Fractional Kelly

Also called half Kelly · quarter Kelly

Staking a fixed fraction of the full Kelly number — typically a half or a quarter — to cut variance at a small cost in growth.

Full Kelly is optimal only if your probability is exact, and it never is. That asymmetry is the entire argument: the growth curve as a function of stake size falls away far more steeply on the oversized side than the undersized side, so an error that makes you overestimate edge is punished much harder than one that makes you cautious.

Half Kelly gives up roughly a quarter of the theoretical growth rate while substantially reducing the volatility of the path you take to get there. Quarter to half is the standard working range for that reason, and it is what the sizing tool here defaults into rather than presenting full Kelly as the recommendation. On top of the fraction sits a hard per-market cap you set yourself, so a single conviction trade cannot become an outsized share of the bankroll no matter what the formula says.

The practical framing that survives contact with real trading: treat any Kelly output as a ceiling to size under, not a target to hit. And honour the total open-exposure limit across positions, because Kelly sizes one trade at a time and says nothing about holding twelve correlated ones at once.

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Definitions describe how Polymarket and Kalshi behave and how WhaleTracks models them; venue rules and fee schedules can change, so confirm anything you size a position on with the venue itself. WhaleTracks is informational analytics, not financial advice. Past performance does not guarantee future results.