Betting Odds Explained: Fractions, Decimals, Margin

Betting odds explained: fractional, decimal, and what the price really means

A price like 5/1 is doing two jobs at once. It tells you what you get paid if the bet lands, and it tells you what the bookmaker reckons the chances are. Most people only ever read the first half.

Read the second half and a fair bit of what looks like a good deal stops looking like one. So here's the plain version: what the numbers actually mean, how to flip between fractional and decimal without much thought, how to turn any price into a percentage, and why the percentages in every betting market add up to more than 100.

The short version

• Fractional odds show profit against stake. 5/1 pays €5 profit for every €1 you risk, and your stake comes back on top.

• Decimal odds show the total return per €1, stake already included. The same price is 6.00.

• To convert a fraction to decimal, divide it and add 1. To go back the other way, subtract 1.

• Any price turns into a percentage chance: divide 100 by the decimal. 6.00 gives 16.67%.

• Add those percentages up across a market and you get more than 100. The extra is the bookmaker's margin, and it's called the overround.

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Fractional odds: profit against stake

The fraction is a ratio of profit to stake. The left number is what you win, the right number is what you put up to win it.

So 5/1 means risk €1, win €5. A €10 bet returns €60: that's €50 profit plus your own €10 back. The stake coming back is where most of the confusion starts, because the fraction never mentions it.

Evens, written 1/1, is a euro for a euro. A €50 bet returns €100.

When the right-hand number is the bigger one, you're into odds-on. 1/5 means risking €5 to win €1, so a €10 bet returns €12. Short prices look mean written down, and they're meant to: 1/5 is the market saying this one wins better than four times out of five.

The awkward-looking ones are just fractions doing what fractions do. €20 at 11/8 gives you €27.50 profit and €47.50 back. €10 at 9/4 gives €22.50 profit and €32.50 back. And 100/30 is an old racecourse survival that never quite died: it's 10/3 in disguise, and €25 on it returns €108.33.

Irish racing still runs heavily on fractions, so if you're at Galway or Leopardstown the board is in eighths and quarters whether it suits you or not.

Decimal odds: total return per euro

Decimal odds fold the stake in. Multiply your stake by the decimal and that's your total return, with no addition at the end.

5/1 becomes 6.00. Ten euro times 6.00 is €60. Same money, less arithmetic.

Evens is 2.00, and that's the line worth committing to memory. Anything above 2.00 is odds-against, anything below is odds-on. So 1.20 is 1/5, 1.50 is 1/2, and 1.91 is roughly 10/11.

Decimal really earns its keep on multiples. Three legs at 2.50, 3.00 and 4.00? Multiply them together and you get 30.00. Try the same thing in fractions and you'll be a while at it.

Flipping between the two

Fraction to decimal: divide the fraction, then add 1. So 9/4 is 2.25, add 1, and you have 3.25.

Decimal to fraction: subtract 1, then tidy up what's left. So 3.50 minus 1 is 2.5, which is 5/2.

The ones you'll meet most often:

• 1/5 is 1.20 and 83.33%

• 1/2 is 1.50 and 66.67%

• 4/5 is 1.80 and 55.56%

• 10/11 is 1.91 and 52.38%

• 1/1 (evens) is 2.00 and 50%

• 11/8 is 2.38 and 42.11%

• 6/4 is 2.50 and 40%

• 2/1 is 3.00 and 33.33%

• 9/4 is 3.25 and 30.77%

• 5/2 is 3.50 and 28.57%

• 3/1 is 4.00 and 25%

• 9/2 is 5.50 and 18.18%

• 5/1 is 6.00 and 16.67%

• 10/1 is 11.00 and 9.09%

• 20/1 is 21.00 and 4.76%

• 50/1 is 51.00 and 1.96%

One small thing that looks like an error and isn't. Decimal prices are normally shown to two places, and plenty of fractions don't fit neatly into two places. 11/8 is exactly 2.375. Shown as 2.38 it's a shade more generous than the fraction, shown as 2.37 it's a shade less. On a tenner that's cents. On a long multiple it adds up, and which way it rounds is the operator's own settlement rule rather than any market-wide standard, so it's worth reading the returns figure rather than assuming it.

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Turning a price into a percentage chance

This is the bit that does the most work, and it's one division.

Implied probability is 100 divided by the decimal odds. That's it.

So 6.00 gives 16.67%. 4.00 gives 25%. 2.00 gives 50%. 1.20 gives 83.33%.

Why bother? Because it turns a vague question into an answerable one. "Is 9/2 a good price on that horse?" isn't a question anyone can settle. "Does that horse win more than 18.18% of the time?" is a question you can actually hold an opinion about, and it's the same question.

It's called implied probability for a reason. It's the chance implied by the price, not the true chance of the thing happening. The price was set to make the bookmaker money, not to publish an honest forecast, and those two aims pull in different directions. Which brings us to the reason the numbers never quite add up.

The overround: why it adds up to more than 100

Take a two-way market with both sides priced at 10/11. That's 1.91 in decimal and 52.38% implied. Two of those comes to 104.76%.

But there are only two outcomes, and between them they have to happen 100% of the time. The market says 104.76%. That extra 4.76 points is the bookmaker's margin, known as the overround, or the vig, or just the book.

A genuinely fair two-way market would be evens and evens: 50% and 50%, adding to exactly 100. Nobody prices that, because there'd be nothing in it for them.

As a rule, more outcomes means more margin. A three-way football market priced 11/10, 23/10 and 12/5 comes to 107.33%. An eight-runner race priced 2/1, 3/1, 5/1, 8/1, 10/1, 16/1, 20/1 and 25/1 adds up to 109.69%.

One precise point, because the numbers invite a wrong reading. A 104.76% book does not hand the bookmaker 4.76% of turnover. Back both outcomes for €1 each and you've staked €2 to get €1.91 back, so you're down 9 cent, which is 4.55% of what you staked. The overround measured in points and the margin measured as a share of money are close, but they aren't the same number.

Two things this is actually useful for:

1. Comparing bookmakers. A lower overround on the same market means better prices across the board, all else being equal. Comparing that margin between firms is what an odds comparison tool is doing under the bonnet.

2. Sanity-checking a boosted price. A special on one selection can sit inside a market whose overall margin hasn't shifted at all, because the other prices in it were quietly trimmed to pay for it.

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Why golf prices look enormous

Same maths, far more names. A tournament field can run past 150 players, so the chance attached to any one of them is small and the price looks huge by comparison. 40/1 is 2.44%. 50/1 is 1.96%.

Spread across a field that size, the margin stacks up in a way it simply can't in a two-way market. That's why, in golf, the each-way terms and the number of places paid usually do more to your returns than the headline price does. It's a topic in its own right and worth reading up on separately before the Irish Open comes round.

What actually moves a price

Prices move for two reasons: information and money.

Information is the obvious one. A non-runner, a team sheet, the going at Punchestown changing after a night of rain.

Money is the less obvious one. If enough is backed on one side, the bookmaker shortens it to stop the liability growing and pushes the other side out to pull money back the other way. Nothing has been learned about the event at all.

That's the thing worth holding on to: a price isn't a forecast that got updated, it's a position that got adjusted. It's why a market can move sharply on a day with no news in it whatsoever.

You'll also see the price returned at the off, the starting price, differ from the price you took earlier. Taking a price early locks in what you took; leaving it to the off means you get whatever the market settles at.

Does the 2% duty come out of your winnings?

No. This is easily the most common Irish misunderstanding about betting prices, and it's worth being exact about.

Betting duty in Ireland is 2%. Per Revenue's excise rates, it applies at 2% to bets a bookmaker takes over the counter from people in the State, and at 2% again to bets taken by remote means. On-course and tote bets are nil-rated. Betting intermediary duty, the exchange model, is 25% of the commission the intermediary charges, not 25% of anything you win.

The key point is who pays it. The duty is charged on the stake and it's payable by the bookmaker to Revenue. It isn't deducted from your return, and it doesn't appear as a line on your settled bet. Whether a given firm prices the cost of it into its margin is that firm's own commercial decision, not a deduction the State makes from you.

Winnings themselves are tax-free for players in Ireland, whoever the operator is and wherever it's licensed.

One thing regulation does not do here is set your price. Since 1 July 2026, online bookmakers serving Irish customers hold a licence from the Gambling Regulatory Authority of Ireland, and Citizens Information sets out that the Authority licenses betting, gaming and certain lottery activities. That covers how an operator behaves towards you. It doesn't cap a bookmaker's margin or dictate which odds format gets displayed, and there's no Irish rule that does.

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Frequently asked questions

What does 5/1 mean in betting?

It means you win €5 in profit for every €1 you stake, and your stake comes back as well. A €10 bet at 5/1 returns €60 in total: €50 profit plus your €10 back.

What is 5/1 in decimal odds?

6.00. To convert any fraction, divide it and add 1: 5 divided by 1 is 5, plus 1 is 6.00. Multiply your stake by 6.00 and you get the total return, stake included.

Are fractional and decimal odds different?

No, they're the same price written two ways. Fractional shows profit against stake, decimal shows the total return per euro. Most bookmakers let you switch the display, and the payout doesn't change either way.

What does €20 at 11/8 pay?

€27.50 profit, so €47.50 back in total. In decimal that price is 2.375, usually displayed as 2.38.

What does odds-on mean?

It means the selection is favoured strongly enough that you risk more than you stand to win. Anything shorter than evens is odds-on, so 1/2 or 1/5 in fractions, or anything below 2.00 in decimal.

What is implied probability?

The chance a price implies, worked out by dividing 100 by the decimal odds. A price of 4.00 implies 25%. It's what the market is saying, not a true forecast, because the price has the bookmaker's margin built into it.

What is the overround?

The amount by which the implied probabilities across a market add up to more than 100%. Two sides at 10/11 come to 104.76%, so the overround is 4.76 points. A lower overround means better prices for you.

Is the 2% betting duty taken out of my winnings?

No. Betting duty is charged on the stake and paid by the bookmaker to Revenue, not deducted from your return. On-course and tote bets carry no duty at all. Gambling winnings are tax-free for players in Ireland.

Safer gambling: knowing what a price pays tells you nothing about what will happen next, and no amount of maths turns a bet into a plan. If gambling is causing difficulty for you or someone you know, the National Gambling Helpline is free and confidential on 1800 936 725, 24 hours a day.