Position Sizing Calculator: How to Calculate the Right Trade Size

Why Position Sizing Determines Your Success

Two traders can use the exact same strategy and get opposite results — one profits consistently while the other blows up. The difference? Position sizing. How much you risk per trade matters more than when you enter or exit.

Position sizing is the mathematical foundation of risk management. Master it and you can survive any losing streak. Ignore it and even the best strategy will eventually destroy your account.

The Position Sizing Formula

Position Size = (Account Balance × Risk %) / (Entry Price - Stop Loss Price)

Example: Stocks

Account: $50,000. Risk per trade: 1% ($500). Buying AAPL at $180, stop loss at $175 (risk = $5 per share).

Position size = $500 / $5 = 100 shares ($18,000 position)

Example: Forex

Account: $10,000. Risk per trade: 1% ($100). Buying EURUSD at 1.0850, stop loss at 1.0800 (50 pips risk).

Position size = $100 / (50 pips × $10/pip for a standard lot) = 0.2 standard lots (20,000 units)

Example: Crypto

Account: $5,000. Risk per trade: 2% ($100). Buying BTC at $65,000, stop loss at $63,000 ($2,000 risk per BTC).

Position size = $100 / $2,000 = 0.05 BTC ($3,250 position)

Risk Percentage Guidelines

• Conservative (0.5%) — Best for beginners and prop firm challenges where survival is priority
• Standard (1%) — The industry standard for retail traders. You can survive a 20-loss streak and only be down 20%
• Aggressive (2%) — For experienced traders with proven strategies and high win rates
• Dangerous (3%+) — Not recommended. A 10-loss streak costs 26%+ of your account

Fixed Fractional vs. Fixed Dollar

Fixed fractional (risking a percentage of current balance) automatically adjusts position size as your account grows or shrinks. After losses, you risk less in absolute terms, protecting against ruin. After wins, you risk more, allowing compounding.

Fixed dollar (risking the same dollar amount every trade) is simpler but does not compound and does not reduce risk during drawdowns.

Recommendation: Use fixed fractional (percentage-based) for accounts you plan to grow over time.

Anti-Martingale: Scale Up Winners

When you are on a winning streak and your account grows, your fixed-percentage risk automatically increases your position size. This is anti-martingale — risking more when winning, less when losing. It is the mathematically optimal approach for positive expectancy systems.

Correlation and Portfolio Risk

If you have three open positions, each risking 1%, your total portfolio risk is NOT necessarily 3%. If the positions are correlated (e.g., three long positions in tech stocks), your effective risk is higher because they will likely win or lose together.

Rule of thumb: limit total portfolio risk to 5-6% maximum across all open positions, accounting for correlations.

Practice Position Sizing

When backtesting, always use your actual position sizing rules. A strategy that works with 1% risk might fail at 3% risk due to different drawdown dynamics. backtestic tracks your balance in real time, making it easy to practice proper position sizing as you replay historical markets.