A position size calculator determines the exact number of lots to trade based on three inputs: account balance, risk percentage, and stop loss distance. The formula is: Lots = (Account × Risk%) ÷ (Stop Loss in Pips × Pip Value). PropWise's calculator adds prop firm-specific features: presets for 12 firms that automatically apply correct drawdown limits, warnings when position size would breach daily DD, and instant calculations for any currency pair.