How to Devig Odds: Multiplicative, Additive, Power and Shin, Compared
Every method for stripping the vig agrees on a -110/-110 line and disagrees everywhere else. Here is what each one does, with the numbers, and why the choice matters more for player props than for game lines.
A sportsbook's odds on both sides of a market add up to more than 100%. That extra is the vig, and it's how the book gets paid. To know what the book actually thinks the probability is, you have to take the vig back out. That's devigging, and it is the first step in every +EV calculation, because the "fair" line you compare a price to is a devigged line.
What most guides skip is that there is more than one way to do it, and they give different answers. On a standard -110 / -110 line every method returns 50/50 and the choice is irrelevant. On a lopsided line, a -300 favourite against a +240 underdog, the methods disagree by a full point of probability, which is the difference between a bet showing +2% EV and showing nothing.
Start with implied probability
American odds convert to implied probability like this: for a negative price, divide the number by itself plus 100; for a positive price, divide 100 by the number plus 100. So -300 implies 75.0% and +240 implies 29.4%. Together that's 104.4%. The 4.4% over 100 is the hold, and it has to be removed from somewhere.
Multiplicative (the default everywhere)
Divide each implied probability by the total. 75.0 / 104.4 = 71.8%, and 29.4 / 104.4 = 28.2%. This is what almost every free calculator does, and it is the method OddsJam and most competitors use.
The problem: it removes the vig in proportion to each side's probability, so the favourite gives up more of it than the underdog. Books don't actually price that way. They shade the underdog harder, because bettors overpay for longshots. Multiplicative devigging therefore tends to overstate the underdog's true probability, which makes underdogs look more +EV than they are.
Additive
Split the hold evenly and subtract half from each side. 4.4% / 2 = 2.2%, so 75.0 becomes 72.8% and 29.4 becomes 27.2%. Simple, and it doesn't punish the favourite the way multiplicative does. But it can push a big longshot's probability below zero on a three-way or wide market, and it has no theoretical basis; it's just a different arbitrary rule.
Power
Raise each implied probability to the same exponent k, and pick the k that makes them sum to exactly 1. For -300 / +240 that k works out to about 1.08, giving 73.3% and 26.7%. Because raising to a power above 1 shrinks small numbers more than large ones, the underdog gives up more of the vig than the favourite. That matches how books actually shade lines, which is why the power method has become the sharp community's default and is what we use on the board.
Shin
The Shin method comes from a 1990s paper modelling a market where some fraction of bettors have inside information. It solves for that fraction and derives the probabilities from it. On two-way markets it lands very close to additive; on our example it gives 72.8% and 27.2%, identical to additive at this precision. It earns its keep on multi-way markets like outrights, where multiplicative and additive both misbehave.
The same line, four answers
| Method | -300 side | +240 side | Fair odds |
|---|---|---|---|
| Implied (with vig) | 75.0% | 29.4% | — |
| Multiplicative | 71.8% | 28.2% | -255 / +255 |
| Additive | 72.8% | 27.2% | -268 / +268 |
| Power | 73.3% | 26.7% | -275 / +275 |
| Shin | 72.8% | 27.2% | -268 / +268 |
Say another book is offering the underdog at +265. Against a multiplicative fair line of +255 that's a +EV bet, about 2.7% on the dollar. Against a power fair line of +275 it's a -EV bet. Same price, same market, opposite conclusion, and the only thing that changed was the devig method.
Why this matters more for player props
Game lines are usually close to -110 / -110, where the methods agree. Player props are not. A "will he score" market at -450 / +320, or an alternate line at -200 / +160, is exactly where the methods diverge most. If you are pricing DFS pick'em legs against a devigged sportsbook line, which is the whole game on PrizePicks and Underdog, the method you choose decides which legs show up as +EV. A multiplicative devig will hand you a steady stream of underdog "value" that isn't there.
Which one to use
Power, for two-way markets, which is nearly everything you'll bet. It's the one that reflects how books shade, and it's the one that won't systematically flatter underdogs. Shin for outrights and other multi-way markets. Multiplicative only if you need to match someone else's numbers. And whichever you pick, use it consistently, because a fair line only means something relative to other fair lines built the same way.
One more thing the method can't fix: a devigged line is only as good as the book you devigged. Pinnacle's -300 / +240 and a soft book's -300 / +240 carry the same hold but not the same information. That's why our fair line is built from many books with sharp ones weighted higher, before the devig is applied, and you can change those weights yourself in Settings.
The free calculator runs all four methods on any two-sided line so you can see the spread for yourself. The Straight Bets board shows the power-devigged fair line next to the best price on every pick.