How Betting Odds Work: A Complete Guide

Betting odds tell you two things: how much you can win, and how likely the bookmaker thinks an outcome is. Understanding odds is the foundation of any betting strategy — whether you're placing your first wager or refining your approach after years of experience. This guide covers the three major odds formats, how to calculate payouts and implied probability, and how to identify value in the market.

Last updated: April 2026

The Three Odds Formats

Odds come in three formats: fractional (used primarily in the UK and Ireland), decimal (standard across Europe, Australia, and Canada), and American (the default in the United States). All three express the same information — the relationship between your stake and your potential return — but they do it in different ways. Knowing how to read all three is essential if you're comparing prices across international bookmakers.

Implied Probability	Fractional	Decimal	American	£10 Stake Returns
50%	1/1 (Evens)	2.00	+100	£20
33.3%	2/1	3.00	+200	£30
20%	4/1	5.00	+400	£50
66.7%	1/2	1.50	-200	£15
80%	1/4	1.25	-400	£12.50

Notice how all three formats describe the same bet — just in different notation. Verdecto's odds converter tool lets you switch between formats instantly.

Fractional Odds Explained

Fractional odds are the traditional format in the UK and Ireland. They show the profit you'll make relative to your stake. The number on the left is your potential profit; the number on the right is the amount you need to stake.

Odds Against (Profit Greater Than Stake)

5/1means £5 profit for every £1 staked. Place £10 and you get £60 back (£50 profit + £10 stake). 7/4means £7 profit for every £4 staked. Place £40 and you get £110 back (£70 profit + £40 stake). These are “odds against” because the bookmaker considers the outcome less likely than not.

Odds On (Stake Greater Than Profit)

1/5means £1 profit for every £5 staked. Place £50 and you get £60 back (£10 profit + £50 stake). The outcome is considered more likely than not — you're risking more to win less. Heavy favourites are always odds-on.

Evens

1/1 (Evens)means your profit equals your stake. Place £10 and you get £20 back. The bookmaker considers this roughly a 50/50 proposition (before their margin).

Common fractional odds you'll see: 2/1, 5/2, 7/4, 11/8, 4/6, 8/13, evens. If you're unfamiliar with the format, the key is always the same — left number is profit, right number is stake.

Decimal Odds Explained

Decimal odds are the default across Europe, Australia, and Canada. They're the simplest format to understand: multiply your stake by the decimal number and that's your total return (including your stake back).

How It Works

3.00 × £10 = £30 total return (£20 profit + £10 stake). 1.50 × £10 = £15 total return (£5 profit + £10 stake). The stake is always included in the calculation — that's the key difference from fractional odds.

Anything above 2.00is “odds against” (the outcome is considered less likely than not). Anything below 2.00 is “odds on”. Exactly 2.00 is evens. Many experienced bettors prefer decimal odds because comparison is instant — higher number means bigger return.

Decimal Odds	£10 Stake	Total Return	Profit
1.25	£10	£12.50	£2.50
2.00	£10	£20	£10
3.50	£10	£35	£25
6.00	£10	£60	£50

American Odds Explained

American odds use plus (+) and minus (-) signs. They're the default format in the United States and the format you'll see on Verdecto's World Cup odds pages.

Plus (+) Odds — The Underdog

+200means $200 profit on a $100 stake. The number tells you exactly how much profit you'd make on a $100 bet. +450 means $450 profit on $100. +1000 means $1,000 profit on $100. The higher the plus number, the bigger the underdog and the bigger the potential payout.

Minus (-) Odds — The Favourite

-150 means you need to stake $150 to win $100 profit. The number tells you how much you need to risk to win $100. -300 means risking $300 to win $100. The larger the minus number, the heavier the favourite and the more you need to stake for the same profit.

+100is the American equivalent of evens — $100 profit on $100 stake. It's the dividing line between favourites and underdogs in American odds notation.

Calculating Implied Probability

Every set of odds can be converted into an implied probability — the bookmaker's assessment (including their margin) of how likely an outcome is. This is the single most useful concept in betting. If you can convert odds to probability, you can compare the bookmaker's view with your own.

Format	Formula	Example	Result
Fractional (5/1)	denominator / (numerator + denominator)	1 / (5 + 1)	16.7%
Decimal (3.00)	1 / decimal odds	1 / 3.00	33.3%
American (-150)	absolute value / (absolute value + 100)	150 / (150 + 100)	60%
American (+200)	100 / (odds + 100)	100 / (200 + 100)	33.3%

These implied probabilities include the bookmaker's margin (also called the vig or overround). That means if you add up the implied probabilities of all possible outcomes in a market, the total will always exceed 100%. The amount by which it exceeds 100% is the bookmaker's built-in profit. More on this below.

Understanding Value in Betting

Value is the most important concept in profitable betting. A value bet exists when your estimated probability of an outcome is higher than the implied probability of the odds. In other words: the bookmaker is offering you better odds than you think are justified.

Value Bet Example

Suppose Spain are available at +450to win the World Cup. The implied probability is approximately 18%. But after your own analysis — squad quality, form, bracket path, tactical setup — you believe Spain have a 25% chance. Since your estimated probability (25%) exceeds the implied probability (18%), this is a value bet. You don't need Spain to win for the bet to have been correct in principle. Over many similar bets, backing value consistently produces a positive return.

Expected Value (EV)

Expected Value is the mathematical expression of value. It represents your average profit or loss per bet if you could repeat the same wager thousands of times. EV = (probability of winning × profit) - (probability of losing × stake). If EV is positive, you have a value bet. If EV is negative, the bookmaker has the edge. Professional bettors are, at their core, people who consistently find positive EV opportunities.

Verdecto's odds hub is designed to help you compare prices across bookmakers — because finding value often starts with finding the best available price for a given selection.

The Bookmaker's Margin (Overround)

Bookmakers don't set fair odds. They build in a profit margin — called the overround or vigorish — on every market. Understanding how this works is essential for spotting value and managing expectations.

A Simple Example

A fair coin flip is 50/50. In a truly fair market, both outcomes would be priced at evens (2.00 decimal, +100 American). The implied probabilities would total exactly 100%. But bookmakers price each side at around 10/11(1.91 decimal, -110 American). The implied probability for each side becomes 52.4%, totalling 104.8%. That extra 4.8% is the overround — the bookmaker's built-in edge.

The Overround Formula

Add up the implied probability of every selection in a market. Subtract 100%. The result is the overround. Overround = (sum of all implied probabilities) - 100%. A typical two-way market runs a 3–5% overround. A three-way market (home/draw/away) might run 5–8%. An outright winner market with 48 teams can run 20–40%.

Margins vary significantly between bookmakers and between market types. Major match markets tend to have the tightest margins because competition between operators is fiercest there. Niche markets — correct score, first goalscorer, outright winner — carry wider margins because fewer bettors are price-comparing in those markets. Shopping around for the best odds is one of the simplest ways to reduce the impact of the overround on your returns.

Odds Converter Tool

Switching between fractional, decimal, and American formats in your head gets tedious — especially when you're comparing prices across multiple bookmakers that use different default formats. Verdecto's odds converter tool handles the conversion instantly. Enter any odds in any format and get the equivalent in all three, plus the implied probability and potential returns for any stake amount.