Kelly Criterion Calculator

Enter your edge and bankroll. We return the mathematically optimal bet size at full, half, and quarter Kelly.

Your Fair Probability (%)

Your model's win probability

Market Price (¢)

Ask price on Polymarket / Kalshi

Bankroll ($)

Total betting capital

Got American or decimal odds? Convert to cents first →

Edge

Expected ROI per $1 bet

No edge — don't bet

Your fair probability is below the implied market probability. Kelly says don't take the trade.

Optimal Bet Size

Full Kelly

Aggressive — high variance

of bankroll

Half Kelly

Moderate

of bankroll

Quarter Kelly

Recommended

of bankroll

Quarter Kelly is the practical default for most traders — it preserves 75%+ of full-Kelly long-run growth while cutting drawdown variance roughly in half.

The Formula

f* = (p × b - q) / b

where:
  f*  = fraction of bankroll to bet
  p   = your fair probability of winning
  q   = 1 - p (probability of losing)
  b   = decimal odds - 1 = (1 / market_price) - 1

Read the full Kelly Criterion guide for derivation, edge cases, and why most traders bet 2-4x too much.

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