Prop firm challenge calculator: a position size for your risk limit
Most challenge failures are not a bad strategy - they are one bad day at the wrong size. The good news: breach risk is far more predictable than profit.
Why most traders fail challenges
Public figures from prop firms show only a minority pass and fewer get paid (for example, one large futures firm reported about 16.8% of traders passing and about a third of funded traders receiving a payout). The usual killer is the daily loss limit: a normal losing streak, traded too big, on one day.
Step 1: size from your own trades
Take your last 60-90 days of trades. The size that keeps your chosen risk is the one where the chance of breaking the daily or total drawdown rule within the challenge stays at about 10% or less. That chance can be estimated from your own trading days and how volatile your trades are - on simulated accounts built from real market prices, our breach model was within about 4 percentage points of what happened on 68,000 forecasts across 39 markets it had never seen.
Step 2: stop for the day at 60% of the limit
If the daily limit is 5%, stop trading for the day at -3%. In our tests on simulated accounts, this 'brake' alone cut breaches sharply, because most breaches are one day that keeps going.
Step 3: adjust size every week
Re-set size each week from your current balance and the days left: a little bigger when you are ahead, smaller when you are behind, never bigger to catch up. On simulated accounts on 146 markets the model never saw, this weekly 'thermostat' passed the simulated challenges 11.4% of the time versus 4.9% for a fixed size, at the same breach risk.
Quick example
| Rule | Example value |
|---|---|
| Account | $100,000 |
| Daily loss limit | 5% ($5,000) → stop for the day at $3,000 |
| Max drawdown | 10% |
| Your trades | 3 a day, typical move 1% |
| Size | the largest size with ≤10% breach chance, re-checked every Monday |
What we never claim: pass odds. Passing depends on your edge continuing, which no backtest can promise. Breach odds, size and the daily stop are what can be measured.
Frequently asked questions
- How much risk per trade fits a prop challenge?
- It depends on your own trade sizes and how many trades you take a day. Many traders keep risk well under 1% per trade; the right number comes from your own history and the firm's rules.
- Why do I keep failing the daily drawdown?
- Usually a normal losing streak at too large a size, or continuing after the first losses. A hard stop at about 60% of the daily limit prevents most of these.
- Should I size up to hit the target faster?
- No. Sizing up raises breach risk much faster than pass chance. Adjust size weekly by the rules instead.
- Can you calculate my size for my risk limit?
- Yes - send your trade list and the challenge rules (daily loss %, max drawdown %, target %, days) and you get breach odds, the size that keeps your chosen risk and a daily stop.
Want your own strategy checked?
Send your trade list. You get a plain-English PDF: verdict, 10 checks, a position size for your chosen risk, a daily stop and a 30-trade paper-test plan. Free in the beta: run it yourself in the Workspace.