📐 Kelly Criterion Calculator

The Kelly Criterion calculates the mathematically optimal percentage of your bankroll to bet. Too small and you under-grow; too large and you risk ruin. Enter your edge and let the formula do the work.

Your Bankroll (R)

Total betting funds available

Bookmaker Decimal Odds

Odds offered by SA bookmaker

Your Win Probability (%)

Your honest estimate

Kelly Fraction

Full Kelly = aggressive

Recommended Stake

0% of bankroll

Full Kelly %

—

Your Edge

—

Break-even Prob

—

Expected Return

—

Potential Win

—

Max Drawdown Risk

—

What is the Kelly Criterion?

The Kelly Criterion is a formula developed by mathematician John L. Kelly Jr. in 1956. It calculates the optimal fraction of your bankroll to bet in order to maximise long-term growth while minimising the risk of ruin.

f* = (bp – q) / b

Where:
b = decimal odds – 1 (profit per R1 staked)
p = your estimated win probability
q = 1 – p (lose probability)

A positive Kelly fraction means there's value. A negative fraction means the bookmaker has the edge — don't bet.

Full Kelly vs Half Kelly

Full Kelly maximises long-run growth but leads to extreme bankroll swings — drawdowns of 50%+ are common. Most professional bettors use Half Kelly (bet half the calculated amount) which captures ~75% of the growth with much lower volatility.

For recreational SA bettors, Quarter Kelly is a sensible starting point. Never bet more than the Full Kelly amount — it's mathematically proven to reduce long-term returns.