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On this page
- Odds Converter and Implied Probability Calculator
- How do you calculate implied probability?
- Why does a betting market add up to more than 100%?
- How do you work out the margin and the “fair” odds?
- Is implied probability the real chance of winning?
- How does implied probability work in accas?
- What do minimum odds in offers mean as a probability?
- Related guides
- Current sports offers
- Frequently asked questions
- Staying in control
- Sources checked
Last checked 21 October 2026 by Tom Perkins. Calculations checked by us against the rules cited below.
Implied probability is the chance of an outcome that the odds suggest. For decimal odds it is 1 ÷ odds × 100: a price of 4.0 (3/1) implies 25%. Because bookmakers build in a margin, the implied probabilities across a whole market add up to more than 100%.
Enter a single price to convert it, or paste every price in a market (comma separated) to see the book percentage and margin.
How do you calculate implied probability?
- Decimal: 100 ÷ decimal odds. 2.5 → 40%.
- Fractional a/b: b ÷ (a + b) × 100. 3/1 → 1 ÷ 4 = 25%. 1/2 → 2 ÷ 3 = 66.7%.
- American: +x → 100 ÷ (x + 100); −y → y ÷ (y + 100). +250 → 28.6%; −200 → 66.7%.
Why does a betting market add up to more than 100%?
Example football match-result market (illustration only):
| Outcome | Fractional | Decimal | Implied |
|---|---|---|---|
| Home | 6/4 | 2.50 | 40.00% |
| Draw | 9/4 | 3.25 | 30.77% |
| Away | 2/1 | 3.00 | 33.33% |
| Total (book %) | 104.10% |
Real probabilities for the three outcomes must sum to 100%. The extra 4.10% is the overround, the bookmaker’s built-in margin. Sometimes it’s called the vig or juice. A lower overround means the prices are closer to “fair”, but it doesn’t tell you who will win.
How do you work out the margin and the “fair” odds?
Add up the implied probabilities of every outcome: anything over 100% is the margin. To strip it out, divide each implied probability by the book total. Using the same example market (illustration only):
| Outcome | Decimal odds | Implied | Margin removed | “Fair” odds |
|---|---|---|---|---|
| Home | 2.50 | 40.00% | 38.42% | 2.60 |
| Draw | 3.25 | 30.77% | 29.56% | 3.38 |
| Away | 3.00 | 33.33% | 32.02% | 3.12 |
| Total | 104.10% | 100.00% |
This simple method spreads the margin evenly; bookmakers don’t always load it that way, so treat the result as a rough guide, not a true probability.
Is implied probability the real chance of winning?
Odds reflect the bookmaker’s view, the money placed and the margin. They aren’t a forecast you can rely on, and no calculation turns betting into a reliable way to make money. Treat these numbers as a way to understand prices, not as a system.
How does implied probability work in accas?
For an acca, multiply the implied probabilities: four legs at 50% each is 6.25%. The margin compounds with every leg, which is why long accas rarely land. See accumulators explained.
What do minimum odds in offers mean as a probability?
Welcome offers set minimum odds for the qualifying bet or the free bets. Turned into implied probability, they look like this:
| Minimum odds | Implied probability | Example from offers we list (checked 21 October 2026) |
|---|---|---|
| 1/2 (1.5) | 66.7% | Spreadex fixed odds qualifying bet |
| 1/1 (2.0) | 50% | Qualifying bets at Sky Bet, BetMGM, BetUK, Unibet, Midnite and Stakemate |
| 2/1 (3.0) | 33.3% | Stakemate free bets (3+ selections, total odds 3.0+) |
| 3/1 (4.0) | 25% | Midnite bet builder free bets (3+ selections) |
| 4/1 (5.0) | 20% | Midnite acca free bet (4+ selections) |
Higher minimum odds mean the free bet is less likely to win. That’s one reason a free bet is worth less than its face value; our guide to how free bets work explains the rest.
Related guides
- Betting odds explained and conversion table
- Each-way betting explained
- Free bets: why a token is worth less than cash
Current sports offers
The offers below are for new customers aged 18+ at UKGC-licensed bookmakers. Each one has its own terms, minimum odds and expiry, so read them before you opt in. Our free bets page explains how tokens work.
New customers. Min £5 deposit. First bet only: min £5 single/each-way at odds 1/1+ within 30 days of registration. 3 x £10 Free Bets (singles/each-way, excl. virtuals), expire after 30 days. Free Bet stakes not returned. Neteller and Skrill deposits excluded. No wagering on Free Bet winnings stated.
Promo code:
AB5G30Three separate £10 Free Bets, usable on singles and each-way.Full T&Cs
New Sky Betting and Gaming customers 18+ only (previous Sky Bet, Vegas, Casino, Bingo or Poker account holders excluded). Register, add card details and make a minimum £5 deposit. Your first bet must be a single or each-way bet of at least £5 on any Sky Bet market at odds of 1/1 or greater; accumulated smaller stakes do not count and only your first bet counts. Staking requirement must be met within 30 days of registration. Rewards: three £10 Free Bet tokens, usable on any single or each-way market excluding virtuals, not with any other offer. Free Bets expire 30 days after crediting, are non-withdrawable and stakes are not included in returns. One claim per customer. Customers depositing with Neteller or Skrill do not qualify. Offer may differ if you arrive via another link. Sky Bet General Promotion Terms apply.
New customers. Opt in at sign-up. Deposit £10+ and place a £10+ bet at odds 1/1+ within 7 days (excl. virtuals, esports, non-UK/IE racing). 4 x £10 Free Bets: 2 for football accas (4+ legs), 2 for sports multiples (2+ legs), valid 7 days. Free Bet stake not returned.
Rewards credited on settlement of one £10 qualifying bet, win or lose.
Full T&Cs
New BetMGM customers 18+. Select the Sports Welcome Offer at sign-up. Deposit at least £10 and place at least £10 on a qualifying bet within 7 days of registration: min odds Evens 1/1 (2.0); each-way bets need £10 each way (£20 total); Bet Builder total odds must be 1/1+. Excludes virtual sports, esports and non-UK/IE horse racing. Free Bets, Profit Boosts, Odds Boosts, void, cashed-out, partially cashed-out and system bets do not qualify. Not with other offers. On settlement you get 4 x £10 Free Bets: 2 x £10 on football accas (4+ selections) and 2 x £10 on sports multiples (2+ selections), selected via the betslip checkbox. Free Bets expire after 7 days; stake not included in winnings. Once per household/computer/IP. Operated by LeoVegas Gaming plc (UKGC 39198). General and promotional T&Cs apply.
New customers. Opt in at sign-up. Deposit £10+ and place a £10+ bet at odds 1/1+ within 7 days (excl. virtuals, esports, non-UK/IE racing). 4 x £10 Free Bets: 2 for football accas (4+ legs), 2 for sports multiples (2+ legs), valid 7 days. Free Bet stake not returned.
Same LeoVegas offer structure as BetMGM, with UK/IE racing allowed for the qualifier.
Full T&Cs
New BetUK.com customers 18+. Opt in by selecting the Sports Welcome Offer at sign-up. Deposit at least £10 and place at least £10 on a qualifying bet within 7 days of registration: min odds Evens 1/1 (2.0); each-way needs £10 each way; Bet Builder total odds 1/1+. Excludes virtual sports, esports and non-UK/IE horse racing. Free Bets, Profit/Odds Boosts, void, cashed-out, partially cashed-out and system bets do not qualify. Once only; not with other offers. On settlement: 2 x £10 Free Bets on football accas (4+ selections) and 2 x £10 on sports multiples (2+ selections). Unused Free Bets expire after 7 days; stake not included in winnings. Operated by LeoVegas Gaming plc (UKGC 39198). General Promotional and General T&Cs apply.
Frequently asked questions
What is the implied probability of evens? 50%. Evens is 2.0 in decimal; 100 ÷ 2 = 50.
How do I calculate a bookmaker’s margin? Add the implied probabilities of every outcome in a market and subtract 100.
Does a high implied probability mean a safe bet? No. Odds-on selections lose regularly, and no bet is safe.
Is implied probability the same on every bookmaker? No. Prices differ between bookmakers, so implied probabilities differ too.
Why is my book percentage under 100%? You have probably mixed prices from different bookmakers or times. A single bookmaker’s market is normally above 100%.
We post new guides and checked offers on Telegram (18+ only):
Staying in control
Set a deposit limit before your first deposit, decide your budget in advance and never chase losses. Calculators show what a bet could return; they do not make any outcome more likely. You can self-exclude from all online gambling sites licensed in Great Britain for free with GamStop, and many banks let you block gambling payments. The National Gambling Helpline (0808 8020 133) is free, confidential and open 24 hours a day. See our responsible gambling page.
Sources checked
- Maths verified with our unit-tested converter
Affiliate disclosure: onlinebetfree.co.uk may receive a commission if you open an account through links on this page. This does not affect the offers we list or how we describe them, and it never changes the terms an operator gives you.
18+ only. Please gamble responsibly. For free, confidential support visit BeGambleAware.org or call the National Gambling Helpline on 0808 8020 133.