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A price is not a prediction
Odds are a way of expressing a price. They tell you how a hypothetical return relates to a stake, but they do not tell you what will happen. A short-priced outcome can fail; a long-priced outcome can occur.
The distinction matters because the number contains more than a view of probability. Commercial pricing and the structure of the market can also affect it. Converting a price into a percentage gives an implied probability, not a measured chance of success.
Three formats, the same underlying price
Decimal odds include the original stake in the return multiplier. Fractional odds express the profit relative to the stake. American odds express either the profit on a hypothetical 100 units, or the units associated with 100 units of profit. These are mathematical descriptions, not recommendations to stake money.
| Decimal | Fractional | American | Implied |
|---|---|---|---|
| 1.50 | 1/2 | −200 | 66.67% |
| 2.00 | 1/1 | +100 | 50.00% |
| 2.50 | 3/2 | +150 | 40.00% |
| 6.00 | 5/1 | +500 | 16.67% |
At even money, +100 and −100 describe the same decimal price of 2.00. For a decimal price of 2.50, subtract 1 to obtain fractional odds of 1.50, or 3/2. The equivalent American price is +150. These formats express the same price.
Convert decimal odds into a percentage
For decimal odds D, the implied probability is 1/D. Multiply by 100 to express it as a percentage.
This is the break-even probability in a simplified model with a win or loss, no extra charges and the stated return. It does not establish that the true chance is 40%. The distinction is particularly important when prices contain a bookmaker margin.
To calculate implied probability, divide 100 by the decimal price. For example, 100 ÷ 2.50 = 40%, while 100 ÷ 2.00 = 50%. These are price-implied percentages, not independent forecasts.
Why a full market can exceed 100%
Imagine three mutually exclusive outcomes that cover every possible result. If their prices are 2.00, 3.50 and 4.00, the implied percentages are 50%, about 28.57% and 25%. Together, they total about 103.57%.
The excess is called the overround. That does not mean the three outcomes can collectively have a true probability above 100%. It means the quoted prices cannot all be treated as fair probabilities. Our guide to market margin explains the distinction.
What the number cannot tell you
Odds alone do not establish the quality of a forecast, the reliability of a data source or a person’s ability to make a profit. A single result also cannot tell you whether an estimate was well calibrated: an event with a 40% chance still happens in some cases.
Nor does understanding the notation remove financial risk. Gambling can cause harm, and a calculation should never be treated as a reason to recover a loss or increase spending. Find support in Ireland through GRAI.
Sources & methodology
All prices and numerical examples are hypothetical; calculations are shown so they can be checked independently.
- This guide distinguishes quoted prices from true probabilities and states the assumptions behind each worked example.
- Mathematical method: reciprocal probability, odds-format identities and, where stated, the multiplication rule for independent events. These are derived in the worked examples above.
- Gambling Regulatory Authority of Ireland: Get Help — local support information.