Position Sizing Math: Risk Per Trade Formulas That Pass Evaluations

Position sizing is the single highest-leverage skill in prop trading. Strategy decides your edge; sizing decides whether the account survives long enough for the edge to express itself.

The core formula

Lot size = (account balance × risk %) ÷ (stop loss in pips × pip value per lot). On a $100,000 account risking 0.5% ($500) with a 40 pip stop where a pip is worth $10 per lot: $500 ÷ $400 = 1.25 lots. Change any input and the size changes proportionally: the same dollar risk on an 80 pip stop is 0.62 lots. Compute your own numbers with the position size calculator.

Pip values and point values

For USD-quoted majors (EURUSD, GBPUSD), one pip is $10 per standard lot. For pairs where USD is the base (USDJPY), pip value floats with the rate and must be computed. For indices and gold, use the dollar value per point per lot from your broker’s contract specifications. Getting this wrong by 2x doubles your actual risk silently, which is how a “1% risk” plan fails an evaluation at trade five.

Which risk percentage passes

With a 10% total drawdown and 5% daily limit, per-trade risk of 0.25-0.5% means it takes 20-40 consecutive losing trades to hit the total limit, which no realistic strategy produces. At 2% risk, five losing trades is a breach, and losing streaks of five happen to every strategy. The risk of ruin calculator shows the exact probability for your statistics.

The four classic sizing mistakes

One: sizing from the stop you want rather than the stop your chart needs (moving stops to “afford” size). Two: rounding lot sizes up. Three: sizing in lots from a previous account balance after a payout reset. Four: increasing size after losses to “get it back”, which is the fastest known route past a daily loss limit. Every one of these is a formula error, not a character flaw, and every one is checkable before the trade.

Educational content only, not financial advice. See our disclaimer.