Position sizing is a risk-management calculation designed to determine how many units of an instrument to trade so that a defined stop-loss distance results in a specific dollar loss — typically expressed as a fixed percentage of account equity.
The core formula: position size = (account equity × risk %) ÷ (stop distance in pips × pip value per lot). Every variable matters. Changing the stop distance without recalculating lot size changes the actual risk taken, even if the percentage feels the same.
Why position sizing determines evaluation outcomes
In a funded evaluation, position sizing interacts with two hard limits simultaneously: the daily loss limit and the maximum drawdown. A correctly sized position keeps both limits intact even if the trade hits its full stop. An oversized position can breach the daily limit on a single loss, ending the evaluation regardless of overall P&L.
The practical constraint is tighter than most traders expect. On a $10,000 account with a 5% daily loss limit ($500) and a 10% maximum drawdown ($1,000), risking 2% per trade ($200) leaves room for two full losing trades before the daily limit is hit. Risking 6% ($600) breaches the daily limit on the first loss.
Evaluation-specific sizing rules
Most programs do not set a maximum position size directly — they set risk limits, and position sizing is the trader’s responsibility. The two constraints that should cap every calculation:
- Daily loss limit: Total risk open at any one time should not exceed the daily limit, accounting for correlation between open positions.
- Maximum drawdown: Cumulative risk across a session should leave enough buffer that a run of losses cannot consume the remaining drawdown room in one day.
Under a trailing drawdown model, available drawdown room shrinks as the account grows — which means the correct position size at a $10,500 equity level is different from the correct size at $10,000 starting balance, even if the percentage feels the same. Recalculate from the current distance to the trailing floor, not from account equity.
Common sizing errors in evaluations
- Sizing from balance instead of equity — open floating losses can push equity below the drawdown floor while balance looks fine.
- Ignoring correlated positions — two trades in the same direction on correlated pairs effectively double the position.
- Fixed lot sizing — a fixed 0.5 lot is a different percentage risk at $10,000 equity versus $11,500 equity.
- Forgetting spread and slippage — actual loss on a stop-out is stop distance plus spread, not stop distance alone.
Run the numbers for your own setup using the position size calculator — it converts account size, risk percentage, stop distance, and pip value into an exact lot size, and shows how many consecutive losses your daily limit can absorb at that setting.
For the limits that position sizing must respect, see daily loss limit, trailing drawdown, static drawdown, and equity vs balance. FundedProp’s exact limits and worked examples publish in the rulebook before any evaluation opens for purchase.
FundedProp provides simulated trading evaluations. All accounts are demo accounts; no real capital is traded by participants. This is not investment advice.