Implied Probability Explained

The formula: turning odds into a percentage

For decimal odds the maths is one line:

Implied probability = 1 ÷ decimal odds

So a Premier League favourite at 1.40 implies 1 ÷ 1.40 = 0.714, or 71.4%. An outsider at 6.00 implies 1 ÷ 6.00 = 0.167, or 16.7%. Multiply the decimal result by 100 to get the percentage.

Fractional odds, still standard on horse racing boards in the UK, use a slightly different formula:

Implied probability = denominator ÷ (numerator + denominator)

A horse at 5/2 implies 2 ÷ (5 + 2) = 2 ÷ 7 = 0.286, or 28.6%. Evens (1/1) implies 1 ÷ 2 = 50%. A short-priced favourite at 4/6 implies 6 ÷ (4 + 6) = 60%.

The quick sense check: shorter odds mean a higher implied probability, because the bookmaker thinks the outcome is more likely and pays out less if it lands.

Why the percentages add up to more than 100%

In a fair market with no profit built in, the implied probabilities of all outcomes would total exactly 100%. Bookmakers price them to total more than that. The extra slice is the margin (also called the overround or the vig), and it is how the operator makes money.

Take a two-way tennis market on the ATP Tour. If both players are priced at 1.90, each implies 1 ÷ 1.90 = 52.6%. Add them together: 52.6% + 52.6% = 105.2%. That 5.2% above 100% is the margin. In a truly even 50/50 contest a margin-free price would be 2.00 on each side, so the 1.90 price shows exactly where the operator's cut sits.

The margin data we hold reflects this directly. Across sampled markets, Betfair shows the lowest average margin in our set at roughly 4.6%, with Coral around 5.7% and Ladbrokes near 5.9%. At the other end, LiveScore Bet and Virgin Bet sit closer to 8.3–8.5%. A tighter margin means the implied probabilities are closer to the true 100%, which leaves more value on the table for you.

Worked example: a Premier League match

Suppose a Premier League fixture is priced like this at one bookmaker:

• Home win: 2.10
• Draw: 3.40
• Away win: 3.80

Convert each to implied probability:

• Home: 1 ÷ 2.10 = 0.476 = 47.6%
• Draw: 1 ÷ 3.40 = 0.294 = 29.4%
• Away: 1 ÷ 3.80 = 0.263 = 26.3%

Add them up: 47.6% + 29.4% + 26.3% = 103.3%. The overround is 3.3%, which is competitive for a three-way football market.

Now strip the margin to see the operator's 'true' estimate. Divide each implied probability by the total (103.3%):

• Home: 47.6 ÷ 103.3 = 46.1%
• Draw: 29.4 ÷ 103.3 = 28.5%
• Away: 26.3 ÷ 103.3 = 25.4%

Those fair figures now total 100%. If your own view is that the home side has, say, a 52% chance of winning against the fair 46.1%, the 2.10 price looks like value. That gap between your estimate and the implied probability is the entire basis of value betting.

Using implied probability to find value

A bet has value when your assessed probability of an outcome is higher than the implied probability of the price on offer. The break-even implied probability tells you the minimum true chance you need for the bet to be worthwhile.

Work it as: value exists if (your probability × decimal odds) is greater than 1.

Example: a horse is priced at 4.00, implying 25%. If you genuinely believe it has a 30% chance, then 0.30 × 4.00 = 1.20. Because that is above 1, the bet carries a positive expected return over the long run. If you only rated it at 20%, then 0.20 × 4.00 = 0.80, below 1, and you'd be overpaying.

This is why line shopping matters. The same outcome can carry different implied probabilities at different bookmakers, purely because their margins and price positions differ. Our sampled best-price share illustrates the point: Betfair topped the best available price on roughly 49% of sampled selections, followed by Coral around 27% and Ladbrokes near 20%, while bet365 and BetVictor topped the price far less often in the same sample. Taking the shorter implied probability (the bigger price) on a selection you fancy is a simple, repeatable edge. You can compare live prices on our betting odds page.

Decimal vs fractional: which to convert from

UK bettors see both formats. Most online betting sites default to decimal odds, and fractional is common on horse racing.

Decimal is easier for implied probability because the formula is a single division. Fractional needs the extra step of adding numerator and denominator. If you prefer fractions, memorising a few conversions speeds things up:

• 1/2 = 66.7% implied
• Evens (1/1) = 50%
• 2/1 = 33.3%
• 3/1 = 25%
• 5/1 = 16.7%
• 10/1 = 9.1%

Most UK bookmakers, including bet365, William Hill and Paddy Power, let you switch the display between decimal and fractional in the settings, so you can pick whichever format makes the mental maths quicker for you.

Common mistakes to avoid

Confusing implied probability with a guaranteed truth. The percentage is the bookmaker's view plus their margin, not the actual likelihood of the event. Your job is to decide whether it's too high or too low.

Forgetting the margin when comparing bets. Two selections at the same nominal price can carry different real value if the surrounding market is tighter at one operator. Always look at the total overround, not just the single price.

Rounding too early. On accumulators, small rounding errors compound. Keep a few decimal places until the final step, especially when multiplying probabilities across several legs.

Ignoring the exchange. On Betfair's exchange the margin behaves differently from a fixed-odds sportsbook, and prices move with supply and demand rather than a set overround. That's part of why its sampled margin runs lower than the traditional bookmakers.