Free tool

Position size calculator

Kelly turns a win rate and a payoff into a fraction of capital. Full Kelly is mathematically correct and practically unholdable — the drawdowns it accepts are not ones a human sits through. What the engine actually applies is quarter-Kelly position sizing with a hard 2% risk cap, on every trade it takes.

Sized by the same function the bot calls, so this page cannot disagree with the product. A negative edge returns no position rather than a small one. Nothing here is financial advice.

Your edge

Account capital

USD

The whole account, not the amount you intend to commit.Win rate

Measured over closed trades, not estimated. Ours is published live on the track record page.Reward-to-risk

: 1

Average winner divided by average loser. This is the number that decides what win rate you need.Stop distance

How far the invalidation sits from entry, as a percent of price. This converts risk into a position size.Closed trades behind those figures

trades

Below 30 the engine refuses to size from a measured edge at all.

What to risk

Break-even win rate

At 3:1 you need this just to stand still

25.0%

Full Kelly

Shown only so the haircut below is legible — never stake this

13.33%

Quarter-Kelly, capped at 2%

The cap is binding here, not the maths

2.00%

Capital at risk

What you lose if the stop is hit

$200.00

Position size

Risking that amount with the stop 2% away

$10,000.00

Risk 2.00% of capital — $200.00.

Quarter-Kelly wants more than 2% here, so the hard cap is what is binding. That cap exists because Kelly is exquisitely sensitive to the win probability, and a win rate is always an estimate.

Kelly assumes your inputs are true. It has no view on whether your win rate will hold, and it is unforgiving if you have overestimated it — overstating an edge sizes you up precisely when you should be sizing down. This is the sizing rule our own engine uses; it is not advice about your account, and nothing here is financial advice.

Why a quarter, and where the 2% cap comes from