Why Understanding Odds Is Essential
Odds are the language of betting. Every wager you place is governed by odds, which tell you two things: how much you can win relative to your stake, and what probability the bookmaker assigns to a given outcome. If you can't read odds fluently, you're betting blind.
There are three main formats used around the world: Decimal (common in Europe, Australia, Canada), Fractional (traditional in the UK and Ireland), and American/Moneyline (used in the United States). Most online platforms let you switch between all three.
Decimal Odds
Decimal odds are the easiest to understand and the most widely used on international platforms. The number shown represents your total return per unit staked, including your original stake.
Formula: Profit = (Stake × Decimal Odds) − Stake
- Odds of 2.00 means a $10 bet returns $20 total ($10 profit).
- Odds of 1.50 means a $10 bet returns $15 total ($5 profit).
- Odds of 3.75 means a $10 bet returns $37.50 total ($27.50 profit).
Any decimal odds below 2.00 means the outcome is considered more likely than not (the "favourite"). Above 2.00 means the outcome is considered less likely (the "underdog").
Fractional Odds
Fractional odds are written as two numbers separated by a slash, such as 5/1 or 3/2. The left number is your profit if you bet the right number.
Formula: Profit = Stake × (Numerator ÷ Denominator)
- 5/1 ("five to one"): Bet $10, win $50 profit + $10 stake back = $60 total.
- 1/2 ("one to two"): Bet $10, win $5 profit + $10 stake back = $15 total.
- 7/4 ("seven to four"): Bet $4, win $7 profit. Bet $10, win $17.50 profit.
When the left number is larger than the right, it's an "odds against" bet (underdog). When the left is smaller, it's "odds on" (favourite).
American (Moneyline) Odds
American odds use a plus (+) or minus (−) sign alongside a number, always based around a $100 reference point.
- Positive odds (+150): You WIN $150 profit on a $100 bet. The team/player is the underdog.
- Negative odds (−150): You must BET $150 to WIN $100 profit. The team/player is the favourite.
For any stake amount: Profit on positive odds = Stake × (Odds ÷ 100). Profit on negative odds = Stake × (100 ÷ |Odds|).
Converting Between Formats
| Decimal | Fractional | American | Implied Probability |
|---|---|---|---|
| 2.00 | 1/1 (Evens) | +100 | 50% |
| 1.50 | 1/2 | −200 | 66.7% |
| 3.00 | 2/1 | +200 | 33.3% |
| 4.00 | 3/1 | +300 | 25% |
| 1.25 | 1/4 | −400 | 80% |
Understanding Implied Probability
Every set of odds carries an implied probability — the bookmaker's assessment of how likely the outcome is. You can convert decimal odds using this formula:
Implied Probability (%) = (1 ÷ Decimal Odds) × 100
If you believe the true probability of an outcome is higher than the implied probability shown in the odds, that bet may offer value. This concept is the foundation of finding value bets.
The Bookmaker's Margin (Overround)
Note that odds on all outcomes in a market will always add up to more than 100% in implied probability — this is the bookmaker's built-in margin (also called the overround or vig). It's how platforms make money regardless of the result. Being aware of this helps you understand the true cost of every bet.