Why Odds Matter
Before you can arb, match bet, or find value, you need to fluently read odds in every format. They're not just numbers — they encode probability and your potential return.
Decimal Odds (Most Common Globally)
Decimal odds represent your total return per unit staked, including your stake.
- Odds of 3.00 on a £10 bet returns £30 (£20 profit + £10 stake back)
- Implied probability = 1 / decimal odds
- 3.00 → 33.3% implied probability
Decimal odds are dominant in Europe, Australia, and Asia. They're the easiest to work with mathematically.
Fractional Odds (UK Traditional)
Fractional odds show profit relative to stake.
- 5/1 ("five-to-one"): win £5 for every £1 staked
- 1/2 ("one-to-two"): win £1 for every £2 staked — the bookmaker thinks this is likely
- Implied probability = denominator / (numerator + denominator)
- 5/1 → 1/6 = 16.7%
Fractional odds are still common on horse racing markets in the UK.
American (Moneyline) Odds
American odds use a +/- system based on a $100 reference unit.
- +200: Bet $100, win $200 profit (underdog)
- -150: Bet $150 to win $100 profit (favourite)
- Positive: (100 / (odds + 100)) = implied probability
- Negative: (odds / (odds + 100)) = implied probability
Converting Between Formats
All formats encode the same information. The conversion formulas:
| Format | To Decimal |
| Fractional (a/b) | (a/b) + 1 |
| American (+) | (odds/100) + 1 |
| American (−) | (100/odds) + 1 |
The Overround — How Bookmakers Make Money
Add up the implied probabilities across all outcomes in a market. You'll always get more than 100% — this excess is the bookmaker's margin (also called the overround or vig).
A typical football match market might total 106–110%. Your job as an arber or value bettor is to find markets where the combined implied probability is below 100%.