Stop-Loss Order

A stop-loss order is an instruction to a broker to sell a security once its price reaches a specified level, called the stop price. Until the market trades at or through the stop price the order does nothing; once it does, the stop-loss becomes a market order and executes at the best price then available. Investors use it to cap a potential loss on a holding, or to lock in part of a gain, without having to monitor the price themselves.

The essential thing to understand is what the stop price controls and what it does not. It controls when the order activates, not the price at which the sale fills. Because the activated order is a market order, the actual execution price can differ from the stop, sometimes substantially. The plain order types a stop builds on, the market order and the limit order, are covered on their own pages; this page is about the stop mechanism, its failure modes, and the stop-limit variant.

A stop-loss is a market order in waiting. Nothing happens while the price stays above the stop. The moment the security trades at or below the stop price, the order is triggered and sent to the market as an ordinary market order, which fills at whatever price is available at that instant. So the stop price is a trigger, not a promise. In a calm market with a liquid stock, the fill is usually close to the stop. In a fast-falling or thinly traded market, it may not be.

Slippage and gaps are the built-in risk. If a stock closes at $50 and opens the next morning at $42 on bad news, a stop-loss set at $48 does not sell at $48. It is triggered when trading opens below the stop and fills near $42, because there was no trading between $48 and $42 for the market order to catch. This gap risk is why a stop-loss cannot be relied on to cap a loss at a precise number, and it is the single most common way the tool surprises the investor who set it. A stop protects against a slow drift down far better than against a sudden crash.

The stop-limit order trades one risk for another. A stop-limit order pairs a stop price with a limit price: when the stop triggers, the order becomes a limit order rather than a market order, so it will not sell below the limit price you set. That removes the slippage risk, but it introduces the opposite one. If the price falls straight through the limit, the order simply does not fill, and you are left holding the position while it keeps dropping, which is the exact outcome a stop was meant to prevent. So the choice is between a sale that is certain but at an uncertain price (stop-loss) and a price that is certain but a sale that is not (stop-limit).

Placement is a judgment about volatility, not just about loss. A stop set just below the current price will be hit by ordinary day-to-day swings and can sell you out of a sound position at a temporary dip, after which the price recovers without you. A stop set far below gives the position room but allows a larger loss before it triggers. There is no correct distance; it depends on how much normal movement the security shows and how much loss the holder is willing to absorb. A stop-loss is a discipline tool, and setting it well is the whole skill in using one.

You set a stop price below the current market price for a holding you own. The order rests with the broker and does nothing while the price stays above the stop. If the security trades down to the stop, the order activates and is sent as a market order to sell, filling at the best available price.

A hypothetical example of slippage. An investor owns a stock trading at $60 and sets a stop-loss at $54 to cap the loss at about 10%. On ordinary weakness the stock drifts to $54, the stop triggers, and it sells near $53.90, close to plan. Now suppose instead the company reports bad news overnight and the stock opens the next day at $47. The stop at $54 is triggered at the open, but the market order fills near $47, not $54, because the price gapped straight past the stop with no trading in between. The loss is about 22% ($60 down to about $47), not the 10% the stop appeared to promise. The stop did its job, activating a sale, but it never controlled the price the sale filled at.

Pros

• It enforces an exit discipline automatically, so a loss cannot run indefinitely while you are not watching the market.
• It removes the in-the-moment emotion from a sell decision made in advance, which is where investors often hesitate.
• It can also protect a gain, by trailing the stop up under a rising price so a reversal locks in some of the profit.

Cons

• It does not guarantee the exit price: in a gap or a fast fall, the market order fills well below the stop, so it cannot cap a loss at a precise number.
• A stop set too close is triggered by normal volatility and can sell a sound position at a temporary dip.
• The stop-limit alternative removes slippage but can fail to sell at all if the price falls through the limit, leaving you holding the loss anyway.