Kelly Betting Calculator & Bankroll Staking Guide
The Kelly criterion answers the most underrated question in betting: not what to back, but how much. Enter your bankroll, the odds and your estimated win chance to get the mathematically optimal stake - plus the safer fractional versions most professionals actually use.
Kelly stake calculator
How the Kelly criterion works
Kelly fraction = (decimal odds × win probability - 1) ÷ (decimal odds - 1). The result is the percentage of your bankroll that maximises long-run growth if your probability estimate is accurate. At $1.90 odds with a 58% win chance, Kelly says stake (1.90 × 0.58 - 1) ÷ 0.90 = 11.3% of bankroll.
The catch is that full Kelly assumes your estimate is exactly right. Real estimates carry error, and overbetting an overestimated edge is far more damaging than underbetting a real one. That is why experienced bettors use half or quarter Kelly - you keep most of the growth with a fraction of the drawdowns.
Flat staking vs Kelly: a practical comparison
| Approach | Stake | Best for | Risk |
|---|---|---|---|
| Flat (1-2% of bank) | Same every bet | Beginners; anyone without calibrated probabilities | Lowest - survives long losing runs |
| Quarter Kelly | Scales with edge | Cautious bettors using model probabilities | Low - drawdowns stay manageable |
| Half Kelly | Scales with edge | Most value bettors - the standard compromise | Moderate - expect 30%+ drawdowns eventually |
| Full Kelly | Aggressive | Only with proven, calibrated estimates | High - 50% drawdowns are routine |
Even a genuinely good model has losing runs. At a 63% strike rate, runs of 4-6 straight losses happen every season. Whatever staking plan you choose, the test is simple: could you survive ten of your average bets losing in a row without changing how you bet? If not, your stakes are too big.
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FAQ
What is the Kelly criterion formula?
f = (bp - q) ÷ b, where b = decimal odds minus 1, p = your win probability, q = 1 - p. Equivalent: (odds × probability - 1) ÷ (odds - 1). The answer is the fraction of bankroll to stake. If it is negative, the bet has no edge - stake nothing.
Why does the calculator say stake $0?
Your estimated win chance is below the implied probability of the odds, so the bet has negative expected value. Kelly's first job is telling you when not to bet. Check the price against your estimate with the value checker.
Should I use full, half or quarter Kelly?
Half Kelly is the standard recommendation: roughly 75% of full Kelly's growth rate with half the variance. Use quarter Kelly if your probability estimates are unproven (most people's are). Full Kelly only makes sense with a long, verified track record of calibrated estimates.
What is an EV betting calculator?
Expected value (EV) = win probability × odds - 1, shown as the "edge per bet" above. A +5% EV means you expect $5 profit per $100 staked over the long run. Kelly then converts that edge into the optimal stake size - the two calculations work together.