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Technical & Quantintermediate

Stop-Loss and Position Sizing

A stop-loss is a pre-set exit price that closes a losing position before the damage grows, the disciplined expression of the rule to cut losses early. Risk per trade is the loss taken if the stop is hit, the gap between entry and stop multiplied by the number of units. Position sizing then chooses that number of units so the loss stays within a fixed share of the account, often near one to two percent. Together these tools convert a vague intention to manage risk into a number set before the trade is placed.

Why it matters

Survival comes before profit. A trader who never lets one trade sink the account can keep playing long enough for an edge to show up, which is why position sizing matters more than picking the perfect entry. The logic runs backwards from the exit. First decide where you are wrong and will get out, the stop, then size the position so that being wrong costs only a small, planned slice of capital. The stop caps the loss per unit and sizing caps the units, so the worst case is known in advance.

Formulas

Risk per unit
Runit=Pentry−PstopR_{\mathrm{unit}} = P_{\mathrm{entry}} - P_{\mathrm{stop}}
For a long position the risk per share is the entry price minus the stop price. A wider stop means a larger loss per unit, which forces a smaller position.
Position size from a risk budget
N=f⋅APentry−PstopN = \dfrac{f \cdot A}{P_{\mathrm{entry}} - P_{\mathrm{stop}}}
With account value AA and a risk fraction ff, the number of units NN is the dollar risk budget f⋅Af \cdot A divided by the risk per unit. Setting ff near one to two percent caps the loss on the trade.

Worked examples

Scenario

An account holds A$50,000 and the trader risks 2 percent per trade. They buy at A$40 with a stop at A$37. How many shares keep the loss inside the budget?

Solution

The risk budget is 2 percent of A$50,000, which is A$1,000. The risk per unit is the entry minus the stop, A$40 minus A$37, which is A$3. Position size is N=10003≈333N = \tfrac{1000}{3} \approx 333 shares. If the stop-loss at A$37 is hit, the loss is about A$1,000, exactly the planned 2 percent, no matter how confident the trader felt at entry.

Common mistakes

  • ✗A wider stop is safer because it is harder to hit. A wider stop raises the loss per unit, so a disciplined trader must take a smaller position. The dollar risk, not the distance, is what gets controlled.
  • ✗A profitable strategy does not need position sizing. Even a strategy with an edge has losing streaks. Oversizing into one can wipe out the account before the edge has time to work.
  • ✗Cutting losses early is the same as being a weak investor. A stop-loss is a risk rule, not a verdict on conviction. It bounds the downside of being wrong on any single trade.
  • ✗A stop-loss guarantees you exit at exactly that price. A market gap can fill the order below the stop. The stop controls intent and discipline, not the exact fill in a fast market.

Revision bullets

  • •A stop-loss is a pre-set exit that caps the loss on a trade
  • •Risk per unit is entry price minus stop price for a long
  • •Risk per trade is risk per unit times the number of units
  • •Position sizing picks units so loss stays within a fixed account share
  • •A common budget is one to two percent of the account per trade
  • •Survival of capital comes before chasing return

Quick check

A trader buys at A$25 with a stop at A$22. The risk per share is

If an account is A$20,000 and the trader risks 1 percent per trade with A$2 of risk per share, the position size is about

Connected topics

Technical AnalysisTrend AnalysisTechnical LimitsQuant Strategy

Sources

  1. Brailsford, Heaney & Bilson (2015), Ch. on technical analysis
    Brailsford, T., Heaney, R., & Bilson, C. Investments: Concepts and Applications. 5th ed. Cengage Learning Australia, 2015.
    Discusses stop-loss discipline and risk control within active trading.
  2. Bodie, Kane & Marcus (2021)
    Bodie, Z., Kane, A., & Marcus, A. J. Investments. 12th ed. McGraw-Hill Education, 2021.
    Reference treatment of risk management and the role of stop orders.
How to cite this page
Dr. Phil's Quant Lab. (2026). Stop-Loss and Position Sizing. Derivatives Atlas. https://phucnguyenvan.com/concept/im-stop-loss
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