How Betting Odds Work: Fractional, Decimal and American Odds Explained

Every betting price answers two questions at once. The first is how much you get back if the bet wins. The second, which bookmakers would rather you did not ask, is how likely they think the outcome is. Once you can read a price both ways, odds stop being a format you tolerate and become the information you bet on.

There are three common formats. They are the same numbers written differently, and the converter above moves between them.

Fractional odds are the traditional British format. The number on the left is the profit, the number on the right is the stake needed to win it. At 5/2 you win £5 for every £2 staked, so a £10 bet returns £25 profit plus your £10 stake, £35 in all. At 1/4 ("four to one on") you win £1 for every £4 staked.

A few prices have names. 1/1 is "evens": you double your money. Anything bigger than evens, such as 3/1, is "odds against"; anything smaller, such as 1/2, is "odds on". Some bookmakers write odds-on prices with the larger number second, which reads oddly until you remember the fraction always means profit over stake.

Decimal odds are standard across Europe, on betting exchanges and in most apps. The number is the total return per unit staked, including the stake. At 3.50, a £10 bet returns £35, of which £25 is profit. Evens is 2.00, 1/4 is 1.25, 10/1 is 11.00. To convert a fraction, divide the top by the bottom and add one: 5/2 becomes 2.5 + 1 = 3.50.

Decimals are easier to compare and easier to multiply, which is why accumulators, exchange trading and odds comparison all use them. The only thing to remember is that a decimal price always includes your stake, so 1.00 is not a price at all, it is your money back.

American odds, also called moneyline odds, use a base of 100. A positive number is the profit on a 100 stake: +250 means a £100 bet wins £250, equivalent to 5/2 or 3.50. A negative number is the stake needed to win 100: −200 means you must stake £200 to win £100, equivalent to 1/2 or 1.50. Evens is written +100.

To convert: positive American odds become decimal by dividing by 100 and adding 1 (+250 → 3.50); negative odds become decimal by dividing 100 by the number and adding 1 (−200 → 1.50).

Divide 1 by the decimal odds and you have the probability the price implies. Evens (2.00) implies a 50% chance. 3.50 implies 1 ÷ 3.5 = 28.6%. 1.25 implies 80%. This is the single most useful calculation in betting, because it turns a price into a claim you can argue with: "is this team really a 28.6% chance?"

For fractional odds the formula is stake ÷ (stake + profit): at 5/2 that is 2 ÷ 7 = 28.6%, the same answer.

Add up the implied probabilities of every outcome in a market and the total comes to more than 100%. A tennis match priced at 1.91 for each player implies 52.4% + 52.4% = 104.7%. That extra 4.7% is the overround, the bookmaker's built-in margin. A football match at 2.10, 3.40 and 3.60 adds up to 47.6% + 29.4% + 27.8% = 104.8%.

The margin is what the bookmaker earns on balanced books, before anyone has been right or wrong about anything. Low-margin bookmakers run at around 2% to 3% on major markets; high-street brands often sit between 5% and 8%, and the margin on obscure markets, accumulators and bet builders can be far higher. Enter a market's prices in the calculator above to see the margin and the "fair" odds you would get with no margin at all.

Because the margin is spread across the outcomes, the fair price of any selection is roughly the quoted decimal odds multiplied by the overround. At 1.91 in a 104.7% market, the fair price is about 2.00: a genuine coin flip.

A bet has value when the chance you assign to it is higher than the implied probability of the price. If you believe a team wins 35% of the time and the price implies 28.6%, the bet is value, regardless of whether it wins tonight. Over many bets, value is the only thing that separates a profitable bettor from an unlucky one, and the bookmaker margin is the headwind every bettor has to beat first.

The simplest way to find value is to compare prices. Different bookmakers have different opinions and different margins, and the best available price on a selection is often 5% to 10% better than the worst. Our odds comparison guide covers how to do that systematically, and our operator reviews note which bookmakers run the tightest margins.

You win £5 for every £2 staked. A £10 bet returns £35 including the stake. As a decimal it is 3.50, and it implies a 28.6% chance.

1.50 is 1/2, "two to one on". A £10 bet returns £15.

Decimal, for anything involving comparison or multiplication. Fractions are fine once you are used to them; American odds only matter if you bet with US-facing books.

Stake £110 to win £100, equivalent to 1.91 or 10/11. It is the standard price on US point spreads and reflects a 4.5% margin when both sides are −110.

No. Your bet settles at the price you took, except where you chose starting price or a best-odds-guaranteed promotion applies.

Related guides: each-way betting explained, accumulators explained, draw no bet and handicap betting, all of which rely on the conversions above.

Sources: Gambling Commission, information for the public and players (official consumer guidance); and Pinnacle, Betting Resources (operator educational material on margins and value). Margins quoted are typical ranges observed on major markets and vary by operator, sport and market.