How FadeTheVig compares the crowd and the house

Crowd vs house

Crowd = prediction markets, Kalshi and Polymarket, where traders set the price. House = US-regulated sportsbooks, which set their own lines and build a margin (the vig) into them.

The split

Crowd price = the midpoint between the best bid and best ask on each liquid Kalshi/Polymarket market (averaged if both are liquid). House fair = the books' no-vig consensus (below). Split = crowd minus house, in percentage points. A big split means the two markets disagree; it is not a prediction of who is right.

House take

For each book, add up the implied probabilities of both sides; the amount over 100% is the book's margin. We show each book's margin and the median across books.

All-in cost per $1 payout (who's cheaper)

Every price becomes what you pay for something that returns $1 if that side wins. Lower is cheaper.

• Kalshi: best YES ask (or the cheaper NO ask on the opponent) + taker fee 0.07 × M × P × (1−P) per contract, M = the series fee multiplier from Kalshi's API (1 for NFL/NBA/NHL/NCAAF games). Kalshi rounds fees up per order, so small orders pay slightly more.
• Polymarket: best ask from the public CLOB order book + taker fee feeRate × p × (1−p) per share, using the fee rate published on each market (Gamma feeSchedule.rate). Makers pay no fee.
• Sportsbooks: best moneyline among US-regulated books, converted to 1 / decimal odds. No separate fee; the vig is inside the price.

No-vig "fair" line

For each book we divide both sides' implied probabilities by their sum (removing the house take), then take the median across books.

What we exclude

Prediction-market quotes with a bid/ask spread wider than 5¢ or less than $25 available at the ask are greyed out and excluded from splits and rankings. Offshore books are never shown. Games that have already started are excluded.

Rules differ

Settlement rules can differ (for example, Polymarket resolves an NFL tie 50-50). Check each venue's rules. Data only, never picks, and not advice.