Prop Firm Position Sizing: The Only Maths That Matters
Your risk per trade isn't a preference under an evaluation — it's derived from the drawdown rule and the losing streak you have to survive.
In a personal account, risk per trade is a preference. Under an evaluation it's an output — determined entirely by the drawdown allowance and the length of losing streak you need to survive.
Get this number right and passing becomes a waiting game. Get it wrong and no amount of setup quality saves the account.
Start from the streak, not the comfort level
A strategy that wins 50% of the time will, over 100 trades, very often produce a run of 6 or 7 consecutive losses. Over 200 trades, 8 or 9 is unremarkable. These aren't disasters — they're the normal shape of randomness.
So the question isn't "what risk feels safe?" It's: how many consecutive losses must I survive with room left to trade?
A defensible answer is 12 to 20. Below 10 you're one ordinary streak from failure. Above 25 the profit target usually becomes unreachable inside the time limit.
The formula
Risk per trade ($) = Usable drawdown ($) ÷ Streak to surviveWorked on a $100,000 account with a 5% maximum drawdown:
| Streak to survive | Risk per trade | As % of account |
|---|---|---|
| 10 | $500 | 0.50% |
| 15 | $333 | 0.33% |
| 20 | $250 | 0.25% |
Then size the position from that dollar figure and your stop distance:
Position size = Risk per trade ($) ÷ Stop distance (per unit)The position size calculator and the stop loss and take profit calculator do both steps; the prop firm risk calculator adds the evaluation rules on top.
Reduce usable drawdown before you divide
The headline allowance is not what you actually get to use. Subtract:
On a $5,000 allowance, a realistic usable figure is closer to $4,000. Divide that.
Check the target is still reachable
Small risk is only correct if it can still get you to the target in time. Test it:
Trades needed ≈ Profit target ÷ (Risk per trade × Expectancy in R)With a $4,000 target, $250 risk and 0.35R expectancy: 4,000 ÷ (250 × 0.35) ≈ 46 trades. If your setup appears three times a week, that's about fifteen weeks — fine for an unlimited-time programme, impossible on a 30-day one.
When the arithmetic doesn't fit, change the programme or the strategy. Never the risk.
Fixed fractional vs fixed dollar
Under a trailing drawdown, fixed dollar is close to mandatory — the floor moves with your balance, so scaling risk with balance leaves your streak tolerance unchanged while the consequences grow.
Scaling: earn it, don't assume it
If you do increase size, tie it to something measurable, not to how you feel:
Written like that, scaling is a rule. Left unwritten, it's just the confidence that shows up before the streak does.
Common sizing mistakes in evaluations
The summary
Take the allowance, subtract costs, slippage and a buffer, divide by 15, and that's your risk per trade. Then check the target is reachable at that size. If it isn't, the problem is the programme or the strategy — and either of those is cheaper to change than an account.
SimpleAlgo does not sell funded accounts, does not operate an evaluation programme, and has no affiliate relationship with any proprietary trading firm. Nothing here is financial advice, and trading carries a substantial risk of loss. Always read the current rulebook of any programme you join — terms change frequently.
Written by
Nikolas MetreveliFounder of SimpleAlgo
Nikolas wrote his first real code at 12 and freelanced software tools and websites for small businesses before finding the markets at 15. He builds and maintains every SimpleAlgo indicator and writes these guides from hands-on use of the tools.
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