Position sizing is one of the first things new traders learn, and it is also a great beginner project for practising Python. In this post, we will build a small risk calculator that tells you how large a trade should be so that a single loss stays within a fixed percentage of your account.

The idea

Before taking a trade, a trader decides how much of the account they are willing to lose if the trade fails. A common rule is to risk only 1% to 2% per trade. The position size then depends on three things:

  • Account balance
  • Risk percentage
  • Stop loss distance (in pips for forex)

The formula is simple:

risk amount = balance x risk percent / 100 lot size = risk amount / (stop loss pips x pip value per lot)

Step 1: Position size function 

def position_size(balance, risk_percent, stop_loss_pips, pip_value_per_lot=10): """Return lot size so a stop loss hit risks only risk_percent of the balance.""" if stop_loss_pips <= 0: raise ValueError("Stop loss must be greater than zero") risk_amount = balance * risk_percent / 100 lots = risk_amount / (stop_loss_pips * pip_value_per_lot) return round(lots, 2)

For a standard lot on many USD-quoted pairs, one pip is worth about 10 USD, so that is the default here. Always check the real pip value for your pair and broker.

Step 2: Risk-reward function 

def risk_reward_ratio(stop_loss_pips, take_profit_pips): return round(take_profit_pips / stop_loss_pips, 2)

Step 3: Try it out 

balance = 1000 risk_percent = 1 stop_loss = 20 take_profit = 40 lots = position_size(balance, risk_percent, stop_loss) rr = risk_reward_ratio(stop_loss, take_profit) print(f"Risk amount: {balance * risk_percent / 100} USD") print(f"Position size: {lots} lots") print(f"Risk-reward ratio: 1:{rr}")

Output:

Risk amount: 10.0 USD Position size: 0.05 lots Risk-reward ratio: 1:2.0

With a 1,000 USD account risking 1%, the maximum loss on this trade is 10 USD, no matter how confident the trader feels.

Ideas to extend the project

  • Take user input with input() instead of hard-coded values
  • Add a warning if the risk percentage is above 2%
  • Build a small command line tool or a Flask web app
  • Log every trade into a CSV file to create a trading journal

Why this matters

The code is easy, but the habit behind it is powerful. Most beginners lose money not because their analysis is bad, but because they size trades emotionally. Turning the rule into a function removes guesswork.

If you want to understand the trading side behind this, such as stop loss placement, risk-reward and charts, you can explore the beginner-focused material at ELY8FX Academy, formed by Ashif KA, a financial analyst and trader with ten years of experience.

Disclaimer: Trading involves risk and you can lose money. This post is for educational purposes only and is not financial advice.