The Sportsbook Vig calculator estimates the bookmaker’s built‑in margin (also called overround or juice) for a two‑sided market using the posted American odds on each side. When you convert both lines to implied probabilities and add them together, a fair market would total 100%. Sportsbooks target a number above 100% so they earn a small edge regardless of outcome. That extra amount is the vig. Knowing the vig helps you compare prices across books, spot unusually sharp or soft markets, and understand how much of a quote reflects true opinion versus house margin.

To use it, enter the two opposing prices (for example, −110 and −110 for an even matchup) and click Calculate. We convert each moneyline into an implied probability with standard formulas, then report (p1 + p2) − 1 as the vig percentage. In the −110/−110 case, each side implies roughly 52.4%, so the sum is about 104.8% and the vig is ~4.8%. Markets that total near 100% are “tight” or low‑margin; totals that are much higher suggest a larger house edge or stale numbers. Keep in mind that rounding, low liquidity, or fast moves can briefly produce totals at or below 100%, which usually correct quickly.

How should you apply this? If two sportsbooks quote the same team at the same odds but one market has meaningfully lower vig overall, the lower‑vig book is generally preferable because more of the price reflects the true belief, not margin. You can also use the vig figure as a sanity check before building parlays or hedges: large vig on a leg means you may be paying an invisible tax that reduces long‑run EV. This calculator doesn’t judge whether a line is good or bad by itself — it simply surfaces the house edge so you can layer it into your decision‑making.

Finally, remember that vig is only one piece of the puzzle. Limits, liquidity, and line movement all affect how tradable a number is in practice. For prediction markets, platform fees play a similar role to vig; our Market Fees and No‑Vig Fair Odds tools can help you adjust those quotes as well. Use these calculators together to build a complete picture of cost, edge, and expected value before you place a bet.