Irish betting sites quote fractional odds by default, exchanges and stats sites live in decimals, and American sportsbooks use plus/minus moneylines. Same bet, three dialects. This page gives the conversion formulas, a reference table, and the one habit worth building: turning any price into an implied probability before you judge it. For quick conversions, our odds converter tool does the arithmetic for you.
The three formats in one line
Fractional 5/2 means “win 5 for every 2 staked”. Decimal 3.50 means “total return of 3.50 per 1 staked, stake included”. American +250 means “win 250 on a 100 stake” (and −150 means “stake 150 to win 100”). All three describe the identical price.
Conversion formulas
Fractional to decimal: divide the fraction and add 1 — 5/2 → 2.5 + 1 = 3.50. Decimal to fractional: subtract 1 and express as a fraction — 1.80 → 0.80 = 4/5. Decimal to American: if the decimal is 2.00 or more, (decimal − 1) × 100; if below 2.00, −100 ÷ (decimal − 1). American to decimal: positive lines are (line ÷ 100) + 1; negative lines are (100 ÷ |line|) + 1.
Implied probability: the only number that matters
From decimal odds, implied probability is simply 100 ÷ decimal. So 2.00 is 50%, 1.50 is 66.7%, 5.00 is 20%. From fractions, probability = denominator ÷ (numerator + denominator): 5/2 is 2 ÷ 7 = 28.6%. Every betting judgement reduces to one question — do you believe the true chance is higher than the implied probability? If you cannot answer that, the format of the odds is the least of your problems.
Reference table
| Fractional | Decimal | American | Implied % |
|---|---|---|---|
| 1/5 | 1.20 | −500 | 83.3% |
| 1/2 | 1.50 | −200 | 66.7% |
| 4/5 | 1.80 | −125 | 55.6% |
| Evens (1/1) | 2.00 | +100 | 50.0% |
| 6/4 | 2.50 | +150 | 40.0% |
| 5/2 | 3.50 | +250 | 28.6% |
| 5/1 | 6.00 | +500 | 16.7% |
| 10/1 | 11.00 | +1000 | 9.1% |
| 33/1 | 34.00 | +3300 | 2.9% |
Why Irish sites show fractions — and why decimals are better for you
Fractions are racing heritage: the on-course boards and generations of Irish and British punters speak 5/2 and 100/30 natively. But fractions hide small price differences — 11/8 vs 7/5 takes a moment’s arithmetic, while 2.38 vs 2.40 is instant. Decimals also make margin visible: add up 100 ÷ odds for every runner and the amount over 100% is the bookmaker’s take. Most Irish sites let you switch the display to decimals in settings; it is worth doing. 18+ — if the fun stops, stop: Gambling Awareness Trust, 1800 753 753, gamblingcare.ie.
Frequently Asked Questions
Divide the fraction and add 1. So 5/2 becomes 2.5 + 1 = 3.50, and 4/5 becomes 0.8 + 1 = 1.80.
The chance of winning that a price assumes: 100 divided by the decimal odds. Decimal odds of 4.00 imply 25%. Compare that with your own estimate of the true chance to judge whether a bet is value.
Tradition from horse racing, where on-course boards have always shown fractions. Almost every licensed site lets you switch to decimal display in your account settings, which makes prices easier to compare.