Reviewed by Darren · Last checked: July 2026
Table of Contents
Odds do two jobs at once: they tell you how much you’ll win, and — less obviously — they tell you what the bookmaker thinks the actual probability of an outcome is. Most bettors only ever learn the first part. Understanding the second part is what actually separates casual betting from informed betting, and it’s genuinely useful whether you’re betting in Ireland (fractional odds) or Canada (increasingly decimal and moneyline). This guide covers all three major odds formats, how to convert between them, and how to read the implied probability behind any price.
The Three Odds Formats
- Fractional (e.g. 5/1) — the traditional Irish and British format.
- Decimal (e.g. 6.00) — the standard format across most of Europe, and increasingly common on Irish and Canadian betting sites via a display toggle.
- American / Moneyline (e.g. +500 or -150) — the standard format in the US and widely used on Canadian sportsbooks, particularly since Ontario’s regulated market opened.
All three represent exactly the same underlying probability and payout — they’re just different ways of displaying it, and most modern betting sites let you switch between them in your account settings.
Fractional Odds Explained
Fractional odds show your profit relative to your stake. 5/1 means you win €5 for every €1 staked, plus your original stake back. A €10 bet at 5/1 returns €60 total (€50 profit + €10 stake). Fractions under 1 (like 1/2) mean you win less than your stake in profit — a €10 bet at 1/2 returns €15 total (€5 profit + €10 stake), reflecting a strong favourite.
Decimal Odds Explained
Decimal odds show your total return per unit staked, including your stake — which makes the maths simpler than fractional. Multiply your stake by the decimal odds to get your total return. A €10 bet at 6.00 decimal returns €60 total (€10 × 6.00) — the same result as 5/1 fractional, just calculated differently. A decimal odds of 1.50 on a €10 bet returns €15 total, matching the 1/2 fractional example above.
American (Moneyline) Odds Explained
American odds use a plus or minus sign relative to a $100 (or €100) baseline. Positive odds (e.g. +500) show how much profit a $100 bet would win — +500 means a $100 bet wins $500 profit, equivalent to 5/1 fractional. Negative odds (e.g. -150) show how much you’d need to stake to win $100 profit — -150 means you need to stake $150 to win $100 profit, reflecting a favourite (equivalent to roughly 2/3 fractional).
Converting Between Odds Formats
| Fractional | Decimal | American |
|---|---|---|
| 1/4 | 1.25 | -400 |
| 1/2 | 1.50 | -200 |
| 4/5 | 1.80 | -125 |
| Evens (1/1) | 2.00 | +100 |
| 5/4 | 2.25 | +125 |
| 2/1 | 3.00 | +200 |
| 5/1 | 6.00 | +500 |
| 10/1 | 11.00 | +1000 |
The general conversion rules:
- Fractional to decimal: divide the fraction, add 1 (e.g. 5/1 → 5 ÷ 1 = 5, +1 = 6.00)
- Decimal to fractional: subtract 1, convert to a fraction (e.g. 6.00 → 5.00 → 5/1)
- Decimal to American (odds ≥ 2.00): (decimal − 1) × 100 (e.g. 6.00 → 5.00 × 100 = +500)
- Decimal to American (odds < 2.00): −100 ÷ (decimal − 1) (e.g. 1.50 → −100 ÷ 0.50 = −200)
Implied Probability: What Odds Are Really Telling You
Every price implies a probability. To calculate it from decimal odds: implied probability = 1 ÷ decimal odds, shown as a percentage.
Example: Odds of 4.00 decimal (3/1 fractional, +300 American) imply a probability of 1 ÷ 4.00 = 0.25, or 25%. That means the bookmaker’s price is effectively saying there’s roughly a 1-in-4 chance of that outcome happening.
This matters because it lets you compare a price against your own view of the actual likelihood. If you think an outcome is genuinely more likely than 25% to happen, but the odds are pricing it at 25%, that’s arguably a value bet — the odds haven’t caught up with what you believe is the real probability.
Understanding the Bookmaker’s Margin
If you add up the implied probability of every outcome in a market, it comes to slightly more than 100% — that difference is the bookmaker’s margin (sometimes called the “overround”), built into every price to ensure profitability regardless of the result. A two-way market (like tennis) priced at 1.91 / 1.91 decimal implies roughly 52.4% + 52.4% = 104.8%, meaning the bookmaker is holding back roughly 4.8% margin. This is exactly why comparing odds across multiple sites matters — different bookmakers build in different margins, and the gap directly affects your long-term returns.
FAQs
Fractional odds (e.g. 5/1) remain the default display on most Irish betting sites, though decimal odds are usually available as a toggle in account settings.
American (moneyline) and decimal odds are both common, particularly since Canada’s regulated online sportsbook market opened — moneyline is especially standard for sites built around the North American sports betting market.
Convert to decimal first (divide the fraction and add 1), then divide 1 by the decimal figure. For example, 3/1 → 4.00 decimal → 1 ÷ 4.00 = 25% implied probability.
That extra percentage is the bookmaker’s built-in margin (the “overround”), which is how bookmakers structure prices to be profitable regardless of the outcome — it’s present in every market, on every operator, to varying degrees.
Neither automatically — value depends on whether the implied probability of the price is lower than the outcome’s actual likelihood, which has nothing to do with whether the odds themselves are short or long.
Want the full breakdown of every core betting term? Check our Betting Basics Glossary.
Related Guides: Betting Calculator: How to Use One · Accumulators Explained · Betting Basics Glossary · Handicap Betting Explained
