PolyMath › Blog › Kelly Criterion on Kalshi

How to Use the Kelly Criterion on Kalshi

April 12, 2026 · PolyMath Team · 10 min read

Kalshi is the only CFTC-regulated prediction market in the United States — and that regulatory legitimacy means more institutional traders, tighter markets, and less low-hanging fruit for casual bettors.

That makes position sizing more important, not less. Getting the math right on Kalshi is the difference between grinding out a consistent edge and blowing up a bankroll on a streak of bad variance.

The Kelly Criterion is the mathematically optimal answer to position sizing. This guide walks through exactly how to apply it on Kalshi — including worked examples, common mistakes, and how to use PolyMath's Kelly Calculator to get the number in seconds.

Why Kalshi Is Different From Polymarket

Before diving into Kelly, it helps to understand what makes Kalshi's market structure unique.

Kalshi is CFTC-regulated. That means US users can trade legally, withdraw in dollars, and use ACH transfers. It also means the platform attracts more sophisticated, institutional-grade market makers.

Kalshi uses a CLOB (Central Limit Order Book). Unlike Polymarket's AMM (Automated Market Maker), Kalshi matches buyers and sellers directly. Prices are set by the order book, not an algorithm. Spreads can be wider on thin markets, narrower on liquid ones.

Fees on Kalshi: Kalshi charges a percentage of winnings (typically 7% of profit for free accounts, lower for higher tiers). This fee changes your effective edge calculation — and therefore your Kelly fraction.

Why this matters for Kelly:You can't just plug your raw probabilities into Kelly on Kalshi without accounting for the fee. We'll show you exactly how.

The Kelly Formula (Quick Review)

For a binary market — YES or NO, resolves at $1 or $0 — the Kelly formula is:

f* = (b × p − q) / b

f* = fraction of bankroll to stake
b = net odds (profit per dollar risked if you win)
p = your estimated probability of winning
q = 1 − p (probability of losing)

Or in a more intuitive form:

f* = Edge / Odds

Accounting for Kalshi's Fee

Say a Kalshi contract is priced at 60¢. You buy YES at 60¢. If it resolves YES, you receive $1 — a profit of 40¢. But Kalshi takes 7% of that profit, so your actual profit is:

40¢ × (1 − 0.07) = 37.2¢

Your effective odds (b) are therefore 37.2/60 = 0.62, not 40/60 = 0.67.

Adjusted Kelly formula for Kalshi:

Effective profit = (1 − contract_price) × (1 − fee_rate)

b_effective = Effective profit / contract_price

f* = (b_effective × p − q) / b_effective

This fee adjustment shrinks your Kelly fraction — and correctly so. The fee reduces your edge, so you should bet less.

Worked Example: A Real Kalshi Market

Scenario: Kalshi has a market on whether the Federal Reserve will cut rates at the next FOMC meeting. The contract is priced at 35¢ (35% market-implied probability). You believe, after reading Fed minutes and economic data, that the probability is 48%.

Step 1 — Calculate effective odds (accounting for 7% fee):

Profit per share if YES: (1 − 0.35) = 65¢

After Kalshi fee: 65¢ × 0.93 = 60.45¢

b = 60.45 / 35 = 1.727

Step 2 — Apply Kelly:

p = 0.48, q = 0.52

f* = (1.727 × 0.48 − 0.52) / 1.727

f* = (0.829 − 0.52) / 1.727

f* = 17.9% of bankroll

Step 3 — Apply fractional Kelly:

Quarter Kelly: 17.9% × 0.25 = 4.5%

Half Kelly: 17.9% × 0.50 = 8.9%

For a $1,000 Kalshi bankroll: Quarter Kelly stake = $45, Half Kelly = $89, Full Kelly = $179. For a single FOMC decision with real uncertainty, Quarter or Half Kelly is the right call.

Use PolyMath's Kelly Calculator

You don't need to run this math manually every time. PolyMath's Kelly Calculator handles it in seconds:

Enter the contract price (what you're paying per share on Kalshi)
Enter your estimated probability (your true belief, not the market's)
Enter your bankroll and fee rate (7% for standard Kalshi)
Get your Kelly fraction, Half Kelly, and Quarter Kelly stakes instantly

Calculate your Kalshi stake now

Enter your probability, the contract price, and bankroll — get your fee-adjusted Kelly fraction instantly.

Open Kelly Calculator →

Common Kalshi Kelly Mistakes

1. Ignoring the Fee

Running naked Kelly math without the 7% fee haircut consistently overstates your edge by 5-8%. On thin edges, this can flip a positive-EV bet into a negative-EV one.

2. Treating All Kalshi Markets as Equally Liquid

Kalshi's order book has wide bid-ask spreads on low-volume markets. Use the ask price — not the mid — in your Kelly calculation when entering positions.

3. Over-Betting Correlated Markets

If you're holding YES on 'Fed rate cut in May' and YES on '10-year yield falls below 4%,' those positions are correlated. Kelly assumes independent bets — treat correlated positions as a single position.

4. Not Accounting for Estimation Error

Kelly math is only as good as your probability estimates. If you're uncertain about your edge, use Quarter Kelly. The cost of under-betting is far lower than the cost of over-betting.

5. Betting on Thin Markets

Kalshi has some markets with very low open interest. Entering a large position can move the price against you. Check liquidity before sizing up.

When to Use Kelly on Kalshi (and When Not To)

Use Kelly when:

You have a genuine, quantifiable edge
The market has enough liquidity
You've estimated probability from data
You're planning to stay until resolution

Skip Kelly when:

Your edge is below 3% after fees
You're trading highly liquid expert-dominated markets
You're uncertain about your estimate
You're new and still calibrating

The Right Framework

Kelly isn't about betting big. It's about betting the right amount — the amount that maximizes long-run compounding without exposing you to ruin.

On Kalshi specifically, the fee drag and the sophistication of competing traders both argue for conservative Kelly fractions. Quarter Kelly is a good default. Half Kelly for your highest-conviction markets. Full Kelly almost never.