What devigging is
A bookmaker's implied probabilities add up to more than 100%: the excess is its margin. Devigging spreads that excess back out so the probabilities add up to exactly 100%. The question is how to spread it: evenly, in proportion, or more on the longshots. Each answer is a method.
Multiplicative (proportional)
Divide each implied probability by the total. It's the simplest method and the one most calculators use. At −110 on both sides (1.91 / 1.91) the total is 104.77%, so each side gets 50% and fair odds of 2.00.
p(i) = q(i) ÷ Σ q where q(i) = 1 ÷ odds(i)
Additive
Subtract the same amount from every outcome: the excess divided by the number of outcomes. It treats a longshot and a favourite the same in percentage points, so it takes relatively more from the longshot.
Power (the one BetTheDrop uses)
Raise every implied probability to the same power k, choosing k so they add up to 100%. Because a power shrinks small numbers more than large ones, the longshot loses more probability than the favourite. That matches how bookmakers really price: they put more margin on longshots (the favourite–longshot bias).
p(i) = q(i)^k with k such that Σ p(i) = 1
Shin
Shin's method models the margin as protection against bettors with inside information. It also takes more from longshots than the multiplicative method, usually a little less than the power method. It's more complex to compute and gives similar answers to the power method in most markets.
Same market, four answers
In a two-way market with a clear favourite, 1.25 and 4.20 (margin 3.81%), the fair odds of the longshot change with the method:
| Method | Favourite (1.25) | Longshot (4.20) |
|---|---|---|
| Multiplicative | 1.298 | 4.360 |
| Additive | 1.280 | 4.565 |
| Shin | 1.280 | 4.565 |
| Power | 1.271 | 4.685 |
And in a 1X2 football market at 1.40 / 4.50 / 8.00 (margin 6.15%):
| Method | Home (1.40) | Draw (4.50) | Away (8.00) |
|---|---|---|---|
| Multiplicative | 1.486 | 4.777 | 8.492 |
| Additive | 1.441 | 4.957 | 9.570 |
| Shin | 1.453 | 4.914 | 9.251 |
| Power | 1.437 | 5.057 | 9.401 |
If your bookmaker offers 9.00 on the away win, the multiplicative method says it has value (9.00 is above 8.49) and the power method says it doesn't (9.00 is below 9.40). On favourites the methods mostly agree; on longshots the choice decides whether a bet looks like value.
The power method step by step
Take the 1.25 / 4.20 market. The implied probabilities are 1 ÷ 1.25 = 80.00% and 1 ÷ 4.20 = 23.81%, 103.81% in total. The power method looks for the exponent k that brings the total to 100%:
- Try k = 1: the total is 103.81%, too high, so k must be larger than 1.
- Raise both probabilities to a larger k: 0.80 and 0.2381 both shrink, the smaller one proportionally more.
- At k ≈ 1.0761 the total is exactly 100%: 0.80^k = 78.65% and 0.2381^k = 21.35%.
- Fair odds are 1 ÷ probability: 1 ÷ 0.7865 = 1.271 for the favourite and 1 ÷ 0.2135 = 4.685 for the longshot.
The favourite lost 1.35 percentage points and the longshot 2.46. With the multiplicative method the longshot would keep 22.94% (fair odds 4.36), almost 1.6 points more than with the power method.
Why the choice matters for value bettors
A value bet is a price above the fair odds, so the fair odds are the line you bet against. If the method makes longshots look more likely than they are, you'll see value where there is none and bet too many long prices. Over hundreds of bets that bias is the difference between a real edge and a slow loss. On favourites and even markets the method matters much less, which is why most of the debate is about longshots, draws and correct scores.
Common mistakes
- Devigging a soft bookmaker: its margin is large and uneven; the fair odds should come from the sharpest market you can find.
- Mixing lines: both sides must be the same line (−1.5 and +1.5, not −1.5 and +1).
- Using stale prices: devig the market as it is now, not the price you saw an hour ago.
- Forgetting the draw: in a 1X2 market you need all three prices, not two.
Which method to use
- Even two-way markets (handicaps, totals near −110/−110): all methods give almost the same answer.
- Markets with a clear favourite or a longshot: use the power or Shin method; the multiplicative method overstates the longshot and makes value appear that isn't there.
- Many outcomes (outrights, correct score): the differences grow; be careful with any single method.
BetTheDrop uses the power method on the sharp reference market for every fair price. The details, and where it can be wrong, are in the methodology.
Try it
Enter any market in the free no-vig calculator and switch between the power and multiplicative methods to see the difference. To see the size of the margin itself, use the vig calculator.