How Betting Odds Work: Fractional, Decimal and American

Every bet starts with a price, and that price does two jobs at once. It tells you how much you get back if you win, and it tells you what chance the bookmaker is giving the outcome. Most beginners only read the first part. The second part is where the useful information sits.

Odds come in three formats: fractional (5/2), decimal (3.50) and American (+250). Those three numbers describe exactly the same bet. Which one you see depends on where you live and which sportsbook you use, and almost every site lets you switch formats in the settings. This guide shows how to read each one, how to convert between them, how to turn a price into a probability and how the bookmaker's margin is hidden inside every market.

Fractional odds: the traditional UK and Irish format

Fractional odds show profit relative to stake. The number on the left is what you win, the number on the right is what you have to stake to win it. At 5/2 (said "five to two") you win £5 for every £2 staked. Your stake comes back on top.

So £10 at 5/2 gives £25 profit (10 x 5 / 2) and a total return of £35. At 1/2, a short-priced favourite, £10 wins only £5 and returns £15. When the left number is smaller than the right one the bet is "odds-on" and you win less than you stake. When the two are equal, 1/1, the price is called evens: £10 wins £10.

Fractions are still the default in UK and Irish horse racing and in many betting shops. Their weakness is that awkward prices such as 8/13 or 15/8 are hard to compare at a glance. Most people need a calculator to see that 4/7 is a shorter price than 8/13, and that is one reason many bettors switch to decimals.

Decimal odds: the total return per unit staked

Decimal odds are used across Europe, Australia, Canada, India and on betting exchanges. The number shows your total return for every 1 unit staked, stake included. Multiply stake by odds and you have the return.

That same 5/2 shot is 3.50 in decimal. A £10 bet returns £35 (10 x 3.50), which is £25 profit plus your £10 back. A price of 1.80 returns £18 from £10. Evens is 2.00. Anything below 2.00 is odds-on.

The main trap for beginners is reading the decimal number as profit. It is not. At 1.50 you do not make 1.5 times your money in profit; you make half your stake in profit. Profit is always stake x (odds minus 1).

American odds: plus and minus moneyline prices

American odds, also called moneyline odds, are built around a $100 reference bet and use a plus or minus sign.

• A plus number shows how much profit a $100 stake makes. At +250 a $100 bet wins $250 and returns $350.
• A minus number shows how much you must stake to make $100 profit. At -150 you stake $150 to win $100, so the return is $250.

You do not have to bet $100. The figure is only a scale. At -150 a $30 bet wins $20 (30 x 100 / 150), and at +250 a $20 bet wins $50 (20 x 250 / 100).

Evens is +100. In theory -100 means the same thing, and you will see both. Where people go wrong is the minus sign: -150 is not a negative return, it simply marks the favourite. The bigger the minus number, the shorter the price. A -400 favourite pays far less than a -120 one.

Converting between odds formats

Once you know the formulas, conversion takes a few seconds. Decimal is the easiest hub: convert everything to decimal first, then out to whatever you need.

• Fractional to decimal: divide the fraction and add 1. 5/2 = 2.5 + 1 = 3.50. 4/5 = 0.8 + 1 = 1.80.
• Decimal 2.00 or higher to American: (decimal minus 1) x 100. 3.50 becomes +250.
• Decimal below 2.00 to American: -100 / (decimal minus 1). 1.80 becomes -100 / 0.80 = -125.
• American plus to decimal: (number / 100) + 1. +400 becomes 5.00.
• American minus to decimal: (100 / number) + 1. -150 becomes 1.67 (1.6667 before rounding).

Here are common prices side by side, with the implied probability covered in the next section:

• 1/2 = 1.50 = -200 = 66.7%
• 4/5 = 1.80 = -125 = 55.6%
• 10/11 = 1.91 = -110 = 52.4%
• 1/1 (evens) = 2.00 = +100 = 50%
• 6/4 = 2.50 = +150 = 40%
• 2/1 = 3.00 = +200 = 33.3%
• 5/2 = 3.50 = +250 = 28.6%
• 4/1 = 5.00 = +400 = 20%
• 10/1 = 11.00 = +1000 = 9.1%

Small differences appear because sites round. 10/11 is really 1.909 in decimal, so a site showing 1.91 has rounded up by a fraction of a cent per unit. It rarely matters on a single bet, but it does matter when you compare prices closely.

Implied probability: what the price says about the chance

Implied probability is the win chance at which a bet would exactly break even over the long run. It is the single most useful thing to take from any price.

For decimal odds the formula is 1 / odds. A price of 1.80 implies 1 / 1.80 = 55.6%. A price of 3.50 implies 28.6%. For fractional odds a/b the formula is b / (a + b): 5/2 gives 2 / 7 = 28.6%, the same answer. For American odds, a plus price gives 100 / (number + 100), so +250 is 100 / 350 = 28.6%. A minus price gives number / (number + 100), so -150 is 150 / 250 = 60%.

This is how you should think about a bet. Backing a team at 1.80 only makes sense if you believe they win more than 55.6% of the time. If your honest estimate is 50%, the price is too short for you, however confident the pundits sound. If your estimate is 60%, the price looks generous.

A quick check of that idea with numbers: say you rate a home side at 50% and the price is 2.10. Over many identical bets, half win £11 profit on a £10 stake and half lose £10. That averages +£0.50 per bet, or 5% of stake. At 1.90 the same estimate averages -£0.50 per bet. The team did not change, only the price did. Of course this rests entirely on your 50% being right, which is the hard part and the subject of our guide to value betting.

The overround: how the bookmaker's margin is built in

Add up the implied probabilities of every outcome in a market and you will get more than 100%. The excess is the overround, sometimes called the vig, juice or margin. It is how the bookmaker earns money regardless of the result.

Take a tennis match priced at 1.91 for each player. Each price implies 52.4%, so the book adds up to 104.8%. A fair coin-flip would be 2.00 each. The missing 0.09 per unit on each side is the bookmaker's cut.

The same works for three-way markets. Picture a football (soccer) match priced at Home 2.10, Draw 3.40, Away 3.60 in the Match Result (1X2) market:

• Home: 1 / 2.10 = 47.6%
• Draw: 1 / 3.40 = 29.4%
• Away: 1 / 3.60 = 27.8%

The total is 104.8%, an overround of about 4.8%. To estimate the "fair" probabilities without margin, divide each figure by 1.048. Home becomes 45.4%, the draw 28.1% and the away side 26.5%, which add back to 100%. Those translate to fair odds of roughly 2.20, 3.56 and 3.77. The gap between 2.10 and 2.20 is what you pay for the bet on the home side.

This simple proportional method spreads the margin evenly. In practice bookmakers often load more margin onto longshots than favourites, so treat the result as an estimate rather than an exact figure.

Where beginners go wrong with odds

Most odds mistakes are not about maths. They come from reading the number too quickly.

The first is treating a short price as a safe bet. A 1.20 favourite implies 83.3%, which still means it loses about one time in six if the price is fair. Stack several of those together in an acca and the losses come quickly; our guide to parlays and accumulators shows how the chances shrink.

The second is ignoring the margin. Markets with many outcomes, such as first goalscorer, correct score or outright winner, often carry a far bigger overround than a two-way market. Add up the implied percentages before you get excited about a 12/1 shot.

The third is not comparing prices. If one licensed sportsbook offers 2.10 and another 2.00 on the same outcome, the first one pays 10% more profit for the same risk. Over hundreds of bets that difference decides a lot. The fourth is forgetting that the price moves. Odds shorten when money comes in or news breaks, so the number you saw in the morning may not be the one you get at kick-off.

Finally, betting should stay entertainment with money you can afford to lose. Sports betting rules differ by country and, in the US and India, by state, so check the law where you live and use operators licensed there. If betting stops feeling under control, see our responsible gambling page.

Before you place the bet: a quick odds checklist

Run through these steps on any price, whatever the sport or market:

• Convert the price to decimal if it is not already, so stake x odds gives your return.
• Work out the implied probability with 1 / odds.
• Write down your own estimate of the chance and compare the two.
• Check the overround of the market by adding the implied percentages of all outcomes.
• Compare the same price at more than one licensed sportsbook.
• Decide the stake before you look at the potential return, not after.

For a worked version: you see a player at 6/4 to win a tennis match. In decimal that is 2.50, so £20 returns £50. The implied probability is 40%. Your own view, based on form and surface, puts the player nearer 45%. The opponent is priced at 1.53, implying 65.4%, so the book totals 105.4%. The margin is a little above average, but your estimate still sits five points above the price. That is the kind of reasoning that turns odds from a payout table into a decision. For the main two-way and three-way markets, read our guide to moneyline bets.