How stock position sizing works
For a stock purchase with a stop below the entry price, the calculation has two parts: convert your risk percentage into dollars, then divide by the planned loss per share.
- Risk budget (US$) = account size (US$) × risk per trade (%) ÷ 100
- Planned loss per share (US$/share) = entry price − stop-loss price
- Shares = risk budget ÷ planned loss per share
- Position value (US$) = shares × entry price
- Estimated money at risk (US$) = shares × planned loss per share
- Position as % of account = position value ÷ account size × 100
For whole-share trading, round the calculated shares down, not up, to stay within the chosen risk budget under the assumed entry and stop prices. The money-at-risk figure describes the planned price difference; it is not a maximum possible loss. A stop order may fill at a different price, particularly if a stock gaps or trading is thin. Fees and other trading costs can add to the loss.
The formula above assumes a long position. For a short position with a stop above the entry, use stop-loss price − entry price as the planned loss per share. If entry and stop are equal, the calculation is undefined. If the stop is on the wrong side of the entry, check the inputs rather than treating a negative share count as meaningful.
Worked example: calculating shares from a risk budget
Suppose, hypothetically, an account holds US$20,000, the selected risk limit is 1%, the planned entry is US$50 per share, and the stop is US$46 per share.
| Calculation | Result |
|---|---|
| Risk budget: US$20,000 × 1% | US$200 |
| Planned loss per share: US$50 − US$46 | US$4 |
| Shares: US$200 ÷ US$4 | 50 shares |
| Position value: 50 × US$50 | US$2,500 |
| Estimated money at risk: 50 × US$4 | US$200 |
| Position as % of account: US$2,500 ÷ US$20,000 | 12.5% |
The example uses exact hypothetical prices and excludes fees, spread, slippage and taxes. Buying 50 shares at US$50 would require US$2,500 in purchasing power. The US$200 figure assumes an exit at exactly US$46; an actual exit could differ.
How to think about risk per trade
Risk per trade is the percentage of account value you choose as a planned loss budget for one position. 1% and 2% are common rules of thumb used to illustrate position sizing, not universal limits or advice. On a US$20,000 account, they correspond to US$200 and US$400 respectively. The appropriate input depends on the account’s constraints and the possibility that actual losses exceed the planned amount.
Risk percentages across positions need to be considered together. Five positions each sized to a 1% planned loss do not necessarily cap the account’s loss at 5%: stops can slip, and related stocks can fall at the same time. The calculator sizes one proposed position; it does not calculate total portfolio risk.
It also does not make a position affordable. A wide stop can produce a share count whose purchase value exceeds the available cash or buying power. Conversely, a very tight stop can produce an unusually large position value even though the estimated stop-based loss stays within the risk budget. Compare the resulting position value with both available funds and the share of the account you are comfortable allocating to one stock.
Why liquidity and concentration still matter
The share-count formula assumes the intended entry and stop prices are attainable. The bid-ask spread—the gap between quoted buying and selling prices—and market impact can make execution more expensive. An order that is large relative to a stock’s typical trading volume may be harder to fill at an assumed price. A stop-loss order does not guarantee execution at its stop price.
Concentration is a separate question. The example’s 50 shares represent 12.5% of the account, despite a planned stop-based risk of 1%. A position can therefore fit its loss budget while still accounting for a substantial share of the portfolio. When reviewing several holdings, consider each position’s weight and whether holdings share exposure to a sector, country or other risk factor.
PreTrAIde provides a free position size calculator, trading cost calculator and portfolio concentration checker on pretraide.com without sign-up. Its pre-trade analysis evaluates a proposed stock basket before orders are placed, including liquidity, estimated spread and market impact, and concentration. Its verdicts are decision support, not recommendations; users place any trades themselves through their own broker.
Using volatility to set a stop
A stop distance can be expressed in dollars or as a percentage of the entry price. Another approach is to relate it to a measure of the stock’s typical price movement, such as average true range (ATR). For example, a hypothetical entry at US$50 and a stop US$5 below it gives a US$45 stop and US$5 of planned risk per share. With a US$200 risk budget, the formula yields 40 shares.
A volatility-based stop is not a prediction of where the price will go or a guarantee that an exit will occur nearby. The chosen measurement period, changing volatility and overnight gaps all matter. A wider stop reduces the calculated share count when the risk budget stays fixed; a narrower stop increases it. Set the entry and stop assumptions first, then use the calculator to see what those assumptions imply for shares, dollars at risk and portfolio weight.
What this calculator does—and does not—tell you
Use the result as a consistent way to translate four inputs into a proposed share count. Check that the position value is affordable, the portfolio weight is acceptable under your own constraints, and the stock is liquid enough for the contemplated order. The output does not assess whether the stock is attractive, whether a stop will fill as planned, or what the trade will earn or lose.