Every price on a horse is a statement about probability, but three different notations are used to express it depending on where in the world you are betting. Fractional odds dominate in the UK and Ireland, decimal odds are the standard across Europe, Australia and most betting exchanges, and American (moneyline) odds are used in the United States. Once you understand how each works, converting between them and reading the underlying probability takes a few seconds.
Fractional Odds
Fractional odds show the profit relative to the stake. A price of 5/2 (read "five to two") means you win £5 for every £2 staked, plus your stake back. A £10 bet at 5/2 returns a profit of £25 and a total of £35. A price of 4/6 is "odds-on": you must stake £6 to win £4, so the horse is considered more likely to win than not. "Evens" (1/1) means the profit equals the stake.
The conversion to decimal is simple: divide the top number by the bottom number and add one. So 5/2 is 2.5 + 1 = 3.50, and 4/6 is 0.667 + 1 = 1.667.
Decimal Odds
Decimal odds show the total return per unit staked, including the stake. A price of 3.50 means a £10 stake returns £35 in total (a £25 profit). This is the easiest format for calculations because the return is simply stake multiplied by the odds. It is also the format used by most analytical models.
To find the implied probability from decimal odds, divide 1 by the price: 1 / 3.50 = 0.2857, or 28.6%. A horse at 2.00 has an implied probability of 50%; a horse at 10.00 has 10%.
American (Moneyline) Odds
American odds are expressed as positive or negative numbers built around a base of 100:
- Positive odds (+150) show the profit you would make on a 100 stake. A +150 price returns 150 profit on 100 staked.
- Negative odds (-150) show the stake needed to make a 100 profit. A -150 price means you must stake 150 to win 100.
To convert decimal odds of 2.00 or more into American odds, subtract one and multiply by 100: 3.50 becomes (3.50 - 1) x 100 = +250. For decimal odds below 2.00, divide -100 by the decimal minus one: 1.667 becomes -100 / 0.667 = -150.
Side-by-Side Comparison
| Fractional | Decimal | American | Implied probability |
|---|---|---|---|
| 4/6 | 1.67 | -150 | 60.0% |
| Evens (1/1) | 2.00 | +100 | 50.0% |
| 5/2 | 3.50 | +250 | 28.6% |
| 4/1 | 5.00 | +400 | 20.0% |
| 9/1 | 10.00 | +900 | 10.0% |
| 20/1 | 21.00 | +2000 | 4.8% |
Implied Probability and Why It Matters
Implied probability is the chance of winning that a price suggests. It is the bridge between the odds and any model of your own. If you estimate a horse has a 25% chance of winning and the price is 5.00 (implied probability 20%), the price is longer than your estimate says it should be, which is the definition of a value bet. If you estimate 15% at the same price, the horse is priced too short. Our guide to value betting develops this idea in more detail.
The Overround: The Bookmaker's Margin
If bookmakers priced every horse at its true chance, the implied probabilities in a race would add up to exactly 100%. They do not. Bookmakers build in a margin, known as the overround or "book percentage", by shortening every price slightly. The implied probabilities therefore add up to more than 100%.
Consider a simplified three-runner race priced at 1.80, 3.50 and 5.00:
- 1 / 1.80 = 55.6%
- 1 / 3.50 = 28.6%
- 1 / 5.00 = 20.0%
The total is 104.1%. The extra 4.1% is the bookmaker's margin. To recover the market's underlying view, divide each implied probability by the total: the favourite becomes 53.4%, the second horse 27.4% and the third 19.2%, which sum to 100%. This process is called normalising or removing the vig, and it is a standard step before comparing a model's output to the market.
Real races have many more runners, and in large-field handicaps the margin can be considerably higher than in our three-horse example. That is one reason it is hard to profit from betting: before you have an edge over the market, you must overcome the margin built into every price.
Starting Price, Early Prices and Best Odds Guaranteed
Three price types appear regularly in UK racing. The starting price (SP) is the price returned at the off, based on the odds offered across the on-course market. An early price is a fixed price taken before the race, which may be longer or shorter than the SP. Best Odds Guaranteed (BOG) is a promotion under which a bookmaker pays at the SP if it is bigger than the price you took, but only if you took an early price. Whether a bet is better placed early or at SP depends on how you expect the market to move, and price movement is the subject of our analysis of market movers.
Tote and Pari-Mutuel Odds
In the United States and in many other countries, most racing bets are placed into a pari-mutuel pool rather than against a bookmaker. The odds are not fixed: the track takes a percentage (the takeout) from the pool and the remaining money is divided among winning tickets. This means the final payout is only known after betting closes, and the odds displayed on the tote board change until the off.
Practical Tips
- Always convert to implied probability before comparing prices across formats.
- Remember that decimal odds include the stake and fractional odds do not.
- Use the overround as a quick measure of how much margin a market carries: the lower the book percentage, the better value the market.
- Compare prices between bookmakers, since a difference of a single price step compounds over hundreds of bets.
Whichever notation you use, the maths is the same. Betting involves risk, and no format changes that fact; bet responsibly and within your means.