To convert fractional odds to decimal, divide the first number by the second and add 1. So 5/2 becomes 2.5 + 1 = $3.50, 6/4 becomes 1.5 + 1 = $2.50, and evens (1/1) is $2.00. The fraction tells you the profit for each unit you stake. The decimal price tells you the total return, stake included, which is what every Australian bookmaker shows.
The rest of this guide covers odds-on prices, the odd-looking ones like 100/30, a full chart for racing, and how to turn any fraction into a probability. Every example price is invented: this is the maths, not a selection.
Why fractions still turn up in Australia
Australian bookmakers quote decimal odds. Decimal is the standard format in Australia, New Zealand, Canada and continental Europe, while fractional odds are mostly used in the United Kingdom and Ireland.
Fractions haven't gone away, though. They're still common in horse racing worldwide, so you'll see them in UK and Irish racing coverage, overseas form, and older racing books. With the Spring Racing Carnival running until 29 November, the Cox Plate at Flemington on 24 October and the Melbourne Cup on Tuesday 3 November, this is the time of year many people hear a price like "9/4" and need it as a decimal.
How fractional odds work
A fractional price is profit over stake. The first number is what you win, the second is what you put on, and you get your stake back as well. Great British Racing puts it simply: at 5/2, "for every 2 units you stake you receive 5 units back plus your initial stake".
A worked example with an invented $20 bet at 5/2:
- Profit: $20 × 5 ÷ 2 = $50
- Stake back: $20
- Total return: $70
The same bet at the decimal price of $3.50: $20 × 3.50 = $70. Same answer. Decimal just has the stake built in, which is why you multiply your stake by the price and stop there.
The conversion formula
Fractional to decimal: decimal = (first number ÷ second number) + 1
- 9/4: 9 ÷ 4 = 2.25, plus 1 = $3.25
- 11/8: 11 ÷ 8 = 1.375, plus 1 = $2.375 (shown as about $2.38)
- 7/2: 7 ÷ 2 = 3.5, plus 1 = $4.50
Decimal to fractional: subtract 1, then write what's left as a fraction.
- $3.75: 3.75 minus 1 = 2.75, which is 11/4
- $1.80: 1.80 minus 1 = 0.80, which is 4/5
- $2.35: 2.35 minus 1 = 1.35, which is 27/20
That last one shows why decimal took over: a bookmaker can price in 5 cent steps without needing a tidy fraction for each. If you'd rather not do it by hand, the odds converter in BetBank's calculators converts between decimal, fractional and American odds and shows the implied probability for each.
Odds-on, odds-against and evens
There are three kinds of fractional price:
- Odds-against: the first number is bigger, like 5/2. You win more than you stake. The decimal is above $2.00.
- Evens: 1/1 is known as evens or even money. You win what you stake, so it's $2.00.
- Odds-on: the first number is smaller, like 4/6. You win less than you stake. The decimal is below $2.00. Great British Racing describes an odds-on runner as one considered very likely to win.
Reading "4 to 1 on"
Odds-on prices are often said backwards. A price of 1/4 can be described as "four-to-one on" or "four-to-one in favour", and 4/1 as "four-to-one against". So when you hear "6/4 on", the fraction is really 4/6:
- 4 ÷ 6 = 0.667, plus 1 = $1.67
- An invented $30 bet returns $30 × 1.67 = about $50, which is $20 profit
If a price is said with "on" at the end, flip the fraction before you convert it.
Why 6/4 isn't written as 3/2
Some fractions are not reduced to lowest terms, and 6/4 is the usual example. It's the same price as 3/2: both are $2.50. 100/30 works the same way. It equals 10/3, so 100 ÷ 30 = 3.33, plus 1 = about $4.33. If a fraction looks strange, do the division and the price is the same whichever way it's written.
Fractional to decimal conversion chart
Decimal = fraction + 1. Implied probability = second number ÷ (first + second), the formula set out in Mathematics of bookmaking. Decimals are rounded to two places and probabilities to one.
| Fractional | Decimal | Implied probability |
|---|---|---|
| 1/4 (4/1 on) | $1.25 | 80.0% |
| 2/5 | $1.40 | 71.4% |
| 1/2 (2/1 on) | $1.50 | 66.7% |
| 4/6 (6/4 on) | $1.67 | 60.0% |
| 4/5 | $1.80 | 55.6% |
| 10/11 | $1.91 | 52.4% |
| 1/1 (evens) | $2.00 | 50.0% |
| 11/10 | $2.10 | 47.6% |
| 6/5 | $2.20 | 45.5% |
| 5/4 | $2.25 | 44.4% |
| 11/8 | $2.38 | 42.1% |
| 6/4 | $2.50 | 40.0% |
| 13/8 | $2.63 | 38.1% |
| 7/4 | $2.75 | 36.4% |
| 15/8 | $2.88 | 34.8% |
| 2/1 | $3.00 | 33.3% |
| 9/4 | $3.25 | 30.8% |
| 5/2 | $3.50 | 28.6% |
| 11/4 | $3.75 | 26.7% |
| 3/1 | $4.00 | 25.0% |
| 100/30 | $4.33 | 23.1% |
| 7/2 | $4.50 | 22.2% |
| 4/1 | $5.00 | 20.0% |
| 9/2 | $5.50 | 18.2% |
| 5/1 | $6.00 | 16.7% |
| 6/1 | $7.00 | 14.3% |
| 8/1 | $9.00 | 11.1% |
| 10/1 | $11.00 | 9.1% |
| 20/1 | $21.00 | 4.8% |
| 33/1 | $34.00 | 2.9% |
| 50/1 | $51.00 | 2.0% |
| 100/1 | $101.00 | 1.0% |
For a fraction that isn't in the table, the formula always works. For a quick probability from a decimal price, divide 100 by the price: $4.00 is 25%.
From fractions to the market percentage
Converting a single price is half the job. The more useful step is adding up the whole field, because that shows the bookmaker's margin.
The Mathematics of bookmaking page gives the method: turn each price into an implied probability and add them up. If the total is over 100%, the extra is the bookmaker's overround. Its worked example prices a market at 4/6, 6/4 and 4/1, which comes to 60% + 40% + 20% = 120%.
Here's an invented five-runner race in fractions:
| Runner | Fractional | Decimal | Implied probability |
|---|---|---|---|
| A | 6/4 | $2.50 | 40.0% |
| B | 5/2 | $3.50 | 28.6% |
| C | 4/1 | $5.00 | 20.0% |
| D | 6/1 | $7.00 | 14.3% |
| E | 10/1 | $11.00 | 9.1% |
| Total | 112.0% |
The book adds up to about 112%, so roughly 12 points of margin are spread across the field. To get a rough fair price for runner A, divide its 40.0% by 1.12 to get about 35.7%, then 100 ÷ 35.7 = about $2.80. That's the price with the margin taken out evenly. Real bookmakers don't spread margin that neatly, so treat it as an estimate, not the truth.
Our guide to market percentage in horse racing goes further on reading and removing the margin from a racing market. If you bet each way, the each way betting guide covers how place terms like a quarter or a fifth of the win odds turn into a place price.
Common mistakes when converting
- Forgetting the +1. 5/2 is not $2.50. It's $3.50. $2.50 is 6/4.
- Missing the "on". "6/4 on" is 4/6, or $1.67, not $2.50.
- Thinking a bigger fraction is more likely. 5/1 pays more than 2/1 because it's less likely: 16.7% against 33.3%.
- Comparing a fraction with a decimal by eye. Convert first. 13/8 is exactly $2.625, so against $2.60 it pays 2.5 cents more per dollar staked, and that adds up over hundreds of bets.
- Treating the implied probability as the real chance. Every price includes the bookmaker's margin, so implied probabilities across a field add to more than 100%.
For more on the three formats side by side, including American odds, see the odds converter guide.
Compare racing prices in decimal
Once a fraction is a decimal, the next question is whether you're getting the best one. BetBank's racing odds screen shows live fixed-odds win and place prices from major Australian bookmakers side by side, with the best price for each runner highlighted, so you can compare prices through the Spring Carnival before you place a bet.
Betting always carries risk, and no price or tool makes a bet a winner. 18+ only. Please gamble responsibly, and if you want to take a break from betting, BetStop is the free national self-exclusion register.