Position Sizing Calculator Guide
How to calculate position size based on account risk percentage and stop distance.
⏱️ ~6 min read
Key Takeaways
- Position size determines how many shares to buy, based on how much you're willing to risk and where your stop-loss is set
- The basic formula is: Position Size (shares) = (Account Size × Risk %) ÷ (Entry Price − Stop Price)
- Position sizing keeps the dollar risk on each trade consistent, even as stop distances vary between trades
- Position size and conviction are sometimes confused — position sizing here refers primarily to risk-based sizing, not a measure of how 'confident' a trader is in a trade
What position sizing answers
Position sizing answers the question: given how much I'm willing to risk on this trade, and where I plan to place my stop-loss, how many shares should I buy? It connects the risk management principle of risking a small percentage of an account per trade (covered in the Risk Management lesson) to a concrete number of shares for a specific trade.
The basic formula
Position Size (in shares) = (Account Size × Risk Percentage) ÷ (Entry Price − Stop-Loss Price). The numerator represents the total dollar amount being risked on the trade; the denominator represents the dollar risk per share (the distance between the entry price and the stop-loss price). Dividing the two gives the number of shares such that, if the stop is triggered, the total dollar loss equals the intended risk amount.
Example: Calculating position size
Account size: $20,000. Risk percentage: 1% ($200). Entry price: $50. Stop-loss price: $47 (a $3 per-share risk).
Position Size = $200 ÷ $3 ≈ 66 shares (rounding down).
Position Value = 66 shares × $50 = $3,300 (16.5% of the account) — even though the dollar risk is only 1% of the account ($200, assuming the stop is hit exactly).
Wider stops mean smaller positions (for the same risk)
Because the formula divides the risk amount by the per-share risk (entry minus stop), a wider stop distance results in a smaller position size for the same dollar risk, and a tighter stop distance allows for a larger position size for the same dollar risk. This means two trades with very different stop distances can carry the same dollar risk, despite having very different position sizes and position values.
Example: Same risk, different stop distances
Same $200 risk amount, but a different stock with entry at $50 and stop at $45 (a $5 per-share risk).
Position Size = $200 ÷ $5 = 40 shares — Position Value = 40 × $50 = $2,000 (10% of the account).
Compared to the earlier example (66 shares, $3,300, with a $3 stop distance), this trade has a smaller position size and dollar value, but the same $200 dollar risk if the stop is triggered.
Position sizing isn't the same as 'conviction'
It's a common point of confusion to think of position size as a reflection of how confident a trader is in a particular trade — 'I'm very confident, so I'll buy a big position.' The risk-based position sizing approach described here instead ties position size primarily to the stop distance and the predetermined risk percentage, with the goal of keeping dollar risk consistent across trades regardless of how 'confident' the trader feels (since confidence doesn't change the actual probability of an outcome, and overconfidence is a commonly cited factor in outsized losses — see the Trading Psychology lesson).
Practical considerations
In practice, position sizes calculated this way are often rounded down to whole shares (since fractional shares may not be available depending on the broker and security), and traders may also apply additional limits — such as a maximum position size as a percentage of the account, regardless of what the risk-based formula produces — to avoid excessive concentration in any single position even when a stop is very tight.
Frequently Asked Questions
What if the calculated position size is larger than I can afford?+
This can happen, particularly with tight stops on higher-priced securities — in that case, the trade may need to be skipped, the stop widened (which reduces position size for the same risk), or the risk percentage reduced for that trade. Forcing a trade that doesn't fit within account constraints undermines the purpose of position sizing.
Does position sizing apply to options or other instruments?+
The same underlying principle — sizing a position so that a defined adverse outcome corresponds to a predetermined acceptable loss — applies broadly, though the specific calculations differ for instruments with different risk profiles (like options, which can have non-linear payoffs) compared to simply buying shares of stock.
Should position size ever exceed 100% of my account?+
Without using leverage (borrowed money, such as margin), position size is limited to the amount of capital available. Using margin to take a position larger than account equity introduces additional risk considerations beyond the basic position-sizing formula, including margin calls and interest costs.