Investing

Kelly Calculator

Calculate optimal bet sizing using the Kelly Criterion

Simulator Parameters
Simulate expected cashflow using your actual Kelly settings

Historical or expected ROI of the tipster

Costs

Subscription or fixed costs deducted monthly

Cashflow Projection
Configure parameters and run the simulation
Click "Run Simulation" to generate bankroll projections
Kelly Playground
Test Kelly calculations with custom inputs. Uses your global Kelly settings.

Historical or expected ROI for this tipster at this confidence level

Actual odds available from bookmaker (if different from advised)

Calculation Result
Recommended stake based on Kelly Criterion with your global settings
Enter values and click Calculate to see results
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How Kelly Criterion Works

The Kelly Formula

The Kelly Criterion calculates the optimal fraction of your bankroll to stake on each bet. The standard formula is f* = (bp - q) / b where b = net odds (decimal odds - 1), p = win probability, and q = loss probability (1 - p).

Our Implementation

This system uses a modified Kelly approach designed for tipster-based betting. Instead of estimating win probability directly, we derive it from the tipster's historical ROI and advised odds: p = (1 + ROI) / advisedOdds. This makes the calculation grounded in actual performance data.

Safety Mechanisms

  • Kelly Reduction: Divides the raw Kelly fraction by a configurable factor (default 3x) for more conservative sizing. This reduces volatility at the cost of slower growth.
  • Kelly Cap: Hard maximum on stake as a percentage of bankroll (default 10%), preventing over-exposure on any single bet.
  • Longshot Reduction: Additional reduction for high-odds bets where variance is highest. Stakes decrease as odds approach the configured threshold.
  • Max Odds Improvement: Limits the benefit when actual bookmaker odds are better than advised odds, preventing over-staking on potentially mispriced opportunities.
  • Edge Validation: Rejects calculations where edge exceeds a threshold (default 60%), flagging likely data errors.

How This Differs From Standard Kelly

  • Uses unit-based edge calculation from tipster ROI instead of estimated win probability
  • Supports variable unit strategies where ROI scales with confidence level
  • Applies progressive longshot reduction (standard Kelly treats all odds equally)
  • Limits benefit from odds improvements (standard Kelly would increase stake proportionally)