Free calculator

Implied Probability Calculator (Odds to %)

Convert fractional, decimal or American odds into implied win probability (and optional overround).

Implied percent is the book price, not a true chance: margin is baked in.

Calculator

Decimaln/a
Fractionaln/a
Americann/a
Implied probabilityn/a
Book overroundn/a

How do you convert odds to implied probability?

Implied probability = 1 ÷ decimal odds. Fractional 5/2 → decimal 3.5 → about 28.57%.

Is implied probability the true chance of winning?

No. It is the book price as a percent, with margin included.

The formula

Implied probability = 1 ÷ decimal odds
Decimal = fractional + 1  (e.g. 5/2 → 3.5)
American positive: decimal = 1 + American/100
American negative: decimal = 1 + 100/|American|
Overround = sum of implied probabilities − 1 across a multi-outcome book.

Worked example

Odds 5/2 → decimal 3.5 → implied probability = 1/3.5 ≈ 28.57%.

American equivalent ≈ +250.

Two-way book at 1.90 / 1.90: 52.63% + 52.63% = 105.26%. Overround ≈ 5.26%.

Methodology

How it calculates

  • Keeps fractional, decimal, American and implied probability in sync from whichever field you edit.
  • Optional multi-outcome overround is the sum of implied probabilities minus 100%.

Formula

Implied probability = 1 ÷ decimal odds
Decimal = fractional + 1  (e.g. 5/2 → 3.5)
American positive: decimal = 1 + American/100
American negative: decimal = 1 + 100/|American|
Overround = sum of implied probabilities − 1 across a multi-outcome book.

Assumptions

  • Implied probability is the book price as a percent, with margin baked in. Not a true chance.

Edge cases

  • Decimal = fractional + 1 (so 5/2 → 3.5, not 2.5).
  • Implied percent is the book price, not a true chance.

Sources

None. Pure maths; no external rate or spec feed.

FAQ

Is implied probability the true chance of winning?

No. It is the bookmaker’s price turned into a percent, with their margin included. Your own probability estimate is a separate input (see expected value and Kelly).

What is overround?

The sum of implied probabilities across outcomes minus 100%. A positive overround is the book’s built-in edge if every price is taken at those odds.

Which odds format should I enter?

Any one field: fractional, decimal, American or probability. The others update to match.

Not financial advice. This calculator is for education only. Double-check results before you make money decisions. Past moves do not predict future ones.