Odds (fractional and decimal)
Odds are the price a bookmaker puts on an outcome, telling you what a winning bet pays relative to your stake, written either as a fraction such as 5/2 or as a decimal such as 3.50. Fractional odds show profit against stake, while decimal odds show the total return for every euro staked, stake included. Both formats describe the same price, and Irish bookmakers usually let you switch between them in the account settings.
How do you convert fractional and decimal odds?
The sum is short: divide the fraction and add one. So 5/2 becomes 2.5 plus 1, which gives 3.50, and evens (1/1) becomes 2.00.
A €10 bet at 5/2 returns €35 if it wins, which is €25 profit plus your €10 stake back. The decimal price gets you there in one step, since 10 multiplied by 3.50 is the same €35.
Implied probability comes from the decimal price as well: one divided by the decimal odds. At 3.50 the bookmaker is pricing the outcome at roughly 28.6%, and at evens it is exactly 50%.
What the price hides: margin and free bets
Add up the implied probabilities across a whole market and the total comes out above 100%, because the bookmaker’s margin, known as the overround, is built into every price.
Take a two-way market priced at 10/11 on each side, which is about 1.91 in decimal. Each side implies around 52.4%, so the book totals roughly 104.8%, and that extra 4.8 points is the bookmaker’s edge before you have backed anything. How wide that margin runs varies from one operator and one market to the next, so do the sum yourself rather than assuming a standard figure.
Free bets are where the two formats trip people up most often. A free bet usually returns the winnings only, without the stake, so a €10 free bet at 5/2 pays €25 rather than the €35 the decimal price suggests.
