How to Implement Dynamic Lot Sizing Based on Account Balance in Mql4

To calculate the lot size based on a percentage of your account balance (risk management), you need to know the Stop Loss distance in pips. Without a Stop Loss, risk cannot be mathematically defined because the potential loss is infinite.

Here is a robust, reusable MQL4 function that calculates the lot size based on 2% risk (or any other percentage) and a specified Stop Loss.

MQL4 Code

//+------------------------------------------------------------------+
//|                                              RiskCalculator.mq4  |
//|                                                                  |
//| Description: Calculates lot size based on % risk of balance      |
//+------------------------------------------------------------------+
#property strict
#property show_inputs

//--- Input Parameters
input double InpRiskPercent = 2.0;  // Risk Percent (e.g., 2.0 for 2%)
input int    InpStopLossPips = 50;  // Stop Loss distance in Pips

//+------------------------------------------------------------------+
//| Script program start function                                    |
//+------------------------------------------------------------------+
void OnStart()
{
   // Calculate the lot size
   double calculatedLots = GetLotSize(InpRiskPercent, InpStopLossPips);
   
   // Output the result
   Print("-------------------------------------------------------------");
   Print("Account Balance: ", DoubleToString(AccountBalance(), 2));
   Print("Risk Percentage: ", DoubleToString(InpRiskPercent, 2), "%");
   Print("Stop Loss: ", InpStopLossPips, " pips");
   Print("Calculated Lot Size: ", DoubleToString(calculatedLots, 2));
   Print("-------------------------------------------------------------");
}

//+------------------------------------------------------------------+
//| Function to calculate lot size based on risk percentage          |
//+------------------------------------------------------------------+
double GetLotSize(double riskPerc, int slPips)
{
   string symbol = Symbol();
   
   // 1. Calculate the amount of money to risk
   double riskMoney = AccountBalance() * (riskPerc / 100.0);
   
   // 2. Get the value of 1 pip for 1 standard lot in the deposit currency
   double tickValue = MarketInfo(symbol, MODE_TICKVALUE);
   double tickSize  = MarketInfo(symbol, MODE_TICKSIZE);
   double point     = MarketInfo(symbol, MODE_POINT);
   
   // Adjust for 3 and 5 digit brokers (Points vs Pips)
   // If a pip is 0.0001, but point is 0.00001, we need to adjust
   double pointsPerPip = 1.0;
   if(Digits == 3 || Digits == 5)
   {
      pointsPerPip = 10.0;
   }
   
   // Calculate the monetary value of the Stop Loss per 1 lot
   // Formula: SL(pips) * PointsPerPip * TickValue
   double lossPerLot = slPips * pointsPerPip * tickValue;
   
   // Safety check to prevent division by zero
   if(lossPerLot == 0)
   {
      Print("Error: Tick value is zero. Cannot calculate lots.");
      return(0.0);
   }
   
   // 3. Calculate raw lot size
   double lots = riskMoney / lossPerLot;
   
   // 4. Normalize the lot size to the broker's step requirements
   double minLot  = MarketInfo(symbol, MODE_MINLOT);
   double maxLot  = MarketInfo(symbol, MODE_MAXLOT);
   double lotStep = MarketInfo(symbol, MODE_LOTSTEP);
   
   // Round down to the nearest lot step
   lots = MathFloor(lots / lotStep) * lotStep;
   
   // 5. Ensure lots are within broker limits
   if(lots < minLot) lots = minLot; // Or return 0 if you don't want to trade below min
   if(lots > maxLot) lots = maxLot;
   
   return(lots);
}

How It Works

1. Risk Calculation: It calculates the monetary value of 2% of your AccountBalance(). For example, if you have $10,000, the risk money is $200.
2. Tick Value Retrieval: It uses MarketInfo(Symbol(), MODE_TICKVALUE) to find out how much 1 tick is worth in your account currency. This handles currency conversion automatically (e.g., trading EURUSD on a GBP account).
3. Pip Adjustment: It detects if your broker uses 5-digit pricing (where 1 pip = 10 points) or 4-digit pricing to ensure the math is accurate.
4. Lot Calculation: It divides the Risk Money by the Cost of the Stop Loss per 1 Lot.
5. Normalization: It rounds the result down to the nearest MODE_LOTSTEP (e.g., 0.01 or 0.10) and ensures the size is within the minimum and maximum allowed lots for that symbol.

How to Use

1. Copy the code into MetaEditor.
2. Save it as a Script (e.g., RiskCalc.mq4).
3. Drag the script onto a chart.
4. A window will pop up asking for the InpStopLossPips. Enter your intended Stop Loss distance (e.g., 20, 50, 100).
5. Check the Experts tab (at the bottom of MT4) to see the calculated lot size printed in the log.

Q&A

Q: Why do I need to provide a Stop Loss distance?
A: Risk is a function of distance. Risking $200 with a 10-pip Stop Loss requires a much larger lot size than risking $200 with a 100-pip Stop Loss. Without knowing the distance, the lot size cannot be calculated.

Q: Does this work for Indices (e.g., US30) and Gold (XAUUSD)?
A: Yes, MODE_TICKVALUE handles different asset classes. However, ensure you input the Stop Loss in "pips" as defined by your broker for that specific symbol (sometimes 1 "pip" on Gold is 10 cents, sometimes 1 dollar).

Q: What happens if the calculated lot size is smaller than the broker's minimum?
A: The code includes a check (if(lots < minLot)). In this version, it defaults to the minimum lot size (usually 0.01) so the trade can still be taken, though the risk will technically be higher than 2% in that specific edge case.