Ireland bets in two languages. The racecourse, the shop board and most casual conversation still speak fractions — 5/2, 11/8, 4/6 — while betting apps and exchanges default to decimals. They encode the same thing, but one of them makes the bookmaker’s take visible at a glance. Learning to flip between them is the single highest-value five minutes in betting education.
What each format says
Fractional odds state profit relative to stake: 5/2 means €5 profit for every €2 staked. Decimal odds state the total return per €1 staked, stake included: the same price is 3.50, because €1 returns €2.50 profit plus your euro back. Conversion is one step each way: decimal = (numerator ÷ denominator) + 1, and fractional is the decimal minus 1, expressed as a fraction.
Conversion table
| Fractional | Decimal | Implied probability |
|---|---|---|
| 1/4 | 1.25 | 80.0% |
| 4/6 | 1.67 | 60.0% |
| Evens (1/1) | 2.00 | 50.0% |
| 6/4 | 2.50 | 40.0% |
| 5/2 | 3.50 | 28.6% |
| 5/1 | 6.00 | 16.7% |
| 10/1 | 11.00 | 9.1% |
The implied probability formula
From decimal odds: implied probability = 1 ÷ decimal odds. A 2.50 shot implies 40%; a 1.25 shot implies 80%. From fractions, convert to decimal first. This number is what the price claims about the world — and the bookmaker’s claims across a whole market always add to more than 100%, which is where the margin hides.
Why decimal exposes the margin faster
Sum 1÷odds for every outcome of a market in decimals and you get the book’s total implied probability in seconds: 1÷1.90 + 1÷3.50 + 1÷4.20 ≈ 105%, so the margin is about 5%. Doing the same from 10/11, 5/2 and 16/5 takes three conversions first. Fractions evolved for calling prices across a betting ring; decimals evolved for comparing value. Use the format that suits the job — and when judging a price, that is decimal.
The Irish habit, kept honestly
Nobody at Leopardstown is switching to 3.50 over 5/2, and there is no need to. Keep the fractions for the craic; keep a converter in your head for the maths. One check worth automating: “odds-on” in fractions (4/6, 8/11) means a decimal below 2.00 — you are risking more than you stand to win, so the implied probability claim is above 50% and deserves extra scrutiny. 18+ — Gambling Awareness Trust, 1800 753 753, gamblingcare.ie.
Frequently Asked Questions
Divide the numerator by the denominator and add 1: 5/2 → 2.5 + 1 = 3.50; 4/6 → 0.667 + 1 = 1.67. Decimal odds include your returned stake; fractions state profit only.
It is the probability a price implies: 1 ÷ decimal odds. Odds of 2.50 imply 40%; odds of 1.25 imply 80%. Summed across a whole market the figures exceed 100% — the excess is the bookmaker’s margin.
Tradition from the racecourse and the shop board — fractions are the spoken language of Irish racing. Every major app lets you switch the display to decimal in settings, which is the better format for comparing prices and spotting the margin.