Event-contract guide
Same Game, Two Prices: Kalshi vs Polymarket
Pull up the same NFL game on Kalshi and Polymarket and you'll regularly see prices a few points apart. Sometimes that's free money; more often it's friction or an illusion. Here's what actually drives the gap — and the traps we've hit measuring it with real data.
1. Two separate crowds, two separate order books
Kalshi is a CFTC-regulated exchange funded by USD bank transfers; Polymarket runs on crypto rails with a different (historically more international, crypto-native) user base. Neither book automatically syncs to the other — prices only converge when someone trades the difference, and moving money between the two venues is slow enough that plenty of gaps persist.
2. When a gap is real arbitrage
If YES on one venue plus NO on the other costs under $1 combined — after Kalshi's fee of 0.07×P×(1−P) rounded up, and using prices you can actually execute at (the ask, not the mid) — you lock a profit regardless of outcome. That is the arithmetic. What we actually observe is that it almost never happens: across roughly 160,000 paired observations we have not recorded a single executable arbitrage. The reason is the word ask. The gap you can see is between two mid prices, and the bid/ask spread on the thinly-traded side is usually wider than the gap itself, so the difference is eaten before you can take it. Treat a headline gap as somewhere to look, not money on the table.
3. Phantom spread #1: stale quotes
The most common fake edge: one side's price is frozen. An event that ended, a book with no recent trades, or a data feed lagging minutes behind will happily show you a 'spread' against a live price on the other venue. Any comparison must check both quotes are fresh — we discard anything older than 15 minutes, and that single rule kills most apparent 'opportunities.'
4. Phantom spread #2: in-play divergence
Once a game starts, Kalshi races toward 1c/99c live while the other venue's pre-game market may freeze or thin out. A settled $1.00 leg against a frozen 51c leg computes as an '88% edge' that never existed as a tradeable moment. Pre-game comparison only, unless you genuinely trade in-play on both venues simultaneously.
5. Phantom spread #3: matching the wrong game
Doubleheaders, teams that play each other repeatedly (MLB series), and one venue listing a different market variant (spread vs moneyline vs series winner) all produce comparisons between non-identical events. Match on team pair AND date AND market type — anything looser eventually pairs the wrong markets and reports nonsense.
6. The honest takeaway
Cross-venue gaps are a real signal worth watching — as an alert to look, not a guarantee of profit. One thing our own history contradicts about how these are usually described: they are not a speed game. Of 841 episodes where the two venues sat three or more points apart, the median lasted about 15 minutes and roughly a third lasted over an hour. You are not in a race with a bot. The durable use is relative value: when one venue is consistently richer for a team, the cheaper venue is the better place to express any view you already hold.
See the numbers live
Polykal runs this exact math continuously — de-vigged sharp consensus vs Kalshi prices for every NFL and CFB game, plus cross-platform spread alerts via Discord + webhooks.
Create an account →Free while paid alerts are still closed. Paste any Kalshi or Polymarket link and we'll find its counterpart and what the gap clears after costs, and keep a watchlist of the markets you care about.
Frequently asked
Which platform has better prices?
Neither, consistently. The gap flips game to game and hour to hour. Fee structure differs too (Kalshi charges a trading fee, Polymarket mostly doesn't), so compare all-in executable cost, not headline price.
Is cross-platform arbitrage legal?
Trading on both platforms where you're eligible is trading, not a legal gray area in itself — but eligibility differs (Polymarket's US access runs through its regulated entity; state-level treatment of sports event contracts is actively contested in 2026). Nothing here is legal advice; know your jurisdiction's status before funding accounts.
How does Polykal fit in?
Polykal indexes both venues continuously, matches games deterministically (team pair + date + moneyline only), applies the freshness and pre-game guards above, and alerts you when a genuine spread opens — plus screens Kalshi prices against sharp sportsbook consensus for one-sided +EV. It's the watching, automated; the trading stays entirely yours.
Can I just see today's gaps without signing up?
Yes — LineGap (linegap.dietsoda.dev) is our free companion site: a page per matched game showing both venues' prices, the gap, and how it moved. No account, nothing to buy. It's the same index this guide describes, published as a read-only view; Polykal is for when you'd rather be told than go looking.
Impersonal market analytics and education computed from public prices. Nothing on this page is investment, betting, legal, or financial advice, or a recommendation to buy any contract. Polykal is not affiliated with Kalshi, Polymarket, or any sportsbook.