Position Sizing
The process of determining how many shares, contracts, or units to trade based on account size, risk tolerance, and the distance to a stop-loss level.
What Position Sizing Is
Position sizing is the process of determining how large each trade should be relative to your account, based on account size, risk tolerance, and stop-loss distance. According to Van Tharp's research in *Trade Your Way to Financial Freedom* (McGraw-Hill, 2007), position sizing accounts for approximately 90% of the variation in portfolio returns across traders using identical entry signals. It is the bridge between a strategy's theoretical edge and an account's ability to survive the variance that comes with it.
The Core Formula
Position Size = (Account × Risk Percentage) / Dollar Risk Per Share
Where Dollar Risk Per Share = Entry Price - Stop Loss Price.
Example: $100,000 account, 1.5% risk, buying at $50 with a $47 stop.
Position Size = ($100,000 × 0.015) / $3 = 500 shares.
Why It Matters More Than Stock Picking
Two traders with identical trade signals will have completely different results based on position sizing alone:
- Trader A risks 5% per trade → faces a 35% drawdown probability over 200 trades
- Trader B risks 1.5% per trade → faces a 4% drawdown probability with the same trades
Position sizing is the single most important factor in trading survival.
Common Methods
| Method | Description | Best For |
|--------|-------------|----------|
| Fixed Fractional | Risk same % of current account per trade | Most traders — simple and self-adjusting |
| Fixed Dollar | Risk same dollar amount per trade | Beginners — simpler but doesn't adapt |
| Volatility-Based | Adjust size by asset ATR | Active traders across multiple instruments |
| Kelly Criterion | Mathematically optimal based on edge | Experienced traders with reliable statistics |
Finding Your Optimal Size
The right position size depends on your specific strategy statistics. Use Monte Carlo simulation to test different risk percentages and find the level where survival probability stays above 90–95%.
Try the Risk of Ruin Calculator to compare position sizes. RangeSight lets you model this with real market data.
Key Takeaway
The process of determining how many shares, contracts, or units to trade based on account size, risk tolerance, and the distance to a stop-loss level. Use RangeSight Studio's Monte Carlo simulator to model position sizing with 10,000 simulated paths and real market volatility.
This explains the concept.
But how position sizing affects your specific strategy depends on your parameters, position sizing, and market conditions.
Test your strategy to see how consistent it actually is → Run survival test
See the full distribution with real market data
10,000 Monte Carlo paths. P5/P50/P95 scenarios. Real ticker volatility.
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The 2% Rule vs. Actual Ruin Probability
Everyone knows the 2% rule. Few know whether it actually works for their specific strategy. Here's how to find out.
Why Most Retail Traders Fail (A Statistical Explanation)
The 90% failure rate in retail trading isn't about bad picks. It's about position sizing and ruin probability.
Related comparisons
Risk of Ruin vs. Kelly Criterion: Which Should Guide Your Position Sizing?
Risk of ruin tells you whether you'll survive. Kelly tells you how fast you could grow. Understanding both is essential for sizing positions correctly.
Fixed Fractional vs. Kelly Sizing: Which Position Sizing Method Wins?
Fixed fractional is the safe default. Kelly is the theoretical optimum. Most traders should use something in between.