Sportsbooks don't need to know who wins. They just need you to bet. The reason is the vig (also called juice or margin): a small cut built into almost every price.
How it works
In a fair coin flip, both sides would be +100: bet $100, win $100. Sportsbooks price it at -110 / -110 instead. Now you bet $110 to win $100 on either side.
If the book gets similar money on both sides, it pays the winners with the losers' money and keeps the difference. Every single time.
How to calculate it
Turn both sides into implied probabilities and add them up.
- -110 → 52.4%
- -110 → 52.4%
- Total: 104.8%
That extra 4.8% is the book's margin. The further over 100%, the worse the deal for you. Props and futures often carry a lot more margin than main lines.
What it means for you
At -110 you need to win 52.4% of your bets just to break even. Winning exactly half your bets = slowly losing money. That's why "I'm basically 50/50" bettors bleed out over a season.
How to pay less of it
- Shop lines. Different books post different odds. Getting -105 instead of -110 on every bet adds up big over a season.
- Lean on main markets over exotic props, where margins are usually higher.
- Skip long parlays, which stack the vig on every leg.
- Only bet when your edge is bigger than the vig. If you think a -110 side wins 53%, that's barely anything. 56%+? Now we're talking.
The bottom line
The vig is the price of every bet you make. You can't avoid it, but you can pay less of it and only bet when your edge is bigger. That's the difference between betting and donating.