Why de-vigging matters
Every price a sportsbook posts has the vig (also called juice, hold, or margin) baked in. The vig is the book's built-in commission, and it's why a market's implied probabilities add up to more than 100%. If you want to know whether a bet is actually good, you first have to strip that commission out to estimate the fair, no-vig price — the price that reflects the market's real opinion of the outcome. De-vigging is how you do that. It's the single most important calculation behind finding positive expected value (+EV) and measuring closing-line value (CLV).
This is a how-to guide. If you want the plain-English concept of what the vig is, that's a separate piece. Here we're doing the math.
Step 1: Convert odds to implied probability
Fair pricing lives in probability space, not odds space, so convert first. For American odds:
- Negative odds (favorite): implied % = |odds| / (|odds| + 100)
- Positive odds (underdog): implied % = 100 / (odds + 100)
A standard -110 becomes 110 / 210 = 52.38%. A +150 dog becomes 100 / 250 = 40.0%.
Step 2: Measure the overround
Add the implied probabilities of every outcome in the market. On a two-way moneyline of -200 / +170:
- Favorite -200 → 200 / 300 = 66.67%
- Underdog +170 → 100 / 270 = 37.04%
- Sum = 103.70%
That extra 3.70% is the overround — the vig expressed as probability. Your job is to redistribute those probabilities back down to a total of 100%. There are three common ways to do it, and they disagree, which is the whole point of this article.
Method 1: Multiplicative (normalization)
Divide each implied probability by the total. This removes the vig proportionally to each side's implied probability.
- Fair favorite = 0.6667 / 1.0370 = 64.29%
- Fair underdog = 0.3704 / 1.0370 = 35.71%
This is the most widely used method because it's simple and always sums cleanly to 100%. Its assumption is that the book applies vig in proportion to each outcome's probability — so more absolute juice is stripped from the favorite. That's a reasonable default and what most tools use by convention.
Method 2: Additive (equal margin)
Subtract an equal share of the overround from each side. With two outcomes, that's 3.70% / 2 = 1.85% each:
- Fair favorite = 66.67% − 1.85% = 64.82%
- Fair underdog = 37.04% − 1.85% = 35.19%
Notice the favorite's fair probability is higher here than under the multiplicative method. Additive assumes the book charges the same flat margin on every outcome. It's intuitive, but it can misbehave on lopsided or three-way markets and even produce negative probabilities on heavy longshots, so use it with care.
Method 3: Power method
The power method finds an exponent k such that raising each raw implied probability to the power k makes them sum to exactly 1 (fair% = pk). For our market, k works out to roughly 1.06, giving about 65.0% / 35.0%.
Why bother? Books don't spread vig evenly. Sharp research on the favorite-longshot bias suggests underdogs carry disproportionately more margin. The power method captures that curvature, pulling more juice off the longshot and typically producing the most accurate fair price on the tails (big favorites, big dogs, and multi-runner futures). The tradeoff is that it has no closed-form solution — you solve for k iteratively — so it's harder to do by hand.
Comparing the three
On this one market, the fair favorite probability lands at 64.29% (multiplicative), 64.82% (additive), or ~65.0% (power). Small differences — but on the tails they widen, and small edges are exactly what separate a +EV bet from a coin flip. Convert 64.3% back to odds and the fair line is about -180 / +180, versus the posted -200 / +170.
Turning fair prices into +EV and CLV
Say a different book prices that same underdog at +200 while your fair estimate is 35.7%. Expected value per $1:
EV = 0.357 × (3.00 − 1) − (1 − 0.357) = 0.714 − 0.643 = +$0.071, or +7.1% EV.
That gap only appears if you (a) de-vig to a fair number and (b) line-shop across books to find the outlier. If that +200 later shortens to the fair +180 at the close, you captured positive CLV — you beat the closing price, the most reliable signal that your bet was priced correctly. None of this is a guarantee; fair prices are estimates, and any single bet can lose.
Where EdgeFinder fits
Doing this by hand for one game is instructive. Doing it across 40+ books and thousands of MLB, NBA, NFL, NHL, college, and 3-way soccer markets in real time is not realistic manually. EdgeFinder automates the full loop: it pulls live odds, de-vigs each market to a fair price, line-shops for the best available number, and surfaces the +EV gaps — plus a self-training model with a public, verifiable track record you can check yourself. See today's edges to watch the math in action, then sign up to unlock every market. Education only, 21+, and bet responsibly.