Kelly Sizing

The fraction of bankroll a Kelly bettor stakes given a known edge. Fractional Kelly trades EV for variance.

b = decimal - 1 = 1.000
0.5 = half-Kelly
Full Kelly
10.00%
Stake: $1000.00
Fractional Kelly (×0.5)
5.00%
Stake: $500.00

Math

f* = (b · p − q) / b where b is net decimal odds, p is win probability, q = 1 − p.

Negative f* means no edge — the calculator clamps stake to 0. Full Kelly maximizes expected log-bankroll growth but is volatile; most practitioners run fractional Kelly (0.25–0.5×) to cut variance.

Kelly assumes you know the edge. If you don't, you don't have a Kelly problem — you have a measurement problem.