes' probabilities would sum to exactly 100% ; the bookmaker's margin is the gap above that.
| Bookmaker / Platform | Typical football margin | Both sides priced at (2-way market) |
|---|---|---|
| Pinnacle | 2–3% | ~1.95 / 1.95 |
| SBOBet | 3–4% | ~1.93 / 1.93 |
| Betfair Exchange | 2–4% (commission on winnings) | Market odds − 5% commission |
| Bet365 | 7–10% | ~1.83 / 1.83 |
| Most soft bookmakers | 8–12% | ~1.80 / 1.80 |
The margin difference compounds significantly over time. A bettor placing 1,000 bets at flat stake into a 10% margin market loses an expected 10% of total turnover. The same 1,000 bets at 2% margin lose an expected 2%, five times better. The margin is the single largest determinant of long-term return for any bettor.
What Value Means in Practice
Value in betting means that the price offered is higher than the true probability of the outcome. If an outcome has a 50% true probability, the fair price is 2.00 decimal. Any price above 2.00 represents positive expected value; any price below represents negative expected value.
In practice, bettors do not know the true probability of outcomes with precision. What professional bettors do is compare the price at a soft bookmaker against the no-vig price at Pinnacle, which functions as the sharpest market reference available. If a soft bookmaker is offering 2.20 on an outcome that Pinnacle is pricing at 2.05 (after removing the vig), that gap of 15 cents in probability terms is where value may lie.
The ability to bet at Pinnacle-level prices (or at exchange odds) is why professional bettors access these platforms via brokers when they are not directly available. Consistently betting into lower margins at sharp odds is the foundation of sustainable long-term profitability.
Frequently Asked Questions
What are decimal odds?
Decimal odds represent the total return per unit staked, including the stake itself. Odds of 2.50 mean that for every €1 staked, the total return is €2.50 : a profit of €1.50 plus the €1 stake returned. Decimal odds of 1.00 represent an event with no payout (the stake is returned but no profit is paid). Decimal odds are standard at Pinnacle, Betfair, and most professional betting platforms. Converting to implied probability is straightforward: divide 1 by the decimal odds. Odds of 2.50 imply a 40% probability (1 ÷ 2.50 = 0.40).
What are fractional odds?
Fractional odds express profit relative to stake. Odds of 3/1 mean a profit of €3 for every €1 staked (total return €4 including stake). Odds of 5/2 mean a profit of €5 for every €2 staked (or €2.50 per €1 staked, total return €3.50). Fractional odds are traditional in UK and Irish horse racing markets. To convert fractional to decimal: divide the numerator by the denominator and add 1. Odds of 3/1 = (3 ÷ 1) + 1 = 4.00 decimal. Odds of 5/2 = (5 ÷ 2) + 1 = 3.50 decimal.
What are American (moneyline) odds?
American odds are expressed as either a positive or negative number, anchored around a €100 unit. Positive American odds (e.g. +250) indicate profit on a €100 stake : odds of +250 pay €250 profit on a €100 stake, total return €350. Negative American odds (e.g. −180) indicate the amount you must stake to win €100 : odds of −180 require a €180 stake to win €100 profit, total return €280. American odds are standard in US sports betting and are less common in professional European markets. Decimal equivalent: positive odds = (American odds ÷ 100) + 1; negative odds = (100 ÷ absolute value) + 1.
What is implied probability?
Implied probability is the probability of an outcome implied by the bookmaker's odds, including the bookmaker's margin. For decimal odds, implied probability = 1 ÷ decimal odds. At odds of 2.50, the implied probability is 40%. If a bookmaker prices both outcomes of a two-way market at 1.90 each, both imply a 52.6% probability (1 ÷ 1.90). The two implied probabilities sum to 105.2% ; the extra 5.2% is the bookmaker's overround (their built-in margin). True probability requires removing this margin to find the no-vig price.
What is the overround and why does it matter?
The overround (also called the vig, juice, or margin) is the bookmaker's built-in profit margin. It is the percentage by which the sum of implied probabilities across all outcomes in a market exceeds 100%. A fair coin toss at true odds would be priced at 2.00 each side (50%/50% = 100%). A bookmaker offering 1.91 each side is pricing each at 52.4%, for a total of 104.7%, a 4.7% overround. This means that on average, the bettor loses 4.7% of every bet in that market over time. Soft bookmakers operate at 7–12% overround on major markets. Pinnacle operates at 2–3%.
How do you identify value in betting odds?
Value exists when the true probability of an outcome is higher than the probability implied by the odds. If a bookmaker offers odds of 3.00 on an outcome you assess at a 40% true probability, the implied probability is 33.3% (1 ÷ 3.00) ; your estimated probability of 40% is higher, meaning the price is above fair value. Identifying value consistently requires either a more accurate probability model than the bookmaker, or access to a sharper price source (such as Pinnacle) to benchmark against. Professional bettors use Pinnacle's odds as the reference price for fair value, then seek any bookmaker offering higher odds on the same outcome.