Removing the margin (devigging)

At the beginner level, you saw that the sum of implied probabilities exceeds 100% because of the margin. Devigging means removing that margin to get probabilities that add up to exactly 100%. These are called fair probabilities, and the corresponding odds are the fair odds.

Why it's central

The most widespread value betting method consists of:

Taking the odds from a highly efficient reference bookmaker (known as sharp, for example Pinnacle or a liquid exchange).
Removing the margin to estimate the true probability.
Looking for odds at other bookmakers that are higher than the fair odds.

Method 1: multiplicative (proportional)

Divide each implied probability by the overround:

p_i^{\text{fair}} = \frac{1/\text{odds}_i}{\sum_j 1/\text{odds}_j}

Example

Running example — Team A vs Team B (2.50 / 3.40 / 3.00, overround 1.0275)

Outcome	Implied	Fair (multiplicative)	Fair odds
Team A	40.0%	38.9%	2.57
Draw	29.4%	28.6%	3.49
Team B	33.3%	32.4%	3.08

If another bookmaker offers a Team A win at 2.70, we get: EV = 0.389 × 2.70 − 1 ≈ +5%.

Method 2: additive

Subtract the same amount from each outcome:

p_i^{\text{fair}} = \frac{1}{\text{odds}_i} - \frac{\text{overround} - 1}{N}

where $N$ is the number of outcomes. In our example: Team A = 39.1%, draw = 28.5%, Team B = 32.4%.

Which one to choose?

When the odds are close (a balanced match), both methods give almost the same result.
When there's a heavy favourite and a big outsider, they diverge. In practice, bookmakers put proportionally more margin on outsiders, and neither of these two methods captures that well.

The advanced level presents finer methods (power, Shin, odds ratio) that correct this bias.

Interactive tool

Method

Outcome 1

Outcome 2

Educational tool. Results are theoretical estimates, not betting advice.

Why it's central

The most widespread value betting method consists of:

Taking the odds from a highly efficient reference bookmaker (known as sharp, for example Pinnacle or a liquid exchange).

Removing the margin to estimate the true probability.

Looking for odds at other bookmakers that are higher than the fair odds.

Which one to choose?

When the odds are close (a balanced match), both methods give almost the same result.

When there's a heavy favourite and a big outsider, they diverge. In practice, bookmakers put proportionally more margin on outsiders, and neither of these two methods captures that well.

The advanced level presents finer methods (power, Shin, odds ratio) that correct this bias.