How to Strip the Vig and Find Out What Odds Are Really Saying
Sportsbooks and prediction markets are not in the business of telling you what they actually think will happen. They are in the business of making money, and they do it by embedding a margin into every set of odds they publish. That margin is the vig, also called the juice or the overround, and it means that if you take the implied probabilities from any set of odds and add them up, you will get something greater than 100%.
That excess over 100% is the house edge. It is invisible unless you know to look for it, but it is always there, silently eroding the expected value of every bet you place. Before you can evaluate whether you have an edge on a market, you need to strip the vig and find the fair odds; the probabilities the market is implying after removing the house take.
The FatNarwhal De-vig tool does exactly that. Paste in a set of odds, and it hands you back the fair implied probabilities as if the vig did not exist.
How to Use It
Go to fatnarwhal.com/devig and enter the odds for both sides of a market.
Take a simple example; a coin-flip-style binary event where a sportsbook is offering -110 on both sides. In American odds format, -110 means you need to bet 100. The implied probability of -110 is 110/210, which is about 52.4%. Add up both sides and you get 104.8%; the excess 4.8% is the vig.
The de-vig tool normalizes these back to 100% so you can see the fair implied probability. In this case both sides come out to 50%, which makes sense for a coin flip; the sportsbook is just extracting 4.8% of every dollar wagered as margin.
Now try it on a real prediction market or a lopsided matchup where one side is a heavy favorite. The vig is usually more interesting when it is not symmetric between the two sides. Knowing the true fair probability lets you compare your own estimate against an honest baseline rather than an inflated one.
The Math Behind It
The standard de-vig method normalizes the raw implied probabilities so they sum to exactly 1.
If side A has implied probability pA and side B has implied probability pB, and pA + pB = 1 + v (where v is the vig), then the fair probability for A is simply:
For multi-outcome markets the same logic applies; each raw implied probability is divided by the sum of all of them.
There are more sophisticated de-vig methods, like the multiplicative method or the Shin method, which make different assumptions about how the house distributes its margin across outcomes. The multiplicative method is generally the most used in practice and is what the FatNarwhal tool applies; it removes the vig proportionally from each outcome rather than removing an equal amount from each.
When to Use It and When Not To
Use it every time you are comparing a market price to your own probability estimate. You want to be comparing against the fair probability, not the boosted one that includes the house margin. Use it when you are looking for arbitrage between two markets, since both sets of fair probabilities need to be on a level playing field for the comparison to be valid.
It is worth knowing that de-vigging does not magically reveal the true probability of an event; it just removes the house's margin to show you what the market participants collectively believe. If the market is wrong about the underlying probability, the de-vigged number will still be wrong; it will just be wrong without the extra margin on top.
Also note that on prediction markets like Polymarket, the implied probability is essentially already the market price (shares trade between 0 and 1), so de-vigging is less of an issue there. It matters most for traditional sportsbooks that publish American or decimal odds with embedded margins.
Try It
Go to fatnarwhal.com/devig and paste in the odds from any sportsbook on an upcoming event. Strip the vig and see the fair probabilities. Then compare those against your own estimate and see if there is a real edge underneath the noise.
The house is always taking a cut. At least now you can see how much.