The vig is the house fee baked into the price
The vigorish (vig, or juice) is the commission a sportsbook charges for taking your bet. You never see it as a line item — it is hidden inside the odds. The standard spread price of -110 on both sides is the classic example.
At -110, the implied probability of each side is 110 / (110 + 100) = 110 / 210 = 52.38%. Add both sides together: 52.38% + 52.38% = 104.76%. A fair coin flip should total 100%. That extra 4.76% is the book overround — the vig.
Strip the juice to find no-vig fair odds
To recover the fair probability, divide each side implied probability by the total of both sides. This "normalizes" the market back to 100% and removes the book fee.
Even example: both sides at -110 imply 52.38% each, totaling 104.76%. No-vig probability = 52.38% / 104.76% = 50.0% per side. So the book true opinion is a pure coin flip; the -110 price is just how it charges you to play.
Lopsided example: Team A is -150 and Team B is +130. Implied A = 150 / 250 = 60.00%. Implied B = 100 / 230 = 43.48%. Total = 103.48%. No-vig A = 60.00% / 103.48% = 58.0%. No-vig B = 43.48% / 103.48% = 42.0%. Those two now sum to 100%, and 58/42 is the book real read on the game once its fee is removed.
Why no-vig numbers matter
The no-vig price is the sharpest single estimate of true probability you can get for free, because it reflects where real money has moved the market. Compare your own probability to the no-vig number, not the raw price, to judge whether you actually have an edge.
It also tells you how much you are paying to play. A market that totals 104.76% costs you more than one that totals 102%. Shopping for lower-vig prices is one of the few edges available to every bettor without any modeling at all.