What Is Vig? The Hidden Tax on Every Bet You Place
Every sportsbook and prediction market charges vig — the built-in margin that guarantees the house profits. Here's how to calculate it and strip it from any line.
What is vig in sports betting?
Every time you place a bet — whether it's on DraftKings, Kalshi, or Polymarket — you're paying a fee. Sportsbooks call it vig (short for vigorish). You'll also hear it called juice, hold, or margin. Prediction markets call it fees or spread. The math is identical.
A fair coin flip should be priced at +100 on both sides. But a sportsbook prices it at -110/-110. That gap is the vig. You're paying $110 to win $100 on a 50/50 proposition.
How to calculate vig
The book sum (also called the overround) tells you everything. Add up the implied probabilities of all outcomes:
- -110 implies 52.4% probability
- -110 on the other side also implies 52.4%
- Total: 104.8%
That extra 4.8% above 100% is the vig. The book is overcharging both sides so they profit regardless of the outcome.
For a more exact number, the hold percentage shows how many cents the book keeps per dollar wagered:
Hold = 1 - (1 / book sum) = 1 - (1 / 1.048) = 4.6%
That means for every $100 wagered across both sides of this market, the sportsbook expects to keep $4.60.
How vig varies by sport and market
Not all markets carry the same vig. Standard lines:
| Market Type | Typical Book Sum | Approx. Hold |
|---|---|---|
| NFL spread (-110/-110) | 104.8% | 4.6% |
| NFL moneyline | 105-108% | 5-7% |
| NBA player props | 108-115% | 7-13% |
| Parlay (3 legs) | ~115% | ~15% |
The pattern is clear: the more obscure or complex the market, the more vig the book charges. Player props and parlays are the most profitable products sportsbooks sell — and the worst deal for bettors. See the math behind parlays for a deeper breakdown of how vig compounds across legs.
Vig on prediction markets
Kalshi and Polymarket work differently — they use order books instead of fixed odds. But the principle is the same. If "Yes" contracts trade at $0.55 and "No" contracts trade at $0.50, the total is $1.05. That extra $0.05 is the spread — prediction market vig.
On top of the spread, Kalshi charges explicit fees on winning trades (typically 5-10% of profit). Polymarket has no trading fees but earns from the bid-ask spread itself. Either way, there's a cost. Read more about how prediction market fees eat your edge.
Why understanding vig matters
Vig is the reason most bettors lose. You don't need to be wrong to lose money — you just need to be not right enough to overcome the built-in tax. On a standard -110 line, you need to win 52.4% of your bets just to break even.
This is also why expected value is the only metric that matters. A bet can look good on paper, but if the vig makes it -EV, you're donating money to the sportsbook over time.
How to strip the vig
Use the de-vig calculator to find the true probability behind any line. The hold calculator shows you exactly how much the book is charging on any market.
Once you know the true odds, you can compare them against your own probability estimates. If your number is higher than the de-vigged line, you have an edge. If it's lower, pass.
The math doesn't care what platform you're on. Once you can see the vig, you can start finding the edge.