Free Kelly sizing tool — see EV%, bankroll fraction and dollar stake in seconds
The Kelly Criterion is a bankroll-sizing formula that tells you what fraction of your roll to risk when you have a repeatable edge. Unlike flat staking, Kelly scales bets with confidence: stronger edges warrant larger wagers, while thin edges stay small—protecting you from ruin during variance.
This free Kelly calculator converts your odds format, win probability, and bankroll into expected value (EV%), optimal Kelly fraction, and dollar stake—instantly. Use fractional Kelly (0.25–0.5×) in real play to smooth drawdowns while still compounding long-term growth.
Choose fractional Kelly, then enter odds, win probability and bankroll — results update automatically.
The Kelly Calculator helps you determine the optimal bet size based on the Kelly Criterion, which maximizes long-term growth while minimizing risk of ruin.
This calculator will show you:
Note: The Kelly Criterion assumes you can accurately estimate your win probability. Be conservative with your estimates to avoid over-betting.
Kelly maximizes long-term logarithmic growth of bankroll when your win probability is accurate. For net decimal odds b and win probability p (lose probability q = 1 − p), the full Kelly fraction is f* = (b·p − q) / b, equivalent to (b·p − 1) / (b − 1). When f* ≤ 0, there is no +EV bet—staking anyway burns bankroll.
Full Kelly is volatile: a few losses can slash balance sharply. Most bettors use fractional Kelly (multiplying f* by 0.25–0.5) and hard caps (e.g., never more than 5% of roll on one play). Pair Kelly with +EV screening so you only size bets that pass expected value checks first.
The Kelly fraction (before your multiplier) for decimal odds \(O\) and win rate \(p\) is:
Stake = Bankroll × (fractional multiplier) × \(f^*\). Expected value % ≈ \((p \cdot (O-1) - q) \times 100\) where \(q = 1-p\).
Scenario: You back an underdog at +150 (decimal 2.50), believe they win 55% of the time, bankroll $10,000, fractional Kelly 0.5×.
| Input | Value | Note |
|---|---|---|
| Decimal odds / p | 2.50 / 55% | +150 American |
| Full Kelly f* | 25% | ×0.5 multiplier → half-Kelly stake |
f* = (2.50·0.55 − 0.45) / 2.50 = 0.25 → 25% of roll at full Kelly (aggressive). Select Half Kelly in the calculator → 12.5% stake → $1,250 on a $10k bankroll.
Estimation error is the silent killer: if you think p = 55% but true p is 50%, full Kelly oversizes and increases risk of ruin. Halving Kelly cuts variance roughly in half while sacrificing only a fraction of theoretical growth—an excellent trade for real markets.
Combine fractional Kelly with flat caps, stop-loss rules, and CLV tracking. Never chase losses by cranking the multiplier after a bad beat—the math assumes independent, identically modeled edges.
Screen plays for +EV first with our expected value calculator—Kelly only sizes bets that already clear value.
Derive fair win probability from any price using the implied probability calculator before you enter p here.
When books diverge, test arbitrage scenarios for risk-free locks—then use Kelly only on discretionary +EV spots.
Only size bets that are +EV after honest probability work. Use fractional Kelly (0.25–0.5×), cap any single wager (often 5% of bankroll max), and track CLV to verify your model. When Kelly fraction is zero or negative, pass—forcing action on -EV prices is how bankrolls die.
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