Implied Probability

Every betting price is a probability statement wearing a costume. Stripping the costume off is the single most useful skill in betting, because a percentage can be argued with and a price cannot.

Until you convert, you are comparing your opinion against a number in a foreign unit. Afterwards, the question becomes concrete: does this happen more often than the price says?

The Two Conversions

Negative American odds: divide the absolute value by itself plus 100.

-150 becomes 150 ÷ 250 = 60.0%.

Positive American odds: divide 100 by the number plus 100.

+200 becomes 100 ÷ 300 = 33.3%.

From decimal odds it is simpler still: divide 1 by the decimal price. 2.50 becomes 1 ÷ 2.50 = 40%.

Removing The Margin To See The Market's Real Opinion

Implied probabilities from both sides of a market always sum to more than 100%, because the book adds a margin. To find what the market actually believes, you have to take it out.

The simplest method is proportional. Suppose a game is priced -140 / +120. That is 58.3% and 45.5%, totalling 103.8%. Divide each by 1.038 and you get 56.2% and 43.8%, which sum to 100%.

Those adjusted numbers are the market's genuine estimate, and they are what your own projection should be compared against. Comparing your 55% estimate to the raw 58.3% would make you think you disagree with the market when you may not.

Finding Value, Concretely

Once both sides are percentages, the process is arithmetic.

Suppose you project a team to win 47% of the time and it is available at +130. The price implies 43.5%. Your 47% is higher, so the bet has positive expected value: for every 100 risked, expected profit is roughly (0.47 × 130) − (0.53 × 100) = 61.1 − 53 = 8.1, or about 8% return.

The same projection at +100 would imply 50%, which is above your 47%, and the bet would be negative expectation despite the identical opinion about the team. The team did not change. The price did. See expected value.

The Discipline This Enforces

Working in probabilities forces you to state opinions in a form that can be wrong, which is uncomfortable and useful.

"I really like this team" cannot be evaluated. "I think they win 58% of the time" can, and it can be compared to the -140 on the screen, which implies about 56% after removing margin. That comparison shows the disagreement is two percentage points, which is inside the range where your own model error swamps any edge.

Most bad bets do not come from bad opinions. They come from opinions never converted into numbers, so their weakness stays invisible. Building the conversion habit is what turns watching sport into handicapping it.

Frequently Asked Questions