kalshikelly.com — optimal bet sizing for Kalshi & Polymarket
Unofficial tool. Not affiliated with Kalshi or Polymarket.
Tired of converting Kalshi percentages to Vegas odds just to run Kelly? This tool skips that. Enter the market price (what Kalshi/Polymarket shows — e.g. 20%) and your fair estimate (what you actually think the probability is — e.g. 35%). The Kelly criterion calculates how much of your bankroll to wager given your edge.
If you have no edge (your estimate ≤ market price), the answer is zero. Scroll down for a full explainer on how Kelly works and why fractional Kelly is often the smarter choice.
The Kelly Criterion is a formula that tells you the optimal percentage of your bankroll to bet when you believe you have an edge. The goal is to maximize long-term bankroll growth while minimizing the risk of ruin.
The classic formula is:
Suppose a market trades at 20¢ — implying a 20% chance — but you think the real probability is 35%.
On Kalshi-style markets, buying at 20¢ means you risk $0.20 to profit $0.80, so your net odds b are:
With p = 0.35 and q = 0.65, plug into Kelly:
Kelly betting balances aggression, growth, and survival:
Full Kelly maximizes theoretical long-term growth, but the volatility is psychologically brutal. Many experienced bettors use Half Kelly (0.5×) or Quarter Kelly (0.25×) because:
Kelly only works if your probability estimate is actually good. If you think a market should be at 35% but reality is 22%, Kelly will massively overbet and destroy your bankroll.
The math is not the hard part. Accurately estimating probabilities better than the market is the hard part.