Why Odds Formats Matter
Odds are the language of betting. They tell you how much you stand to win and what probability the bookmaker assigns to an outcome. Understanding all three major formats — decimal, fractional, and American (moneyline) — allows you to bet confidently on any platform, anywhere in the world.
Decimal Odds
Decimal odds are the most common format used by bookmakers in Europe, Australia, and most of the world. They represent the total return per unit staked, including your original stake.
Formula: Payout = Stake × Decimal Odds
For example, odds of 3.00 on a €10 bet returns €30 (€20 profit + €10 stake). Odds of 1.50 return €15 on the same stake (€5 profit).
Converting to implied probability: (1 / Decimal Odds) × 100
Fractional Odds
Traditional in the UK and Ireland, fractional odds show the profit relative to stake. The left number is your profit; the right is your stake.
- 5/1 (five-to-one): Win €5 profit for every €1 staked. Total return: €6.
- 1/2 (one-to-two, or "odds-on"): Win €1 profit for every €2 staked. Total return: €3 on a €2 bet.
- Evens (1/1): Double your money — €10 stake returns €20.
Converting fractional to decimal: add 1 to the fraction. So 5/1 = 6.00 in decimal.
American Odds (Moneyline)
Used primarily in the United States, American odds use a baseline of $100 and are expressed as positive or negative numbers.
- Positive odds (+150): You win $150 profit on a $100 bet. This represents the underdog.
- Negative odds (-200): You must bet $200 to win $100 profit. This represents the favourite.
Converting positive American odds to decimal: (Odds / 100) + 1 = Decimal. So +150 = 2.50.
Converting negative American odds to decimal: (100 / |Odds|) + 1 = Decimal. So -200 = 1.50.
Quick Comparison Table
| Outcome | Decimal | Fractional | American | Implied Probability |
|---|---|---|---|---|
| Strong Favourite | 1.40 | 2/5 | -250 | 71.4% |
| Slight Favourite | 1.80 | 4/5 | -125 | 55.6% |
| Even Money | 2.00 | 1/1 | +100 | 50.0% |
| Slight Underdog | 2.50 | 6/4 | +150 | 40.0% |
| Heavy Underdog | 5.00 | 4/1 | +400 | 20.0% |
The Overround: What the Bookmaker Takes
If you add up the implied probabilities of all outcomes in a market, they'll total more than 100%. This excess is the bookmaker's margin — also called the overround or vig. A typical two-way market (e.g., tennis) might show 105–107% total implied probability, meaning the bookmaker keeps roughly 5–7% of all money wagered on that market over time.
Shopping across multiple bookmakers for the best available odds is the simplest way to reduce the impact of the overround on your returns.
How to Use This Knowledge
Once you can convert between formats and calculate implied probability, you unlock the ability to assess whether a bet has value. Compare the bookmaker's implied probability to your own assessment. When your estimate is meaningfully higher, you've potentially found a value bet.