### Probability

A straightforward introduction to the concept of probability as it’s used in sports betting.

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Bookmakers have developed various different systems for expressing odds. Among the odds formats offered by most online bookmakers are:

Fractional odds are the most widely used odds format in the United Kingdom. Decimal odds are popular in Europe, and moneyline odds are used mostly in the United States.

Bookmakers offer odds on just about all competitive sports. As an example, say there’s an estimated 20% chance of a horse winning a race outright. Another way of putting this is that there’s a one in five chance of the horse winning.

In betting odds, the probability of 20% (or one in five) is expressed in one of three ways.

**The probability of 20% is expressed in fractional odds as 4/1 (or as 4:1, or sometimes just as 4 – the 1 is assumed)**.

Statistically, this implies that a horse with odds of 4/1 would lose four times and win once out of every five races.

If you bet £1 on a selection with fractional odds of 4/1, it means that:

- if your bet wins, you’ll be paid £4 plus the £1 you staked – so in total, you’ll be paid out £5
- if your bet loses, you’ll lose the £1 you staked.

**The probability of 20% is expressed in decimal odds as the number 5**.

This is calculated as 100 divided by the probability of 20%.

Unlike fractional odds, decimal odds include your stake.

If you bet £1 on a selection with decimal odds of 5, it means that

- if your bet wins, you’ll be paid £5 – this includes your refunded stake of £1
- if your bet loses, you’ll lose the £1 you staked.

**The probability of 20% is expressed in moneyline odds as +400**.

Moneyline odds, or US odds, are tricky for anyone who’s not used to them. If a plus sign appears before these odds or they’re expressed as whole numbers, it means that the odds tell you how much you’ll win if you stake £100 on a selection and your bet wins.

If a minus sign appears before the odds, it means they tell you how much money you have to stake in order to win £100.

Moneyline odds of +400 (or just 400) mean that if you bet £100 on a selection and your bet wins, you’ll win £400 and your stake of £100 will be refunded. So if you stake £1 and your bet wins, you’ll be paid £4 plus the £1 you staked – a total payout of £5.

In sports betting, it’s always important to shop around for the best odds. More favourable odds will mean you’ll get paid out more if a bet you place wins, and lose less if your bet loses.

Especially for major sporting events, odds are offered by large numbers of competing bookmakers. Generally it’s best to stick to well-known, established bookmakers – see our guidelines on avoiding blacklisted bookmakers for more information.

Even if you rule out less reputable bookmakers, you’ll be left with a fairly large selection of bookmakers, and it can be time-consuming to visit each of these bookie’s sites online. Instead save time and ensure you find the best, most competitive odds in all sports betting markets by

- checking odds from top bookmakers
- using one of several other odds comparison web sites.

A final option is to investigate the odds on betting exchanges like Betfair. On a betting exchange, you can set your own odds for an event or accept odds set by another user. Sometimes this means you can get better odds than are available from traditional bookmakers.

When a betting exchange attracts a high enough volume of users, continual trading results in ongoing changes to the odds on offer, much like market movements affect the prices in a financial stock exchange. This can result in odds that are highly accurate. For more information about betting exchanges and how to use them, visit our betting exchanges page.

William Hill offers prices in all popular odds formatsA straightforward introduction to the concept of probability as it’s used in sports betting.

You might be familiar with fractional and decimal odds, but what are moneyline odds and how do they work?

Fractional odds are the most popular in the UK. Learn how to interpret them.