Position sizing turns a trade idea into a controlled financial decision. This guide shows how to calculate a stock position from account equity, a chosen risk percentage, entry price, stop-loss price, fees, and portfolio concentration limits, with a worksheet you can reuse before each trade.
Overview
Position sizing answers a practical question: how many shares can you buy while keeping the planned loss within an amount you can accept? The answer depends less on how confident you feel about a stock and more on the distance between your entry price and your invalidation point.
The core calculation is:
Position size in shares = allowed trade risk ÷ risk per share
For a long trade, risk per share is usually the entry price minus the stop price. For a short trade, it is usually the stop price minus the entry price. The result should be rounded down to a whole share unless your broker supports fractional shares and your trading plan allows them.
A position-sizing calculation is not a guarantee that the trade will lose only the planned amount. Market gaps, fast-moving prices, order execution, spreads, taxes, commissions, and other charges can make the realised result different from the estimate. Treat the formula as a planning limit, not a promise.
How to estimate
Use the following sequence before placing an order:
- Determine account equity. Use the value relevant to your trading plan, such as the current account value or the portion of capital allocated to active trading.
- Choose a risk percentage. Convert the percentage into a dollar amount: account equity multiplied by the selected risk percentage.
- Set the entry and stop prices. The entry may be a planned limit price or an estimated execution price. The stop should reflect where the trade thesis is no longer valid, rather than an arbitrary distance chosen only to increase position size.
- Calculate risk per share. Subtract the stop price from the entry price for a long position. Add an allowance for fees and expected slippage where appropriate.
- Divide allowed risk by risk per share. Round down the result.
- Check the position value and concentration. A trade can fit the loss limit but still represent too large a portion of the portfolio.
A more complete version of the formula is:
Shares = (account equity × risk percentage − fee allowance) ÷ (entry price − stop price + expected slippage per share)
For a short position, replace the price difference with stop price − entry price. If the calculation produces zero or a negative number, the trade does not fit the inputs as written.
Inputs and assumptions
Record these fields in a spreadsheet, calculator, or trading journal:
- Account equity: The capital base used for the calculation.
- Risk percentage: The fraction of equity you are prepared to lose if the stop is reached. There is no universally correct percentage; consistency and affordability matter more than copying another trader’s figure.
- Allowed trade risk: Account equity multiplied by the risk percentage.
- Entry price: The expected fill price. If the market is moving quickly, consider calculating a less favourable entry as a sensitivity check.
- Stop price: The level that invalidates the setup. A stop should account for normal price movement and the stock’s trading characteristics.
- Risk per share: Entry price minus stop price for a long trade, plus any estimated slippage or per-share cost.
- Share quantity: The risk-based result, rounded down.
- Maximum planned loss: Share quantity multiplied by risk per share, plus estimated fees.
- Position value: Share quantity multiplied by entry price.
- Portfolio concentration: Position value divided by account equity. Compare this with your own maximum allocation rule.
Fee assumptions should be current for your broker and account. If trading costs are not known in advance, use a conservative allowance rather than assuming they are zero. Also consider spread and slippage, particularly for thinly traded stocks, premarket movers, after-hours stock movers, and trades around earnings or major economic announcements.
Risk limits should be considered across related positions. Several technology stocks, for example, may respond to the same market signal, interest-rate change, or earnings news. Each trade may pass an individual position-size test while the combined exposure remains larger than intended.
Worked examples
Example 1: A long stock trade
Assume the following planning inputs:
- Account equity: $20,000
- Risk percentage: 0.5%
- Entry price: $50
- Stop price: $47.50
- Fee and slippage allowance: $0.10 per share
Allowed trade risk is $20,000 × 0.005, or $100. Price risk is $50 − $47.50, or $2.50 per share. Adding the allowance produces estimated risk of $2.60 per share.
Shares = $100 ÷ $2.60 = 38.46. Rounding down gives 38 shares. The estimated position value is $1,900, and the estimated planned loss is $38 × $2.60, or $98.80. The position value is 9.5% of the account, so it should also be compared with the investor’s concentration limit.
Example 2: A wider stop means a smaller position
Keep the same account and risk limit, but use a $44 stop with a $50 entry. Price risk is now $6 per share. With the same $0.10 allowance, estimated risk is $6.10 per share.
Shares = $100 ÷ $6.10 = 16.39. Rounding down gives 16 shares. The wider stop reduces the share count even though the account risk remains approximately the same. This is the central relationship in stop-loss position sizing: a wider invalidation distance generally requires fewer shares.
Example 3: The concentration check changes the answer
Suppose a calculation permits 100 shares at a $40 entry, creating a $4,000 position in a $10,000 account. The risk limit may be acceptable, but the position represents 40% of the account. If your written plan caps any single position at 20%, the maximum position value is $2,000, or 50 shares at the assumed entry price. The final order size should be the lower of the risk-based quantity and the concentration-based quantity.
When to recalculate
Recalculate before every new trade rather than reusing an old share count. Update the worksheet when account equity changes materially, when the entry or stop moves, or when fees, spreads, and execution conditions differ from your assumptions.
Revisit it especially before earnings, major economic releases, or other events that can increase gap risk. A stop order may not fill at its exact trigger during a sharp move, so consider reducing size or declining the trade when the planned loss no longer reflects a plausible execution outcome. The same caution applies to stocks with limited liquidity and automated strategies that can submit multiple orders during a fast market.
Use this repeatable checklist:
- Write down current equity and the maximum risk percentage.
- Define the entry, stop, and estimated per-share costs.
- Calculate risk-based shares and round down.
- Calculate position value and compare it with concentration limits.
- Add existing positions with similar market or sector exposure.
- Record the final quantity, assumptions, and reason for the trade in your journal.
Position sizing works best when it is part of a complete plan: a clear setup, a logical stop, an exit process, and a review of actual execution. It can support discretionary trading, stock signals, or a trading bot, but automation does not remove the need to set risk limits and test how orders behave in fast markets.
