Betting odds explained: how to read fractional, decimal & American odds

Betting odds, in the simplest terms, are a bookmaker’s price for an outcome. Each price tells you two things: how likely the outcome is, and how much you get if your bet wins.

You might see odds written as decimals or fractions, and this betting guide covers all three formats. By the end you’ll be able to read fractional, decimal and American odds, swap between them, work out your returns, work out the bookmaker’s margin, and turn odds into a percentage.

How do betting odds work? Guide to probability and payouts for beginners

Every price you see is really two pieces of information stitched together.

• The first is probability. The lower the odds, the more likely the odds compilers believe the outcome is. The higher the odds, the less likely they think it is. That’s why a title favourite is priced short and a rank outsider is priced long.
• The second is payout. Short odds pay out less because the outcome is expected; long odds pay more because it isn’t. So when you look at a price, you’re reading a probability and a potential return at the same time.

Here’s an example. Put a £20 bet on Argentina to win the FIFA World Cup 2026 and you’d find it under Outrights in the betting menu. At 9/1 (10.00, +900), a winning bet returns 9 times your stake in profit, which comes to £200 including your original stake: £180 profit on top of your £20 back.

Odds are not a pure reflection of real-world probability, hence why other World Cup betting sites might offer different odds.

The bookmaker’s margin is baked into the price, which is why the implied probability (i.e. the likelihood of that outcome occurring) you calculate is always a little higher than the true chance. More details on that in the overround section below, but first, let’s look at the three types of betting odds formats.

The three types of betting odds, explained: fractional, decimal & American

You may have already seen odds written as fractions, decimals or whole numbers. While the outcome (the implied probability) is ultimately the same, the way you read fractional, decimal, and American odds differs.

The table below shows the same set of prices in all three formats, then the corresponding implied probability. The second shows what each price actually pays on a £10 stake: the probability side and the payout side, split out.

Fractional odds explained: Left number is winnings, right number is stake

Fractional odds, also referred to as British odds, UK odds or traditional odds, tell you the potential winnings with the number on the left in the fraction. The number on the right in the fraction tells you the stake required for that profit.

So, a bet with odds of 2/1 gives you £2 for every £1 you stake.

The thing to remember is that fractional odds show winnings only. To get your total return (if the bet wins), subtract the amount you staked.

Here are a few examples:

A winning fractional bet returns your profit plus your original stake.

Decimal odds explained: Your stake x decimal odds equals your total return

Decimal odds are extremely easy to calculate. All you have to do is take the amount you plan to wager and multiply that by the decimal odds for your bet. The resulting amount is your total return, meaning the winnings plus your stake. Decimal odds differ from fractional odds in that the stake is included.

Here are a few examples:

Decimal returns already include your stake, so profit is the return minus £10.

American odds explained – Positive = profit on £100 stake, negative = stake needed to win £100

You’re unlikely to see them in the UK, but American odds, also referred to as the moneyline, are calculated using a £100 stake. Positive odds, which are given to the underdog, show you the potential return you’d get from a £100 stake. Negative odds, which are given to the favourite, show you how much you’d need to stake to win £100. 

Here are a few examples:

How to convert between odds formats

The easiest way to convert odds is by using an odds converter. You can use the one below, or a quick search online will bring one up. 

In case you don’t have a betting odds converter nearby, here’s how to convert the odds yourself for each type of odds. 

How to convert decimal odds to fractional, and fractional odds to decimal

To convert decimal odds to fractional, the formula is Fractional odds = (Decimal odds − 1) expressed as a simplified fraction. Here’s how it works step by step. 

1. Subtract 1 from the decimal odds
2. Put the result over 1 as a fraction
3. Multiply the top and bottom numbers by the same number so you have whole numbers on each side of the fraction. Use the lowest common denominator

Here are a couple of examples. 

If the decimal odds are 5.00, subtract 1 to make it 4 and put that over 1. The fractional odds are 4/1. 

If the decimal odds are 4.5, subtract 1 to make it 3.5, put that over one to make it 3.5/1. Multiply both sides by 2 to get a whole number on top and bottom. The fractional odds are 7/2. 

Converting fractional odds to decimal odds is much simpler. The formula is Decimal odds = 1 + (numerator/denominator). Here’s how it works. 

1. Divide the number at the top of the fraction (numerator) by the number at the bottom (denominator)
2. Add 1 to the outcome so that your stake is included (remember, decimal includes your stake being returned, fractional does not). 

For example, if you’re converting 5/2 to decimal: 

1. 5 ÷ 2 = 2.50
2. Add 1. 2.50 + 1 = 3.50
3. 3.50 is the decimal odds

If you’re converting 1/4 to decimal: 

1. 1 ÷ 4 = 0.25
2. Add 1. 0.25 + 1 = 1.25
3. 1.25 is the decimal odds 

How to convert decimal odds to American, and American odds to decimal: Step by step 

Decimal odds are converted to American using two different formulas, depending on the odds given. One formula is for decimal odds above 2.00, and the other is for odds below 2.00. 

For odds of 2.00 or higher, the formula is (Decimal odds – 1) × 100 = (American odds). 

For example, to convert 3.50 to American odds:

1. 3.50 – 1 x 100 
2. 3.50 – 1 = 2.50 
3. 2.50 x 100 = +250
4. +250 is the American odds

For odds lower than 2.00, the formula is -100 ÷ (Decimal odds – 1) = (American odds).

For example, to convert 1.25 to American odds:

1. -100 ÷ (1.25 – 1)
2. 1.25 – 1 = 0.25
3. -100 ÷ 0.25 = -400
4. -400 is the American odds

To convert American odds to decimal, there are two formulas, one for +100 odds or higher, and one for -101 or lower.

To convert +100 odds or higher to decimal, the formula is (American odds ÷ 100) + 1 = decimal odds. 

For example, to convert +250 to decimal:

• 250 ÷ 100 = 2.50
• Add 1. 2.50 + 1 = 3.50
• 3.50 is the decimal odds

To convert -101 odds or lower to decimal, the formula is (100 ÷ American odds) + 1 = decimal odds. 

For example, to convert -225 to decimal:

• Remove the minus sign (225)
• Divide 100 by the odds. 100/225 = 0.44
• Add 1. 0.444 + 1 = 1.44
• 1.44 is the decimal odds

How to convert fractional odds to American & vice-versa

To convert fractional odds to American odds, there are two separate formulas. One is for odds of 1/1 (EVS) or higher, and one for odds lower than 1/1. 

To convert odds of 1/1 (EVS) or greater to fractional, the formula is (Numerator ÷ denominator) × 100 = American odds. So if the top number (numerator) is equal to or greater than the bottom number (denominator), you’ll use this formula. 

For example, to convert 5/2 to American odds: 

• 5 ÷ 2 = 2.5
• 2.5 x 100 = 250
• +250 is the American odds

When the fractional odds are below 1/1, the formula is -100 ÷ (Numerator ÷ denominator) = American odds. 

For example, to convert 1/4 odds to American odds: 

• -100 ÷ (1 ÷ 4)
• 1 ÷ 4 = 0.25
• -100 ÷ 0.25 = -400
• -400 is the American odds

To convert American odds to fractional, there are two equations again. One is for positive odds, and one for negative. 

To convert positive American odds, the formula is American odds / 100 (then reduce). 

For example, to convert +220 to fractional: 

• 220/100 = 11/5
• 11/5 is the fractional odds

To convert negative American odds to fractional, the formula is inverted: 100 / American odds (then reduce)

For example, to convert -325 to fractional: 

• 100/325 = 4/13 [100 ÷ average 25 = 4, 325 ÷ 25 = 13]
• 4/13 is the fractional odds

Prices in practice: the betslip and bet builder

When you pick a selection, it drops onto your betslip (the little panel that shows your odds, the stake box, and the potential returns before you confirm and place your bet).

I am placing a few World Cup bets on William Hill and here’s what my betslip looks like. You’ll see the fractional odds and potential returns for winning bets:

A bet builder lets you combine several selections from a single match (say, a team to win, over 2.5 goals, and a named player to score) into one bet. Each leg’s odds are multiplied together, so the combined price is longer and the payout bigger, but every leg has to land. Bet builders are most popular on football, and you can find the deepest markets at the best football betting sites.

Betting odds and implied probability: converting odds to percentages

Now for the technical stuff. To convert betting odds to a percentage, you can use formulas to calculate it yourself. Here’s the formula for each and how to use them. 

How to convert decimal odds to implied probability

To convert decimal odds to a percentage, the formula is (1 / Decimal odds) × 100.

For example, to convert 2.50: 

1. 1/2.5 x 100 = 40
2. Implied probability: 40%

How to convert fractional odds to implied probability

To convert fractional to a percentage, the formula is Denominator ÷ (Numerator + denominator). 

For example, to convert 6/4:

1. 4 ÷ (4 + 6)
2. 4 ÷ 10 = 0.4
3. Implied probability: 40%

How to convert American odds to implied probability

To convert American odds to implied probability, you’ll need two different formulas. One is for American odds of +100 or greater, and one is for -101 or less. 

To convert positive American odds to a percentage, the formula is 100 ÷ (American odds + 100) x 100. 

For example, to convert +250:

1. 100 ÷ (250 + 100) x 100
2. 100 ÷ (350) x 100
3. 100 ÷ 350 = 0.285714285714
4. 0.285714285714 x 100 = 28.57%
5. Implied probability: 28.57%

To convert negative American odds to a percentage, the formula is American odds ÷ (American Odds + 100) × 100.

For example, to convert -300:

1. Remove the minus.
2. 300 ÷ (300 + 100) x 100
3. 300 ÷ (400) x 100
4. 300 ÷ 400 = 0.75
5. 0.75 x 100 = 75%
6. Implied probability: 75%

What is the bookmaker’s margin? Overround, vig & juice explained

The bookmaker’s margin (a.k.a. overround, vig, or juice) is whatever amount exceeds 100% after you calculate the sum of all implied probabilities in one betting market. It’s basically how the bookie makes money.

The sum of all implied probabilities goes over 100% because bookies deliberately price each outcome shorter than its true chance, which builds in a profit margin. That’s why bookmaker odds aren’t actually the real-world probability of a bet. 

How to calculate a bookmaker’s overround

Figuring out what the bookie’s cut is will help you to figure out how good the odds you’re being given are, and which bookmakers are giving you the best odds. It’ll take a bit of maths, but it’s definitely worth it if you’re the type of punter who bets frequently. 

1. Work out the implied probability of every possible outcome using the formulas above.
2. Add them together to get the total market percentage.
3. Subtract 100.

Whatever’s left is the bookmaker’s margin.

For example, to calculate the overround for a 3-way football market, figure out the implied odds for all 3 potential outcomes (home wins, match draw, or away wins). Add them up and then subtract 100. Whatever is left over is the bookie’s overround.

Here are the odds from a real three-way football market that already took place: Panama v England at the World Cup 2026, priced by Sky Bet.

Overround: 12.5% + 15.38% + 80% = 107.88%, then 107.88% − 100% = 7.88%. Sky Bet’s overround for this market is 7.88%.

Now, the point of doing this maths is comparison. Run the same sum on the same market at another bookmaker, and you’ll see who’s charging the smaller cut.

Below is an example only. The figures aren’t indicative of a live price, just a demonstration of how a second book (compare, for instance, the BetFred review price on the same market) might stack up.

Which UK betting sites offer the best odds with the lowest margins?

The lowest margins are generally from betting exchanges. Betfair’s Exchange and Smarkets both claim to run under 2% on popular markets, with Betfair Exchange averaging less than 1% in my testing, detailed in the Betfair review.

You’re betting against other punters and paying commission on winnings rather than that overround baked into the price. The trade-off is that exchanges are less beginner-friendly, so they tend to suit more experienced punters.

Among traditional UKGC-licensed books, no site’s always cheapest. Margins move by market, by sport and by day, so the honest answer is “it depends”.

That’s exactly why it’s worth knowing how to calculate the overround maths yourself. Patterns do show up when you test the same market across many sites at once.

In our latest odds testing (Premier League match-result prices across all 10 Matchday 38 fixtures, sampled in May 2026), three bookies consistently priced below the 6% margin mark, and bet365 posted the most best-priced selections of any operator we checked.

At the other end, the same test flagged some noticeably wider prices. Virgin Bet at 11.39%, BetMGM at 8.89% and NetBet at 8.49% on that Matchday 38 sample. A “big brand” doesn’t automatically mean “best price.”

A few asterisks:

• These numbers are just a snapshot, not a permanent value indicator. Generally, the more unpredictable the market, the wider the margin.
• Weigh up special features like odds boosts and Best Odds Guaranteed alongside the raw margin.
• Only use UK Gambling Commission–licensed sites. Even if offshore or “non-GamStop” operators show better prices, you run more risks with your money and data.

Over hundreds of bets, a two-point difference in margin compounds into a big impact on returns. It won’t change a losing bet into a winning one, but you shouldn’t need to pay more than necessary to make the exact same bet.

What are “good” betting odds (understanding value)?

Sports betting is all about identifying value. However, to do that, you need to have an intimate understanding of the matchup. That includes players, what their strengths and weaknesses are, how they match up with the other team, and the list goes on. 

Once you have an idea of how likely an outcome is through research, you can make your own estimation of its probability. Assigning your own implied probability to a bet will allow you to identify whether or not you’re getting a good deal from the bookmaker. 

For example, you’ve done your homework on the Panama v England match and you’ve estimated that England has an 85% chance to win (this is hypothetical). 

Sky Bet is offering 1/4 odds, which, after doing the maths, converts to 80% odds. Since you believe England has an 85% chance to win, you’re getting a bargain from the bookie. 

You will not always win your bet. Key words: estimated and believe.

Bookmakers employ trading teams who specialise in identifying and eliminating opportunities for punters to take advantage of the odds. They look for things like arbitrage opportunities and prices that haven’t moved to reflect a shift in an outcome’s likelihood, so you won’t find them first.

For the average punter: putting your own percentage on an outcome before you check the price is the most useful habit in this guide. It changes betting from “who do I fancy?” to “is this price worth taking?“

With that said, it is possible to find valuable betting opportunities on UK betting sites. 

Why do betting odds change? Steamers, drifters & injuries

Odds move for two main reasons: things that change the real chances in the game, and the money coming in that the bookmaker has to balance.

External factors that change the real chances:

Betting action from bettors: the bookmaker wants the money balanced, so that winning bets are mostly paid out of losing ones and the overround is left as profit. If everyone backs the over on England’s goals, the bookmaker might cut the over and boost the under to make the under more tempting and even things out.

This is where the Best Odds Guaranteed feature comes in. Because prices move between now and the start, some bookmakers promise that if you take an early price on a horse and the starting price is bigger, they’ll pay you at the bigger price.

Betting action from punters: How betting money and market moves shift the odds

As mentioned, a bookmaker’s goal is ultimately to make a profit from the overround and let the punters essentially pay each other out. If the betting action is perfectly balanced, the bookies won’t have to dip into their own pocket to pay off winning wagers. In that case, the money that winning punters get will come from the losing wagers. 

With this in mind, bookies will try to balance out the betting odds so that the betting action is balanced between the two (or three) sides of a wager. 

For example, if most punters are taking the over on 2.5 goals for England against Panama, they might reduce the odds for that wager and increase the odds for the under. That way, the under will look more attractive and incentivise more punters to choose the under. 

Common mistakes beginners make with odds

Newcomers to the world of sports betting can get mixed up rather easily. There are a few things in particular that sometimes confuse beginners. 

Confusing decimal and fractional odds 

As we covered earlier, decimal odds include your stake along with the winnings. Fractional odds do not. You have to add the stake to the winnings yourself if you want to know the total return of a wager. 

Not comparing prices across multiple bookmakers

Many newbies don’t shop around for the best odds when they’re ready to make a bet. The best thing to do is compare prices for the same bet if you can find it across multiple bookmakers. Then, if you have an account at each of them, you can simply place the bet where you’re getting the best odds. 

An added benefit to this approach is that you can claim the welcome offer for multiple bookmakers, which increases your potential bankroll from the outset. 

Treating implied odds as the actual probability 

Since bookies need to make a profit, the overround gets factored into the odds. Because of this, when you convert the odds for a bet (any kind of odds) into implied probability, you won’t get a real-world percentage. You’ll simply get the bookie’s odds converted into a percentage. 

Ignoring the bookie’s margin

Since the bookmaker is trying to profit from the difference between their odds and the real probability, you won’t break even by winning some and losing some. That’s why it’s even more important to be able to identify value and make informed betting decisions.

Beginners’ glossary of betting odds terms

Betting odds FAQ

Responsible gambling

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