Sports is now the main event on prediction markets. Kalshi sports betting volume runs ahead of every other category on the exchange, and Polymarket's sports books fill up every weekend. But calling it betting hides the interesting part: you are not placing a bet with a house. You are trading a contract with another person, on a regulated exchange, at a price that is simply a probability wearing a dollar sign.
A contract priced at 62 cents says the market thinks the outcome has about a 62 percent chance. If it happens, the contract settles at one dollar. If it does not, it settles at zero. You can sell any time before settlement, which is where it starts to feel more like a stock market than a sportsbook. Prices are data first; the earnings calendar here runs on the same logic.
An exchange, not a sportsbook
At a sportsbook, the house sets the odds, takes the other side of your bet, and builds its margin into the price. That margin, the vig, is why a fair coin flip costs you 110 to win 100 on both sides. On Kalshi there is no house on the other side: the exchange runs the order book, matches buyers with sellers, and charges a fee for the match. Polymarket works the same way on its central limit order book, settling in USDC instead of dollars. Prices move constantly as traders react to news. Winning traders are not limited or banned, because the exchange earns fees whoever wins. And positions are liquid: a futures contract bought in September can be sold in October, at the market price.
What sports you can actually trade
Kalshi lists game winners, spreads and totals for the major US leagues, season futures, player props, and combos that bundle several outcomes into one contract, the exchange version of a parlay. Polymarket covers the same US sports and adds deeper international soccer, tennis, MMA and esports. The biggest books sit around the events everyone watches: the Super Bowl alone drew about 1.6 billion dollars of combined trading earlier this year, per platform figures compiled by DeFi Rate.
Kalshi vs sportsbooks, side by side
| Feature | Kalshi and Polymarket | Sportsbooks |
|---|---|---|
| Who is on the other side | Other traders, matched on an exchange | The sportsbook itself |
| How the price is set | Supply and demand; 62 cents means about 62 percent | Set by the book, with the vig built in |
| The cost of trading | An explicit fee per contract, published in advance | About 4.8 percent hold on a standard -110 line |
| Getting out early | Sell at the market price any time before settlement | Cash out only when the book offers, at the book's price |
| Winning traders | No limits for winning; the exchange earns fees either way | Books often limit or close winning accounts |
| Regulator | CFTC at the federal level | State gaming commissions, state by state |
What it costs: the kalshi sports fees formula
Kalshi publishes its fee schedule, and the formula is worth learning: the taker fee is 0.07 times the number of contracts times the price times one minus the price. The fee peaks where uncertainty peaks. One hundred contracts at 50 cents cost 1.75 dollars in fees; the same hundred at 90 cents cost 63 cents, and at 99 cents just 7 cents. Orders that rest on the book are free on most markets. There is no settlement fee, bank transfers are free, and card deposits can carry a fee of up to 2 percent. Polymarket historically charged no trading fee on most markets, has added taker fees on some categories including crypto and select sports, and its separate US app charges a small published taker fee.
Are kalshi sports contracts legal?
Kalshi is a designated contract market regulated by the CFTC, and it argues its sports contracts are financial products under federal law. Several states disagree and have sued or moved to restrict sports contracts, so the map is unsettled and changes as cases move. Polymarket's US access runs through a separate CFTC-regulated app still rolling out state by state. Federally regulated, contested locally: check your own state before you trade, because availability varies by state and by product.
Read prices as probabilities and these sports books are some of the cleanest public forecasts in the world. The companion pieces dig into the two skills that matter once you are in: when to sell a winning contract, and where the underlying data comes from.