Kelly Criterion Calculator
If you think a selection has a higher true probability than the odds suggest, Kelly tells you what fraction of your bankroll to stake. We default to fractional Kelly (half-Kelly) because full Kelly is brutal on a bad run.
Kelly Criterion Calculator
Your edge sets the stake. Default is half-Kelly because full-Kelly is brutal.
Why half-Kelly, not full
The full Kelly formula tells you the theoretically optimal stake to maximise long-run growth. In practice, you never know your true probability precisely — and full Kelly punishes estimation errors brutally. Half-Kelly gives up a bit of upside for a much gentler drawdown curve.
If Kelly returns a negative fraction, the odds are offering worse than your estimated probability. Skip the bet. Don't stake on negative-edge markets to 'chase' a loss.
The editor's notes on this calculator
What this calculator does
Kelly Criterion is a bankroll-management formula that tells you the optimal fraction of your bankroll to stake on a bet where you have a stated edge. Enter your bankroll, the decimal odds offered and your estimate of the true probability. The calculator returns the full-Kelly stake and the quarter-Kelly stake (we recommend the quarter).
The formula
Full Kelly = (bp - q) / b, where b = decimal odds minus 1, p = your estimated probability of winning, q = 1 - p. If the result is negative, do not bet (the price has no value).
Why we recommend quarter-Kelly
Full Kelly is mathematically optimal if your probability estimates are exactly right. They never are. Using quarter-Kelly gives you 50% of Full Kelly's expected growth rate while taking on dramatically less drawdown risk. It is the standard for professional bettors.
Worked example
Bankroll: N100,000. You think Chelsea's probability of winning at home is 65%. Bookmaker offers 1.65. Full Kelly = (0.65 x 0.65 - 0.35) / 0.65 = 0.085 = 8.5% of bankroll = N8,500. Quarter-Kelly = N2,125.
Nigerian caveat
Kelly only works if you genuinely have an edge. Most casual bettors overestimate their probability of winning by 5 to 10 percentage points. If you use Kelly with inflated probability estimates, you will over-stake and bust your bankroll faster than a flat stake would. Start by tracking your bets honestly for 30 days before trusting any Kelly output.
Responsible play
Calculators compute returns, not outcomes. Every bet can lose. If betting is stressing you out or costing more than you can afford, please stop. Get help.