Tool
Kelly position sizing
Given an entry, stop, target, and your win-probability estimate, compute the bet size that maximizes long-run geometric growth. Half-Kelly and Quarter-Kelly are practical defaults — full Kelly is mathematically optimal but volatile in real life.
Kelly assumes you know p (win probability) and b (odds). You don’t. Most retail traders overestimate p systematically. Use Half-Kelly or Quarter-Kelly to compensate, or treat the output as an upper bound rather than a recommended size. Not investment advice.
Trade setup
Trade math
Reward / share
$20.00
Risk / share
$5.00
Risk / reward (b)
4.00:1
Favorable
Expected value
+175.0%
per $1 risked
Position sizing
Full Kelly (+43.8%) exceeds your safety cap of +10%. Sizes shown are capped.
Full Kelly+43.75% fraction · +10.0% of capital
Position size
$10,000.00
Shares
100
Max loss (at stop)
$500.00
Maximum-growth size. High variance — most practitioners avoid.
Half Kelly+21.88% fraction · +10.0% of capital
Position size
$10,000.00
Shares
100
Max loss (at stop)
$500.00
Common practical default — ~75% of Kelly's growth at ~50% of variance.
Quarter Kelly+10.94% fraction · +10.0% of capital
Position size
$10,000.00
Shares
100
Max loss (at stop)
$500.00
Conservative — for high-uncertainty estimates of p and b.
Sensitivity to win probability
p -10%
+31.3%
$10,000.00
p -5%
+37.5%
$10,000.00
p +0%
+43.8%
$10,000.00
p +5%
+50.0%
$10,000.00
p +10%
+56.3%
$10,000.00
How much your sizing depends on your win-probability estimate. If a 5-percentage-point error halves your size, your edge is fragile.