Three ways to write the same price
Every bet is a price. The price can be written three ways, and understanding all three means you can read any bookmaker, any tipster and any article without being confused. Decimal odds (2.50) are the most common worldwide and show your total return per unit staked. Fractional odds (3/2) show profit relative to stake and are traditional in the United Kingdom and Ireland. American odds (+150 or -200) show profit on a 100 stake for positive numbers and the stake needed to win 100 for negative numbers.
| Decimal | Fractional | American | Implied chance |
|---|---|---|---|
| 1.50 | 1/2 | -200 | 66.7% |
| 2.00 | 1/1 (evens) | +100 | 50.0% |
| 2.50 | 3/2 | +150 | 40.0% |
| 3.00 | 2/1 | +200 | 33.3% |
| 5.00 | 4/1 | +400 | 20.0% |
| 11.00 | 10/1 | +1000 | 9.1% |
Odds converter
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Implied probability: the number behind the price
Implied probability is the chance the price assumes, before any margin. For decimal odds divide 1 by the price: 2.00 is 50%, 4.00 is 25%, 1.25 is 80%. It is the single most useful habit in betting because it replaces a vague feeling ("it is a good price") with a number you can compare with your own estimate of how likely the outcome really is. If you believe an outcome happens more often than the implied probability says, the bet has positive expected value in your view; if less, it does not.
The margin: how the bookmaker is paid
If a market were perfectly fair, the implied probabilities of all outcomes would add up to exactly 100%. They do not. A football match priced at 2.10 for a home win, 3.40 for a draw and 3.60 for an away win implies 47.6%, 29.4% and 27.8%. Add them up and you get 104.8%. The extra 4.8% is the margin, sometimes called the overround or vig, and you pay it on every bet whether it wins or loses. Use the calculator to test any three-way market.
Expected value in one line
Expected value (EV) is the average result of a bet if you could place it many times: EV = (probability of winning × profit) - (probability of losing × stake). With true odds equal to the price, EV is zero before margin. Because of the margin, a typical bet at a bookmaker has negative EV. Betting is entertainment with a cost, not an income source, and the sooner you accept that the better your decisions about stakes and limits become.
Why accumulators cost more than they look
An accumulator multiplies the odds of several selections, but the margins multiply too. If each leg carries a 5% margin, a four-leg accumulator has an effective house edge of about 18%, because 1.05 to the power of 4 is roughly 1.216 and 1 divided by 1.216 is about 0.82. That is why accumulators produce spectacular winnings occasionally and steady losses over time.
Practical habits
- Convert the price to an implied probability before you bet.
- Compare the same market across several bookmakers; small differences compound.
- Avoid accumulators with many legs unless you accept the cost.
- Stake a small, fixed fraction of a bankroll you can afford to lose, and never chase losses.
- Remember that bonuses carry conditions; see bonuses and wagering before you accept one.
Frequently asked questions
What do decimal odds of 2.50 mean?
A 10 unit stake returns 25 units in total if the bet wins: 15 profit plus your 10 stake back. The implied probability is 1 divided by 2.50, or 40%.
How do I convert fractional odds to decimal?
Divide the first number by the second and add 1. Fractional 5/2 becomes 5 divided by 2 equals 2.5, plus 1, giving 3.50 decimal.
What is the bookmaker's margin?
The built-in advantage in a market. Add the implied probabilities of all outcomes: anything above 100% is the margin. A sum of 105% means a 5% margin.
Why are accumulator bets worse value than they look?
Each leg carries its own margin and the margins multiply. Four legs at 5% margin each leave an effective house edge of roughly 18%.
Can odds tell me who will win?
Odds show the market's price, including the margin. They are a useful estimate of likelihood but they do not predict outcomes.