Sports betting is a game of price. The prices you see on a sportsbook are not "fair" — every line has a built-in markup, known as the vig or juice, which is how the bookmaker makes money. To look for candidate +EV bets, you first need to strip away this markup to estimate the "true" price of a bet.

This process is called de-vigging. It is a foundational skill for measuring estimated value and looking for an edge. Sports betting is not risk free and results are never guaranteed.

This guide walks through how it works. SmartStake explains what the vig is, how to remove it with a simple example, and the different methods traders use to estimate fair prices. For more on how SmartStake uses this to surface candidate bets, see SmartStake's Positive EV Guide.

What is the Vig, and Why Does It Matter?

When a bookmaker sets odds, the implied probabilities of all outcomes will always add up to more than 100%. That extra amount is their profit margin.

Let's take a simple example: a tennis match between Player A and Player B. A bookmaker offers the following odds:

• Player A: 2.10 (+110)
• Player B: 1.75 (-133)

First, we convert these odds into implied probabilities using the formula 1 / odds:

• Player A's Implied Probability: 1 / 2.10 = 47.6%
• Player B's Implied Probability: 1 / 1.75 = 57.1%

Now, add them together: 47.6% + 57.1% = 104.7%.

This 104.7% is the overround. The extra 4.7% is the bookmaker's vig. It guarantees them a profit if they take balanced action on both sides. Our job is to remove that 4.7% to find the fair, 100% market.

How to De-Vig: Estimating the True Price

De-vigging means scaling those probabilities back down so they add up to 100%. The simplest way to do this is the Multiplicative (or proportional) method.

Using the example above:

1. Find the total probability (overround): 104.7% (or 1.047)
2. Divide each outcome's probability by the total:
   • Player A's Estimated True Probability: 47.6% / 104.7% = 45.5%
   • Player B's Estimated True Probability: 57.1% / 104.7% = 54.5%
3. Convert the estimated true probabilities back to fair odds:
   • Player A's Estimated Fair Odds: 1 / 0.455 = 2.20
   • Player B's Estimated Fair Odds: 1 / 0.545 = 1.83

So, after removing the vig, the estimated true price for Player A is 2.20, not 2.10. Any price you find at another sportsbook that is higher than 2.20 represents a candidate Positive Expected Value (+EV) opportunity — though individual bets can still lose.

Choosing Your Tool: A Guide to De-Vigging Methods

The Multiplicative method is a great starting point, but it's not the only tool available. Different methods distribute the vig differently, which can be useful in certain market conditions, especially when there's a heavy favorite and a big underdog (a situation known as the favorite-longshot bias).

Here’s a quick, non-mathy breakdown of the most common methods:

• Multiplicative (Proportional): The simplest approach. It assumes the bookmaker applies their margin proportionally to each outcome. It’s stable, reliable, and a great baseline for most situations.

• Additive: This method removes an equal slice of the margin from each outcome. It's effective at correcting for some of the favorite-longshot bias and often produces results very similar to more advanced models in two-way markets.

• Power: A more flexible method that uses an exponent to control how much the vig removal is skewed. It's a good middle-ground between the Multiplicative and more aggressive methods.

• Shin: This method assumes the bookmaker's profit comes from balancing their books against informed and uninformed bettors. It's particularly effective in markets with three or more outcomes (like a soccer match) and behaves much like the Additive method in two-way markets.

• Probit: Uses a statistical approach (the normal distribution) to adjust the probabilities. It's especially useful for spread and totals betting, where small differences around the 50% mark are critical.

• Worst-case (Conservative): Not a single method, but a strategy. You calculate the fair odds using a few different methods and pick the most conservative price for each side. This reduces the risk of finding false positives but means you might pass on some borderline +EV bets.

When to Use Which Method

See it in Action: Interactive Calculator

Use the calculator below to see how these methods work in practice. Change the odds and switch between methods to see how the fair price moves. Notice how the differences become larger when one side is a heavy favorite.

Once you have the estimated fair odds, you can compare them to other sportsbooks. If you find a book offering a better price than the estimated fair odds, you may have found a candidate edge. If two books disagree enough, you may have found an arbitrage opportunity — though execution risk, line movement, and user error mean outcomes are not guaranteed. See SmartStake's Arbitrage Guide for more on that.

Further Reading

• Joseph Buchdahl (Football-Data/Books). Accessible explanations of overround allocation and devigging in fixed-odds markets. football-data.co.uk
• Hyun Song Shin (1992 and related). Insider trading and price formation—the foundation behind the Shin adjustment.
• Practitioner notes on probit/logit link-based margin removal used in sports trading and statistical modeling.

Related SmartStake Guides & Tools

• Guide: The Sharp Bettor’s Playbook — Positive EV
• Guide: The Ultimate Guide to Sportsbook Arbitrage
• Tool: De‑Vigging Calculator
• Tool: Odds Converter
• Tool: Arbitrage Calculator