Polymarket Position Sizing: Kelly Criterion & Fixed Fractional
How much to put on a trade matters more than most people realize — often more than which trades you pick. Good position sizing keeps you in the game through losing streaks; bad sizing ends your account on a single bad run.
Why sizing beats picking winners
Two traders with the same strategy can have opposite outcomes purely from sizing. Oversize, and one normal losing streak wipes you out before your edge can play out. Sizing is the bridge between “a good strategy” and “surviving long enough to benefit from it.”
Fixed-fractional sizing
The simplest robust method: risk a fixed small percentage of your bankroll per trade — say 1–2%. As your bankroll grows or shrinks, position sizes scale with it. It is boring, transparent, and very hard to blow up with.
The Kelly criterion
Kelly calculates the bet size that maximizes long-term growth given your edge and odds. In theory it is optimal; in practice it depends on knowing your true edge precisely — which you almost never do in prediction markets.
Full Kelly vs fractional Kelly
Full Kelly is brutally volatile and assumes your edge estimate is exactly right. Overestimate your edge and full Kelly massively oversizes. Most practitioners use fractional Kelly (a half or a quarter of the formula) precisely because edge estimates are uncertain.
Sizing relative to liquidity and uncertainty
Even if your risk budget allows a large position, liquidity may not. Cap size so you can exit without heavy slippage, and size down when your confidence (or the market) is shakier.
Automating sizing in a bot
Sizing rules are perfect for automation because they remove the human temptation to “go bigger” on a trade that feels certain. A bot applies the same formula to every trade — see risk management for bots.
A sizing framework you can adapt
- Default to fixed-fractional (1–2% risk per trade).
- If using Kelly, use a fraction of it, not the full figure.
- Cap every position by available liquidity.
- Reduce size during drawdowns and high uncertainty.
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Frequently Asked Questions
Disclaimer: This article is for educational purposes only and is not financial, investment, or legal advice. Prediction-market trading carries a real risk of loss. Automation does not guarantee profit, and past performance never guarantees future results. Only trade funds you can afford to lose, and confirm that Polymarket is available and legal in your jurisdiction before trading.