Odds are the language of betting. They express potential return, encode probability, and embed bookmaker margin — all at once. Understanding odds in all three formats is the foundation of every analytical betting approach.
Europe, Australia, exchanges
Decimal
2.50
Total return per £1 staked (profit + stake). £10 at 2.50 returns £25 total (£15 profit).
Implied prob = 1 ÷ decimal odds
UK, Ireland (traditional)
Fractional
3/2
Profit per units staked. 3/2 means £3 profit for every £2 staked. £10 at 3/2 = £15 profit + £10 stake back = £25.
Decimal = (num ÷ den) + 1
USA, Canada
American / Moneyline
+150 / -200
Positive: profit on £100 stake. Negative: stake needed to profit £100. +150 → £250 return on £100. -200 → stake £200 to profit £100.
+odds: decimal = (odds ÷ 100) + 1
All three formats showing the same probability and what kind of outcome each represents.
| Decimal | Fractional | American | Implied prob | Context |
|---|---|---|---|---|
| 1.50 | 1/2 | -200 | 66.7% | Heavy favourite |
| 2.00 | Evens | +100 | 50.0% | Coin flip |
| 2.50 | 3/2 | +150 | 40.0% | Slight underdog |
| 3.00 | 2/1 | +200 | 33.3% | Moderate underdog |
| 4.00 | 3/1 | +300 | 25.0% | Long shot |
| 6.00 | 5/1 | +500 | 16.7% | Big outsider |
| 10.00 | 9/1 | +900 | 10.0% | Major outsider |
| 21.00 | 20/1 | +2000 | 4.8% | Rank outsider |
A typical Premier League 1X2 market might look like this:
| Outcome | Decimal odds | Implied probability | Return on £10 |
|---|---|---|---|
| Home win | 2.10 | 47.6% | £21.00 |
| Draw | 3.60 | 27.8% | £36.00 |
| Away win | 3.80 | 26.3% | £38.00 |
| Total implied probability | 101.7% | Margin = 1.7% | |
The 1.7% overround is relatively low for this market (tighter-priced Premier League games). Cup markets, lower leagues, and exotic markets carry 5–15% margins.
The margin (overround, vig) is built into the odds. If you backed all outcomes on a market, you would guarantee a loss equal to the margin percentage of your total stake. Removing the margin reveals the bookmaker's true probability estimate — called the fair odds or no-vig probability.
Fair probability = Implied prob ÷ Sum of all implied probs
Fair odds = 1 ÷ Fair probability
Example: Home 47.6%, Draw 27.8%, Away 26.3% → sum = 101.7%
Fair home prob = 47.6% ÷ 1.017 = 46.8% → Fair odds = 2.14 (was 2.10)
Value exists when your estimated true probability exceeds the fair probability implied by the odds. Use the Fair Odds Calculator to strip margin and compare any set of odds to your own probabilities.
Betting odds express how much you will be paid if your bet wins, relative to your stake. They also encode the bookmaker's estimate of the probability of that outcome — although this probability is inflated by a margin to ensure the bookmaker makes profit. Higher odds = lower implied probability = higher potential return.
Decimal odds (e.g. 2.50) represent your total return including your stake, per unit staked. Fractional odds (e.g. 3/2) represent your profit only, per unit staked. To convert: decimal odds = (fractional numerator ÷ denominator) + 1. So 3/2 = 1.5 + 1 = 2.50. Decimal odds are now standard in most of Europe and on betting exchanges.
Implied probability = 1 ÷ decimal odds. For fractional odds: implied probability = denominator ÷ (numerator + denominator). For American odds: if positive, probability = 100 ÷ (odds + 100). If negative, probability = |odds| ÷ (|odds| + 100). These probabilities sum to more than 100% due to the bookmaker margin.
The bookmaker margin (also called overround or vig) is the percentage by which the sum of implied probabilities across all outcomes exceeds 100%. A typical Premier League match margin is 4–8%. This means for every £100 you bet, the bookmaker takes £4–8 in margin over the long run. The margin is why recreational bettors lose money on average, even with good selections.
American odds are expressed relative to a £100 (or $100) stake. Positive odds (e.g. +250) show your profit on a £100 bet. Negative odds (e.g. -150) show how much you must stake to profit £100. +250 is equivalent to decimal 3.50 (profit £250 on £100). -150 is equivalent to decimal 1.67 (stake £150 to profit £100).
18+ only. For informational purposes only. Please gamble responsibly. BeGambleAware.org
