Kelly Stake
The bet size that maximises long-run growth.
Kelly fraction
Recommended stake
£50.005.00% of bankroll · half-Kelly
- Full Kelly f*
- 10.00%
- Edge / unit
- +10.00%
- Growth / bet
- +0.375%
- Implied prob
- 50.0%
- Break-even odds
- 1.82
- vs full-Kelly growth
- 75%
Same 180 bets · full vs half vs quarter Kelly
Full 2.02×Half 1.78×Quarter 1.41×
Full Kelly grows fastest in theory but swings hard and can crater on a bad run. Half and quarter give up a little growth for far calmer rides — which is why most people bet a fraction.
The Kelly criterion gives the stake that maximises the long-term growth rate of a bankroll. It scales with your edge and shrinks as the odds get shorter: f* = (b·p − q) / b, where b is the odds minus one, p your win probability and q is 1 − p.
Full Kelly is aggressive and assumes your probability estimate is exact. Because it rarely is, most people bet a half or a quarter of the Kelly fraction — giving up a little growth for a large reduction in swings.
FAQ
- What is the Kelly criterion?
- A formula for bet sizing that maximises the expected logarithm of wealth. In practice it produces the fastest sustainable bankroll growth without risking ruin.
- Why use half-Kelly?
- Full Kelly is very sensitive to overestimating your edge, which causes painful drawdowns. Half-Kelly keeps about three-quarters of the growth with far smaller swings.
- What if the calculator returns zero?
- It means the inputs imply no edge (or a negative one) at that price. Kelly says the correct stake is nothing — don't bet.