Updated · Methodology: named formula library
Kelly Criterion Calculator
Optimal bankroll fraction for a bet.
(1 × 55.0% − 45.0%) / 1 = 10.0% of bankroll.
Why This Calculation Matters
The Kelly Criterion Calculator turns performance into programmable targets, splits, loads, and paces that make training decisions objective.
How to Use This Calculator
- Enter your values in the input fields, each one has a label and help text explaining what to type.
- Results appear instantly as you type; there's no "calculate" button to press.
- Change any input to compare scenarios side by side.
All math happens in your browser. Nothing you type is sent to a server, saved, or shared.
How to Use
Enter values in the fields on the left. Results update as you type, no submit button needed.
Understanding Results
Each output shows the calculated figure plus a breakdown of contributing inputs. Compare scenarios by editing any value.
Accuracy Notes
Every Kelly Criterion Calculator on CalcIntel uses a documented formula. Results are estimates, real outcomes depend on assumptions and market conditions not captured in a simplified calculation.
Formula
ROI = (gain − cost) ÷ cost × 100. Annualized ROI adjusts for holding period: ((1 + ROI)^(1/years) − 1) × 100.
Worked Example
55% win prob, even-money odds (1:1)
- winProb
- 55
- odds
- 1
- Result
- 10% of bankroll
(1 × 0.55 − 0.45) / 1 = 10%. Half-Kelly: bet 5% for less variance.
When to Use This Calculator
- Track personal records, splits, or performance targets.
- Program workouts or training schedules with concrete targets.
- Compare results across events, distances, or competitors.
Limitations & Common Mistakes
- Results are estimates, real-world outcomes depend on factors not captured in a simplified calculation.
- Always verify critical numbers against an authoritative source or domain expert before acting on them.
Frequently Asked Questions
What is Kelly Criterion?
Optimal bet-sizing formula: f* = (bp − q) / b, where b = net odds, p = win probability, q = 1−p. Tells you the fraction of bankroll to risk to maximize long-term log-growth. Negative result = don't bet (negative expected value).
Is Full Kelly too aggressive?
For most practical purposes, yes. Full Kelly maximizes geometric growth but produces high volatility (40%+ drawdowns are routine). Most bettors and investors use Half-Kelly or Quarter-Kelly to capture most of the growth with much less variance.
Where does Kelly apply?
Sports betting (when you have a true edge), poker, options trading, and any repeated-bet scenario with known win probabilities and payouts. Don't use Kelly for one-shot decisions or when probabilities are unknown — it assumes you have an honest probability estimate.
What if my probability estimate is wrong?
Kelly is highly sensitive to p. Overestimating your edge by 10% can lead to bet sizes that go negative-EV. Best practice: discount your stated probability by 10–20% to account for estimation error, or just use Half-Kelly throughout.
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