Kalshi & Polymarket Fees Explained: What MLB Traders Actually Pay

The biggest reason serious MLB bettors moved to event-contract exchanges is cost. Sportsbooks bake a 4–6% margin into every line; Kalshi and Polymarket charge a fraction of that. But "lower fees" isn't free — both venues have a fee schedule, and the structure looks nothing like a sportsbook's vig. Here's what you actually pay when you trade MLB markets on Kalshi and Polymarket, and how it stacks up against a traditional book.

Kalshi fees: the short version

Kalshi charges a per-contract trading fee calculated on the price and size of your trade. There's no deposit fee, no withdrawal fee on ACH, no monthly minimum, and no inactivity fee.

The fee formula on Kalshi's general markets:

fee = round_up( 0.07 × contracts × price × (1 − price) )

Translated to plain English: you pay roughly 7 cents per contract on a 50¢ market, scaling down as the contract gets closer to $0 or $1. A few examples on an MLB market:

You pay the fee on both sides — once when you enter, once when you exit (or it settles). Markets trading near 50% are the most expensive; deep-in-the-money and deep-out-of-the-money trades are nearly free.

Sports-specific markets

Kalshi runs a separate lower fee tier on most sports markets, including MLB futures and game-day contracts. The exact formula gets refreshed periodically, but the sports-tier rate has consistently come in 30–50% cheaper than the general formula above. Always check the live fee on the order confirmation screen before you submit — Kalshi shows the exact cents you'll pay before the trade goes through.

Settlement, deposits, and withdrawals

• Settlement: free. When a market resolves, winning contracts pay $1.00 with no haircut.
• ACH deposits and withdrawals: free.
• Debit card deposits: ~1.5% fee for instant funding.
• Wire transfers: Kalshi covers incoming wires; outgoing wires may carry a bank fee on your end.

Polymarket fees: the short version

Polymarket's headline trading fee is zero. There's no per-contract charge when you take a price or post a limit order. The catch is that Polymarket runs on Polygon (an Ethereum layer-2), and the on-chain mechanics carry their own costs.

What you actually pay on Polymarket

1. Gas fees on Polygon. Each trade is a blockchain transaction. Polygon gas is cheap — typically $0.01–$0.10 per trade — and Polymarket subsidizes most of it via a gasless meta-transaction layer for standard order sizes. For most users, you'll never see a gas charge directly.
2. USDC bridge cost. Funding your wallet means buying USDC on a centralized exchange (Coinbase, Kraken) and withdrawing it to Polygon. Coinbase charges $0 to send USDC on Polygon; Kraken charges ~$1. This is one-time per deposit, not per trade.
3. Spread. Polymarket's bid/ask spreads on MLB markets are typically 1–4 cents on liquid futures, 5–10 cents on niche props like no-hitter contracts. That spread is your real cost on every trade — you cross it once to enter, once to exit.
4. Withdrawal cost. Sending USDC back to a centralized exchange is gas-priced, again pennies on Polygon. If you bridge to Ethereum mainnet, gas can spike to $5–$30 depending on network conditions.

The hidden tax: bid/ask spread

Polymarket's "0% fee" headline hides the fact that spread is the real fee on any order book. A 2-cent spread on a 50¢ market is a 4% round-trip cost — comparable to or higher than Kalshi's per-contract fee on the same market. On thin niche markets, the spread can blow out to 6–10 cents, which is worse than sportsbook vig.

The rule: Polymarket is cheaper than Kalshi on liquid futures with tight spreads, and more expensive on thin niche markets where you're crossing a wide spread.

Side-by-side: trading $1,000 on the Dodgers to win the World Series at $0.22

On a liquid futures market with a tight spread, Polymarket comes out ahead. On a market with a 5¢ spread, Kalshi wins. Always check the live order book before assuming one is cheaper.

How both compare to sportsbook vig

A typical sportsbook World Series future is priced with a 4–6% baked-in margin across the field. On a Dodgers +400 ticket (implied 20%), the "true" probability is closer to 22% — that 2-point gap is the vig, and you pay it whether you win or lose. On a $1,000 bet, the sportsbook's edge is $40–$60 round-trip before you ever see a result.

Both Kalshi ($34 round-trip in our example) and Polymarket ($20 round-trip) come in meaningfully cheaper than sportsbooks on futures. Over a full season of futures trading, the savings compound: every cent of vig you don't pay is a cent that stays in your bankroll for the next position.

Where fees actually matter for your strategy

• Short-hold, high-turnover trades. If you're flipping in and out of a market multiple times in a week, Kalshi's per-contract fee compounds fast. Polymarket's zero fee plus tight spread is the better venue for active trading on liquid contracts.
• Long-hold futures. If you're buying a preseason World Series contract and holding to October, the fee is paid twice — entry and settlement. The all-in cost is small relative to your hold period. Either venue works; pick the one with the better price.
• Niche markets. Polymarket's wide spreads on no-hitter and milestone markets are the real fee. Don't be fooled by the "0%" label. Calculate the spread cost before sizing in.
• Account minimums. Neither venue has one. You can start with $50 and trade.

A simple fee-aware workflow

1. Always price both venues before entering a futures position. Kalshi may be 22¢ on the Dodgers; Polymarket may be 21¢. After fees, one is cheaper than the other.
2. For high-frequency trading, prefer Polymarket on liquid contracts. Zero per-contract fees mean turnover is nearly free as long as the spread is tight.
3. For one-and-done long holds, use Kalshi's sports tier. The fee is real but small relative to the bid/ask edge you get on regulated U.S. liquidity.
4. For niche markets (no-hitter, perfect game, milestone props), check Polymarket's spread carefully. If the bid/ask is wider than 4 cents, the "free" trade is more expensive than it looks.
5. Avoid debit card deposits. ACH on Kalshi and direct USDC transfers on Polymarket are free. The 1.5% debit card fee on Kalshi is the single most expensive cost you can voluntarily take on.

What to read next

• Kalshi MLB Guide — the full platform walkthrough.
• Polymarket MLB Guide — the crypto-native venue for niche baseball props.
• Kalshi vs Polymarket — choosing between the two for any given market.
• MLB Event Contracts vs Sportsbooks — the full side-by-side comparison.

Fees are the most underweighted cost in MLB betting. A sharp bettor who saves 3% per trade across a 200-trade season is up six full units before any edge in handicapping. Knowing exactly what Kalshi and Polymarket charge — and where their costs hide — is the foundation for trading either venue profitably.