Decimal · Fractional · American · Implied probability · Expected value
What the numbers on an operator's board actually mean, how they convert between formats, what implied probability tells you, and why the overround matters more than the headline price. Tennis-specific examples throughout.
Decimal odds are the default display format for all five UKGC-licensed operators in our benchmark, and the standard across most European online betting platforms. A decimal odd of 3.20 means that for every £1 wagered, you receive £3.20 back in total if the bet wins — comprising your £1 stake plus £2.20 profit. The formula: return = stake × decimal odds; profit = stake × (decimal odds − 1).
Tennis example: Djokovic at 3.20 in a Wimbledon quarter-final. A £20 stake returns £64.00 (£20 × 3.20), representing £44.00 profit. A heavily favoured player at 1.45 would return £29.00 on a £20 stake — £9.00 profit. Decimal odds of exactly 2.00 represent an evens bet: £1 wagered returns £2.00, and the implied probability is exactly 50%. Any decimal below 2.00 represents a "odds-on" position (the market implies the outcome is more likely than not); any decimal above 2.00 is "odds-against".
Fractional odds are the legacy format from UK high-street bookmakers and remain available as an alternative display option on most UKGC-licensed platforms. The fraction represents profit relative to stake: 9/4 means £9 profit for every £4 wagered, plus your stake returned. Converting to decimal: divide the numerator by the denominator and add 1. So 9/4 = (9 ÷ 4) + 1 = 2.25 in decimal. Evens (1/1) = 2.00 decimal. Odds-on examples: 4/6 = (4 ÷ 6) + 1 = 1.67 decimal.
Tennis context: a Wimbledon favourite at 4/6 is being priced as highly likely to win — the market-implied probability is 60% (see implied probability section below). An underdog at 9/4 carries a market-implied probability of approximately 30.8%. Fractional odds do not display stakes cleanly for non-round numbers; a 9/4 bet becomes awkward to calculate at £13 stake (£29.25 profit, £42.25 return). This is why most UK online bettors now default to decimal, which scales naturally to any stake amount.
American odds, also called moneyline odds, are not the standard format for UKGC-platform displays but appear on US-facing data feeds and some aggregator tools used by tennis traders. Positive moneyline (+200) represents profit per $100 wagered; negative moneyline (-150) represents the amount you must wager to win $100. Converting +200 to decimal: (200 ÷ 100) + 1 = 3.00. Converting -150 to decimal: (100 ÷ 150) + 1 = 1.67. Tennis-specific example: Djokovic at +220 on a US-market feed is equivalent to 3.20 decimal; a tournament favourite at -180 is 1.56 decimal. The format is primarily relevant if you are comparing prices across international data aggregators or using a tennis statistics tool that displays American lines alongside ATP market data.
Every set of odds encodes an implied probability — the market's assessment of the likelihood of the outcome occurring. The formula: implied probability = 1 ÷ decimal odds. So 3.20 decimal implies a probability of 1 ÷ 3.20 = 0.3125, or 31.25%. A price of 1.45 implies 1 ÷ 1.45 = 0.6897, or approximately 69%.
The overround is the mechanism by which operators ensure a structural margin. If you sum the implied probabilities of all outcomes in a market (in a two-player tennis match, the two match-winner prices), the total exceeds 100%. A match priced at 1.80 / 2.30 sums to: (1 ÷ 1.80) + (1 ÷ 2.30) = 0.556 + 0.435 = 0.991... wait — actually: 1/1.80 = 55.6%, 1/2.30 = 43.5%, total = 99.1%... In practice operators price to above 100%, for example: 1.75 / 2.10 = 57.1% + 47.6% = 104.8%. The 4.8% excess is the overround — the operator's built-in edge. Betfair Exchange works differently: the overround is replaced by a commission on net winnings (typically 5% for most markets), which can produce better net prices than fixed-odds books on liquid markets.
Expected value (EV) is the central concept for assessing whether any bet carries long-run positive or negative value. The formula: EV = (probability of winning × profit) − (probability of losing × stake). If you believe Djokovic has a 40% chance of winning a Wimbledon quarter-final match where the operator prices him at 3.20 (implying 31.25%), then: EV = (0.40 × £2.20) − (0.60 × £1.00) = £0.88 − £0.60 = +£0.28 per pound staked. Positive EV means the bet offers value relative to your probability assessment; negative EV means the operator's implied probability exceeds your assessed probability of the outcome.
The challenge is that assessing true probability requires reliable data, surface-specific form, and an understanding of market-making dynamics. An operator's price reflects the aggregate of money placed across all bettors, not necessarily the most accurate probability estimate. At major tournaments like Wimbledon, the money placed on the betting market is large enough that gross mispricing is rare — but in early-round matches featuring less-tracked players, the market is thinner and the gap between true and implied probability can be material. This is the operational environment where the analytical framework in scoring-the-serve.php becomes most relevant.
When you use cash-out on an open tennis bet, the operator's system calculates the current value of your position using the live market odds at that moment, then applies a discount that embeds their margin on the settlement. If you backed Djokovic to win a match at 3.20 with a £20 stake (implied return: £64) and he is now leading by two sets, the operator might offer a cash-out value of £54. The difference between £54 and your full settlement of £64 represents both the probability that he might still lose (however small) and the operator's margin on the cash-out transaction. The cash-out value is not a neutral fair price — it is always discounted relative to the true expected value of the bet at current market odds.
The UKGC's April 2019 stake limit for B2 gaming machines (fixed-odds betting terminals, FOBTs) reduced the maximum stake per spin to £2. This regulation targeted high-frequency electronic roulette and fixed-odds terminals in licensed betting offices and does not apply to online sports betting, where no equivalent stake cap exists for adults who have completed age and identity verification. The regulatory change is sometimes misrepresented as applying to online tennis betting; it does not. Online sports bets in the UK are regulated under the UKGC's Remote Gambling and Software Technical Standards (RTS), which cover fairness, randomness, and consumer protection without a maximum stake cap for sports markets.
However, since 2022, UKGC guidance has required operators to conduct enhanced customer due diligence (CDD) checks on customers who show patterns consistent with unaffordable spend. This means high-stakes bettors on tennis and other sports may be subject to source-of-funds requests from operators under the UKGC's Customer Interaction Requirements — not a regulatory stake cap, but an operational check that can affect large single-match bet acceptance. For editorial methodology transparency, see scoring-the-serve.php.