What is vig?
Vigorish — vig, juice, or the overround — is the sportsbook's margin. On a 50/50 game both sides will price at roughly -110, meaning the implied probability of each side is about 52.4%. Together that's 104.8% — the extra 4.8% is the book's edge regardless of outcome.
Step-by-step de-vigging
Given a two-way market (both sides quoted):
- Convert each side's American odds to raw implied probability. American +160 → 100/(160+100) = 38.5%. American -180 → 180/(180+100) = 64.3%.
- Sum them: 38.5% + 64.3% = 102.8%. The 2.8% over 100% is the vig.
- Divide each side by the sum to normalize: 38.5/102.8 = 37.4% and 64.3/102.8 = 62.6%. Those are the no-vig probabilities — they sum to 100% and represent the book's true assessment of each side.
Why de-vig before comparing to Kalshi?
Kalshi has no vig — YES + NO always sums to exactly 100%. Comparing a Kalshi YES at 65¢ to a sportsbook's -200 (which implies 67% raw) is misleading. After de-vigging, the sportsbook's true assessment might be 64% — meaning Kalshi at 65¢ is actually slightly more expensive than the book's fair line, not cheaper.
SportsBookISH does this for you
Every book quote we ingest gets de-vigged in real time. The 'Books median' column on every event page is the median across all books' de-vigged probabilities — your fair-line reference for comparing to Kalshi. The 'Best book' column is the de-vigged probability at the cheapest book (longest American odds).
Multi-way markets
Three-way markets like soccer 1X2 (home / draw / away) work the same way: convert each outcome's American odds to raw probability, sum them, divide each by the sum. The total before normalization tells you the vig: typically 3–5% across the three sides.