Betting Odds Explained

Odds are the language of betting, and three different dialects of it are in use: decimal in Europe and most of the world, fractional in Britain and Ireland, and American (moneyline) in the United States. All three say the same thing in different ways, and all three hide the same two facts: the probability the bookmaker has assigned to the outcome, and the margin it has built in. This guide explains how to read each format, how to convert between them, how to extract the implied probability, how to measure the bookmaker's margin, and what the common market types mean. It is written for people entering the industry as much as for bettors.

What odds are

Odds express the relationship between the stake and the payout on a winning bet, and, implicitly, the probability the price-setter assigns to the outcome. A price of 2.00 in decimal odds (evens, or +100) means a winning 10 stake returns 20, including the stake, and implies a probability of 50 per cent. Longer odds (a bigger payout) mean a less likely outcome; shorter odds (a smaller payout) mean a more likely one.

The two things to hold onto: odds are a price, set by a bookmaker to attract money and make a margin, not an objective statement of probability; and the sum of the implied probabilities across all outcomes in a market is always more than 100 per cent, and the excess is the bookmaker's margin.

Decimal odds

Decimal odds state the total return per unit staked, including the stake. A price of 3.50 returns 3.50 for every 1 staked (2.50 profit plus the 1 stake). Evens is 2.00; odds-on prices are between 1.00 and 2.00 (1.50 returns 1.50 per 1, a 0.50 profit); a 1.01 price is a near-certainty paying almost nothing.

Implied probability from decimal odds is 1 divided by the odds: 3.50 implies 28.6 per cent; 2.00 implies 50 per cent; 1.50 implies 66.7 per cent.

Decimal is the format used in continental Europe, Australia, Canada and most online markets worldwide, and the format most industry systems use internally because it makes the arithmetic trivial: return equals stake times odds.

Fractional odds

Fractional odds state the profit relative to the stake. 5/2 ("five to two") means 5 profit for every 2 staked, so a 10 stake wins 25 and returns 35 including the stake. 7/4 means 7 profit per 4 staked. Evens is 1/1. Odds-on prices put the larger number second: 1/2 ("two to one on") means 1 profit per 2 staked.

To convert fractional to decimal, divide the fraction and add 1: 5/2 is 2.5 plus 1, so 3.50; 1/2 is 0.5 plus 1, so 1.50. Implied probability is the denominator divided by the sum of numerator and denominator: 5/2 is 2 divided by 7, or 28.6 per cent.

Fractional is the traditional British and Irish format, still the default in horse racing and on high-street betting shops, and increasingly displayed alongside or replaced by decimal online.

American odds

American (moneyline) odds use a plus or minus sign with a base of 100. Positive odds state the profit on a 100 stake: +250 means 250 profit for 100 staked (3.50 decimal, 5/2 fractional). Negative odds state the stake needed to win 100: minus 200 means 200 staked to win 100 profit (1.50 decimal, 1/2 fractional). Evens is +100 (or, by convention, minus 100 on some boards).

Conversion: for positive odds, decimal is (American divided by 100) plus 1; for negative odds, decimal is (100 divided by the absolute value) plus 1. Implied probability: for positive odds, 100 divided by (American plus 100); for negative odds, the absolute value divided by (the absolute value plus 100). Minus 200 implies 66.7 per cent; +250 implies 28.6 per cent.

American odds are the format of the United States market and the one new entrants find least intuitive; the sign flip at evens is the source of most confusion.

Converting between formats

The decimal format is the bridge. Any price can be converted to decimal and then to the other formats:

Decimal 3.50 is fractional 5/2 and American +250. Decimal 1.50 is fractional 1/2 and American minus 200. Decimal 2.00 is fractional 1/1 and American +100. Decimal 1.91 (the standard price on a point spread) is fractional 10/11 and American minus 110.

For fractional odds that do not reduce neatly, bookmakers use conventional approximations (decimal 1.91 is displayed as 10/11; 1.83 as 5/6; 2.10 as 11/10).

Implied probability and the overround

The implied probability of a price is what the bookmaker's odds say the outcome's chance is, and it is always higher than the bookmaker's true estimate, because the margin is built into the price.

Take a football match priced at 2.50 home, 3.40 draw, 2.90 away. The implied probabilities are 40.0, 29.4 and 34.5 per cent, which sum to 103.9 per cent. The excess over 100, here 3.9 percentage points, is the overround (also called the vig, juice, margin or book). It is the bookmaker's expected gross margin if money is staked in proportion to the prices. A market with an overround of 103.9 pays out, on average, 100 divided by 103.9, or 96.2 per cent of stakes, so the bookmaker's expected margin is 3.8 per cent of turnover.

Overrounds vary by market and bookmaker: 2 to 5 per cent on major football and American sports match markets at competitive online bookmakers, 5 to 10 per cent on less liquid markets, 15 to 25 per cent or more on multi-runner horse races and on accumulators, where the margins on each leg compound. Comparing overrounds is the fastest way to compare bookmakers' prices, and the industry's own analysts use it as a measure of competitiveness. The How Bookmakers Set Odds and Make Money guide explains where the prices come from and how the margin is managed.

To find a bookmaker's true probability estimate from its prices, remove the overround: divide each implied probability by the sum. In the example, the home price's 40.0 per cent becomes 38.5 per cent of a fair book.

The main market types

Match result (1X2, moneyline). Which side wins; in sports with draws, three outcomes; in American sports, two, with overtime included and ties handled by the rules.

Point spread (handicap). One side is given a head start of points or goals so that the market is close to even; the bettor takes the favourite to win by more than the spread or the underdog to lose by less (or win). Priced at around minus 110 each side (1.91), which is a 4.5 per cent overround. The dominant market in American sports. Asian handicap is the football version, with quarter-goal lines that split the stake and refunds on a push.

Totals (over/under). Whether the combined score is above or below a line, priced around minus 110 each side.

Outright (futures). Who wins a competition; a multi-runner market with a large overround because the bookmaker prices every contender.

Each-way. A horse-racing (and golf) bet that is two bets: one to win and one to place, with the place part paid at a fraction of the win odds (a quarter or a fifth) for a specified number of places.

Accumulators (parlays). Several selections combined; all must win; odds multiply. The bookmaker's margin compounds with each leg, which is why accumulators are the most profitable product for bookmakers and the most marketed.

Props and specials. Player and event markets (first scorer, number of cards, a player's points). Less liquid, wider margins, and the growth area of American betting.

In-play (live). Any of the above, repriced continuously during the event.

Cash out. An offer to settle a bet before the event ends at a price reflecting the current state of the market, with a margin in the bookmaker's favour.

What price movements mean

Odds move for three reasons: new information (a team sheet, an injury, the weather), money (the bookmaker shortens the price on a selection taking a lot of stakes and lengthens the others, to balance its liabilities), and the sharp market (the bookmaker follows the prices of the exchanges and the bookmakers that accept the largest bets from the best-informed customers). The opening price is the bookmaker's model; the closing price, just before the event, is the market's consensus, and beating the closing price consistently is the measure of a skilled bettor and the signal bookmakers use to restrict accounts.

Betting exchanges

An exchange matches bettors against each other: one backs an outcome at a price, another lays it (bets against it) at the same price, and the exchange charges a commission on net winnings (2 to 5 per cent) instead of building a margin into the price. Exchange prices are therefore closer to true probabilities and are used across the industry as the reference for fair odds. Laying is the exchange's distinctive feature: a bettor can act as the bookmaker.

Odds and the industry

Prices are the product. A sportsbook's trading function sets and manages them; its risk function decides how much to accept at each price and from whom; its marketing sells the margin-rich products (accumulators, props, boosts) and the margin-thin ones (competitive match prices) as the loss leaders that bring customers in. The regulator's interest is in fairness (prices honoured, rules published, settlement correct) and in the marketing of enhanced odds and boosts, which are treated as promotions in most markets. The Sportsbook Trading course covers the discipline in full.

Frequently asked questions

What do odds of 5/2 mean? Five profit for every two staked: a 10 stake wins 25 and returns 35. In decimal, 3.50; in American, +250; implied probability 28.6 per cent.

What does minus 110 mean? Stake 110 to win 100 profit. It is the standard price on each side of a point spread or total, equal to 1.91 decimal or 10/11 fractional, and implies 52.4 per cent, so the two sides together carry a 4.8 per cent overround.

How do I convert odds to probability? Decimal: 1 divided by the odds. Fractional a/b: b divided by (a plus b). American positive: 100 divided by (odds plus 100); negative: odds divided by (odds plus 100), ignoring the sign.

What is the overround? The amount by which the implied probabilities in a market exceed 100 per cent. It is the bookmaker's built-in margin. A 105 per cent book is a 5 point overround and pays out about 95 per cent of stakes on average.

Are shorter odds a better bet? Not in themselves. A short price is a likely outcome with a small payout; the question is whether the price is better than the true probability, which the odds alone do not tell you.

Why do odds change after I bet? Because of new information, the weight of money on each side, or moves in the sharp market the bookmaker follows. A bet is settled at the odds taken, not the odds at kick-off, unless the terms say otherwise (starting price in racing).

Related on iGaming Times

How Bookmakers Set Odds and Make Money is the companion guide on the price-setter's side. RTP, House Edge and Margin Explained covers the same concept in casino terms. Prediction Markets Explained covers contracts priced as probabilities from 0 to 100, the format prediction markets use instead of odds, and Sports Integrity the course on what unusual price movements can indicate.

---

Regulation, tax and market figures move quickly, sometimes mid-year. Where this guide gives a number, treat it as a starting point and confirm the current position with the named primary source before you rely on it.