Almost every betting mistake starts in the same place: treating odds as a payout rather than as a probability. A price of -150 is not just “risk $150 to win $100”. It is the market telling you something specific about how likely an outcome is — and about how much the sportsbook is charging you to take a position on it.
Once you can move fluently between formats and strip out the built-in margin, you can compare your own estimate against the market’s. That comparison is the entire basis of value betting. Without it, you are guessing.
Decimal odds are the easiest to work with because they include your stake. A price of 1.91 returns $1.91 for every $1 risked, of which $0.91 is profit. Implied probability is simply 1 divided by the decimal price.
American odds split into two rules. For a negative price, implied probability equals the odds divided by the odds plus 100, using the absolute value. For -110, that is 110 / (110 + 100) = 0.5238, or 52.38%. For a positive price, implied probability equals 100 divided by the odds plus 100. For +150, that is 100 / 250 = 0.40, or 40%.
Fractional odds convert to decimal by dividing the fraction and adding one. 10/11 becomes 0.909 + 1 = 1.909, which is the same price as -110.
| American | Decimal | Fractional | Implied probability |
|---|---|---|---|
| -200 | 1.50 | 1/2 | 66.67% |
| -150 | 1.67 | 2/3 | 60.00% |
| -110 | 1.91 | 10/11 | 52.38% |
| -105 | 1.95 | 20/21 | 51.22% |
| +100 | 2.00 | 1/1 | 50.00% |
| +120 | 2.20 | 6/5 | 45.45% |
| +150 | 2.50 | 3/2 | 40.00% |
| +250 | 3.50 | 5/2 | 28.57% |
Take a standard two-way market priced at -110 on both sides. Each side implies 52.38%. Added together, that is 104.76% — which is impossible, because the two outcomes cover every possibility and must sum to exactly 100%.
That extra 4.76% is the sportsbook’s margin, known as the vig, juice, or overround. It is not a fee you pay separately. It is baked into the price, which is why so many bettors never notice it. The book’s expected hold on a balanced two-way market at -110 is roughly 4.55% of total handle — the overround divided by the total implied probability.
The standard method is proportional normalisation: divide each side’s implied probability by the sum of both sides. It assumes the margin is spread evenly across the market, which is a reasonable approximation for most two-way lines.
Worked example. A moneyline is posted at -140 for the favourite and +120 for the underdog.
Those two fair figures now sum to 100%, and they represent the market’s genuine estimate once the house edge is stripped away. The fair price on the favourite would be 1 / 0.5620 = 1.78 decimal, or about -178 in American terms — but you are being offered 1.71, or -140. On the underdog, the fair price would be 1 / 0.4380 = 2.28, and you are offered 2.20. Both sides are shaded against you, and the difference between the fair price and the posted price is where the book makes its living.
Two practical consequences follow.
First, your break-even rate is higher than you think. At -110 you need to win 52.38% of your bets just to stand still. Not 50%. A bettor going 51% at standard juice is losing money while feeling like they are ahead.
Second, a bet only has value when your estimate beats the no-vig number, not the posted number. If you think the favourite in the example above wins 58% of the time, and the no-vig market says 56.20%, you have a genuine edge of about 1.8 percentage points. If you think they win 55%, you are behind the market even though the posted price of -140 looks generous next to your estimate.
This is the discipline that separates a model from a hunch. A prediction that does not come with a probability attached cannot be checked against a price, and therefore cannot be shown to have value.
Proportional normalisation gets less reliable as markets get less liquid. Soccer three-way markets, player props and exotic derivatives frequently carry margins several times larger than a standard side or total, and the margin is often loaded asymmetrically onto the outcome the book expects recreational money to favour. Calculate the overround before you assume a soft-looking prop is soft. A market with an 8% margin needs a very large edge before it becomes profitable.
No. Raw implied probabilities always include the sportsbook margin, which is why they sum to more than 100%. Only after removing the vig do you get the market’s fair estimate — and even that is an estimate, not a fact.
At -110 you need 52.38%. At -105 you need 51.22%. At -120 you need 54.55%. The higher the juice, the steeper the hill, which is one of the strongest arguments for shopping for better prices.
For the bettor, yes — a lower margin means more of the price is returned to you. Reduced-juice books and exchanges typically post lower overrounds than standard retail sportsbooks, though they may offset this with commission or lower limits.
The same way: sum all three implied probabilities, then divide each by the total. Be aware that proportional normalisation tends to understate the fair price on heavy longshots, so treat the output as an approximation rather than a precise fair value.
Knowing what a price implies is the first step. The second is having a probability to compare it against. Members get one pick a day with the odds we took and the reasoning behind the estimate.
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69 Advisory provides informational sports analysis only. Nothing above is a guarantee of results and past performance does not indicate future outcomes. Only stake what you can afford to lose.
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