Risk-Based Lot Size Calculator Script with On-Chart Panel - script for MetaTrader 5

This script answers one question before every trade: how many lots should I open so that a stop loss hit loses exactly the percentage of my account that I decided to risk?

You drop it on a chart, set the risk percent and the stop loss distance in points, and it shows the lot size in a panel on the chart. The calculation uses OrderCalcProfit(), so the tick value, the contract size and the account currency are handled by the platform. It works on forex pairs, gold, indices and crypto without any symbol specific setting.

How the lot is calculated

The script takes the risk base (equity, balance or free margin) and multiplies it by the risk percent to get the amount of money you accept to lose. It then asks the platform how much 1 lot would lose if the price moves from the entry price to the stop loss, and divides the two numbers. The result is rounded down to the volume step of the symbol and kept between its minimum and maximum lot, so the real risk never goes above what you asked for.

If the exact lot is below the broker minimum, the panel shows a warning, because the minimum lot risks more than you asked. A second warning appears when the free margin is not enough for the calculated lot.

Input parameters

• Risk per trade, % of the base below - the percent of the risk base you accept to lose if the stop loss is hit (default 1.0).
• Risk base - equity, balance or free margin.
• Direction - true for a buy, false for a sell. The entry price is the current Ask for a buy and the current Bid for a sell.
• Stop loss distance, in points - distance between the entry price and the stop loss (default 200).
• Take profit as a multiple of risk - optional. Enter 2.0 to see the take profit level at 2R. Leave 0 to hide it.

How to use

1. Copy RiskLotCalculator.mq5 to the MQL5\Scripts folder and compile it in MetaEditor.
2. Drag the script from the Navigator onto the chart of the symbol you want to trade.
3. Set the inputs and press OK.
4. Read the lot size, the money at risk and the margin needed in the panel. Press Close to remove the panel.

The script does not open, modify or close any trade. It only reads the account and symbol information and draws the panel.