Kelly criterion calculator
Kelly fraction of capital per trade and the growth it gives, full or fractional.
Example inputs, not market prices: change them to your trade.
Result
How it is computed
f* = p − (1 − p) ÷ b
p is the win rate and b the payoff ratio. Kelly gives the share of capital that maximises long-run growth; most traders use a fraction of it because the inputs are estimates.
Worked example
Inputs
- Win rate (%)
- 55
- Average win / average loss
- 1.2
- Share of Kelly used
- 0.5
Result
Size used
8.75%
- Full Kelly
- 17.5%
- Growth per trade
- +1.3804%
Computed on the server by the same code as the form above (calculator table 2026-10-04.1).
Questions
Why use half Kelly?
It keeps about three quarters of the growth with much smaller swings, and forgives an overestimated edge.
What if Kelly is zero?
The inputs give no edge: the criterion says not to take the trade.
Is Kelly a risk per trade?
It is the share of capital put at risk on the trade, so close to a risk per trade when the stop is the whole loss.
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A calculation, not a forecast. Not investment advice.