Stock Position Size Risk Calculator
Calculate exact share purchase quantities based on account portfolio equity, dollar risk percentage, and share stop loss price.
Precision Position Sizing for Equity Investors and Traders
When purchasing individual stocks or ETFs, calculating share quantities based on arbitrary dollar amounts or unconsidered capital fractions leads to severe portfolio volatility. Professional equity traders use a disciplined risk-budgeting model: determining how much total account capital to risk on a trade (e.g., 1.0% of portfolio equity) and calculating exact share volume based on share stop-loss distance.
The Share Quantity Sizing Equation
Number of Shares to Buy = Total Dollar Risk Allowance ÷ (Entry Price per Share − Stop Loss Price per Share). For example, if your total allowed risk is $500 on a stock trading at $150 with a stop loss at $140 ($10 per share risk), you should purchase exactly 50 shares ($7,500 position value). If the stock triggers your stop loss at $140, your realized dollar loss is exactly $500 (1.0% of a $50,000 account).
How to Use This Stock Position Size Calculator
Enter total portfolio value ($), risk percentage allowance (or fixed dollar risk), target stock entry price, and share stop-loss price. The calculator computes exact number of shares to purchase, total position cost basis, position size as a % of portfolio, and potential dollar loss.
Avoiding Portfolio Over-Concentration Hazards
A common mistake among stock traders is opening tight stop losses on highly volatile mid-cap stocks, which results in calculated share quantities that exceed 50% of total portfolio equity. To maintain diversification, set maximum position capital caps (e.g., capping any single stock position at 20% of total portfolio value) alongside stop-loss sizing.
Frequently Asked Questions
How much portfolio equity should I risk per stock trade?
Financial advisors recommend risking between 0.5% and 2.0% of total portfolio equity on any single stock trade.
What is a maximum position concentration cap?
A position cap limits the total dollar value allocated to a single stock (e.g. max 15% to 20% of total portfolio) to prevent single-stock crash risk.
Can I use this calculator for fractional share purchases?
Yes, if your brokerage supports fractional shares, you can buy exact decimal share quantities output by the calculator.