Implied probability explained

The formula: decimal odds to implied probability

For decimal odds, the calculation is simple:

Implied probability (%) = (1 ÷ decimal odds) × 100

Worked examples: - 2.00 → 1 ÷ 2.00 = 0.50 = 50% - 1.50 → 1 ÷ 1.50 = 0.667 = 66.7% - 3.50 → 1 ÷ 3.50 = 0.286 = 28.6% - 6.00 → 1 ÷ 6.00 = 0.167 = 16.7%

The shorter the price, the higher the implied probability — the bookmaker thinks the outcome is more likely. A price of 1.20 implies 83.3%; a price of 11.00 implies just 9.1%.

This is the single most useful skill for reading a market. Once every price is a percentage, you can compare outcomes on the same scale, whether it's a Premier League match, a hurdle at Leopardstown, or an ATP Tour first-round tie.

Fractional odds: the same idea, different notation

Irish and UK bookmakers still show fractional odds on horse racing and GAA markets. Convert to implied probability like this:

Implied probability (%) = denominator ÷ (numerator + denominator) × 100

Worked examples: - 2/1 → 1 ÷ (2 + 1) = 33.3% - 5/2 → 2 ÷ (5 + 2) = 28.6% - 1/2 → 2 ÷ (1 + 2) = 66.7% - 10/3 → 3 ÷ (10 + 3) = 23.1%

If it helps, turn fractional into decimal first (numerator ÷ denominator + 1), then use 1 ÷ decimal. For example, 5/2 = (5 ÷ 2) + 1 = 3.50, and 1 ÷ 3.50 = 28.6%. Same answer, two routes.

The overround: why the percentages add up to more than 100

Take a two-way market — say a tennis match with no draw. Fair odds for a genuine 50/50 would be 2.00 each, giving 50% + 50% = 100%. But a bookmaker won't offer that. They might price both players at 1.90.

1 ÷ 1.90 = 52.6%, and 52.6% + 52.6% = 105.2%.

That extra 5.2% is the overround (also called the margin or the vig). It's the bookmaker's built-in edge. Across a full book, the total implied probability always exceeds 100% for the operator to make money regardless of the result.

The lower the overround, the better value the market — you're paying less to the house. This is where price comparison earns its keep, because two bookmakers can price the same match with very different margins.

A three-way worked example: Premier League match odds

Football match odds have three outcomes: home, draw, away. Suppose a bookmaker prices a Premier League fixture:

• Home 2.10 → 1 ÷ 2.10 = 47.6%
• Draw 3.40 → 1 ÷ 3.40 = 29.4%
• Away 3.80 → 1 ÷ 3.80 = 26.3%

Total = 47.6% + 29.4% + 26.3% = 103.3%

The overround here is 3.3%. To strip the margin out and see the bookmaker's 'true' view, divide each implied probability by the total:

• Home: 47.6 ÷ 103.3 = 46.1%
• Draw: 29.4 ÷ 103.3 = 28.5%
• Away: 26.3 ÷ 103.3 = 25.5%

Those now sum to 100%. This adjusted figure is what the bookmaker actually reckons before their cut. If your own assessment of the home win is meaningfully higher than 46.1%, the 2.10 price may hold value. If it's lower, you'd be backing an outcome the market rates more generously than you do.

Using implied probability to find value

Value betting comes down to comparing two percentages: your estimate of an outcome versus the implied probability in the price.

• If your estimate is higher than the implied probability, the odds are longer than they should be — potential value.
• If your estimate is lower, you're being offered a worse deal than you'd give yourself — avoid.

Example: you rate a GAA outsider at roughly 30% to win. The bookmaker offers 4.50, which implies 1 ÷ 4.50 = 22.2%. Because your 30% is well above the 22.2% implied by the price, that bet looks like value on your read.

The honest caveat: this only works if your estimate is realistic. Implied probability tells you what the market thinks; it doesn't tell you what will happen. Treat your own figure with scepticism, especially in markets you don't follow closely.

Margins differ by bookmaker — and it shows in the numbers

Because the overround is baked into every price, a tighter-margin bookmaker gives you a better implied deal on the same outcome. Our sampled odds data across Irish-facing operators shows meaningful spread in average margin:

• Betfair: average margin around 4.29% (sampled 213 markets), the lowest in our sample, and it took the best price on 53.1% of sampled markets.
• Unibet: around 5.73% (191 markets).
• Ladbrokes: around 5.83% (193 markets), best price on 22.3% of markets.
• Betway: around 6.32% (195 markets).
• Paddy Power: around 6.93% (195 markets).
• William Hill: around 6.96% (219 markets).
• bet365: around 7.44% (30 markets sampled).
• BoyleSports: around 7.51% (195 markets).
• BetVictor: around 7.51% (188 markets).
• LiveScore Bet: around 8.37% (193 markets), the highest average margin in our sample.

Read these as tendencies, not guarantees on any single match. A lower average margin means the implied probabilities across that operator's book sit closer to 100%, so you're paying less to the house on average. This is why serious bettors check more than one price before staking. See our comparisons and betting odds pages to line up the same market side by side.

Which bookmaker suits a value-focused bettor

If implied probability and margin are what you care about, the choice narrows:

• Choose Betfair if you want the tightest sampled margin (≈4.29%) and it takes the best price on most markets in our sample; its odds rating is 8.9. Note it does not list a Bet Builder in its features here, so it's less suited if that's your main product.
• Choose bet365 if you want broad market coverage (rated 9.6) and depth (9.5) alongside Bet Builder, Cash Out and Live Streaming, accepting a higher sampled average margin.
• Choose Ladbrokes or Unibet if you want low margins (≈5.83% and ≈5.73%) from operators that also carry Live Betting, Cash Out and Bet Builder.
• Avoid leaning on a single high-margin book for value plays; the extra overround compounds across accumulators.

All operators listed here are licensed. Paddy Power, BoyleSports and Novibet are noted as regulated by the Gambling Regulatory Authority of Ireland (GRAI) in our data. Betting is 18+ in Ireland.

Implied probability and accumulators

Margins hurt most on multiples because the overround stacks. Combine three selections each carrying a 5% margin, and you're effectively paying that edge three times over.

Quick illustration: multiply the decimal odds to get the accumulator price. Three legs at 2.00, 1.80 and 3.00 give 2.00 × 1.80 × 3.00 = 10.80, an implied probability of 1 ÷ 10.80 = 9.3%. If each leg carried no margin, the fair combined price would be longer, so your real edge shrinks with every leg you add. Fewer legs at tighter prices generally leave more of the value with you than long-shot accumulators at wide margins.