run position-size
Position Size & Kelly Calculator
Risk-based position sizing plus the Kelly criterion — see what over-betting does to a simulated account.
New to this? Start here
Position sizing answers a different question than 'is this a good trade' — it answers how much of the account to put on any single trade so that a losing streak can't wipe the account out. The Kelly formula turns a win rate and a payoff ratio into a mathematical bet-size ceiling; the simulator below shows what happens to an account betting that full amount versus smaller or larger fractions of it, using the exact same sequence of wins and losses for each.Recommended shares
100
$5,000 position, risking $500
Kelly fraction
8.3%
of account, per the formula
Half-Kelly
4.2%
the common "safer" fraction
Betting 16.7% per trade still ended ahead at $52,039, but only after much wilder swings along the way; betting 4.2% per trade grew more steadily — position size, not picks, decided who survived.
Half KellyFull Kelly2x Kelly
View as table
| Trade # | Half Kelly | Full Kelly | 2x Kelly |
|---|---|---|---|
| 0 | $10,000 | $10,000 | $10,000 |
| 20 | $8,791 | $7,359 | $4,457 |
| 40 | $11,676 | $12,286 | $10,055 |
| 60 | $13,988 | $16,712 | $15,124 |
| 80 | $13,632 | $15,094 | $10,111 |
| 100 | $14,730 | $16,730 | $10,139 |
| 120 | $19,565 | $27,930 | $22,875 |
| 140 | $21,141 | $30,958 | $22,938 |
| 160 | $34,516 | $77,846 | $116,444 |
| 180 | $41,350 | $105,895 | $175,149 |
| 200 | $32,785 | $63,496 | $52,039 |
Methodology & assumptions
- Risk-based sizing assumes the full distance from entry to stop is the realized loss if the stop is hit.
- The Kelly formula assumes a fixed, known win rate and payoff ratio — real trading edges drift and are never known in advance.
- The simulated equity curves assume independent trades and pure geometric compounding of a fixed bet fraction; they use one representative random sequence per fraction, not a distribution of outcomes.
- This is a math and risk-of-ruin illustration, not a recommendation to size any real position or bet at any fraction of Kelly.