What it means
A stop order stays inactive until its stop price is triggered. It then becomes a market order, which seeks execution at the available market price. The stop price is a trigger, not a guaranteed fill price.
A sell stop is generally placed below the current market price; a buy stop is generally placed above it. A sell stop may be used to limit exposure after a decline, while a buy stop may be used to enter after a rise or to close a short position. Neither type guarantees that a loss will be limited to a chosen amount.
A broker or trading venue determines the precise triggering rule, such as whether it uses a trade price or a quotation. Check that rule, along with the order's time-in-force, before submitting it. A stop order differs from a stop-limit order: once triggered, a stop-limit order becomes a limit order rather than a market order.
How it is calculated
A stop order has no formula that calculates its execution price. For a simple trigger illustration, let P_ref be the price used under the broker's triggering rule, in US$ per share, and P_stop be the selected stop price, also in US$ per share:
- Sell-stop trigger:
P_ref <= P_stop - Buy-stop trigger:
P_ref >= P_stop
These comparisons illustrate the direction of a trigger; the broker's actual rule controls when it occurs. After activation, the fill price depends on available liquidity and any price movement. To measure the difference afterward, use Fill difference = |P_fill - P_stop| × Q, where P_fill is the fill price in US$ per share and Q is the number of shares filled. The result is in US$. It measures distance from the stop price, not the investor's total gain or loss; for a partial fill, calculate using the shares actually filled.
Worked example
Suppose, hypothetically, that a stock trades at US$50 per share and an investor places a sell-stop order for 100 shares with a US$48 stop price.
- While the broker's reference price remains above US$48, the order has not triggered.
- Suppose the reference price reaches US$48 under the broker's rule. The stop activates, and the order becomes a market sell order for 100 shares.
- Suppose the available market allows all 100 shares to fill at US$47.80. Proceeds are
100 × US$47.80 = US$4,780, before any fees. - The difference from the stop price is
|US$47.80 - US$48.00| × 100 = US$20. A fill at US$48 would instead have produced US$4,800, before fees.
If the stock gaps past US$48 and the first available price is much lower, the fill could be farther from the stop. This example does not imply that any particular fill is likely.
Why it matters before you trade
A stop order separates when an order activates from the price at which it executes. That distinction matters most when prices move quickly, liquidity is thin, or trading resumes after a halt or overnight gap. A market order created by the trigger may fill across several prices or only partly if available liquidity is insufficient.
Before placing an order, a pre-trade check can distinguish the intended stop price from the potential execution cost. Relevant inputs include the bid-ask spread, order size relative to daily volume, volatility and market impact. None of these makes a stop price a guaranteed exit price. Order availability and triggering rules can also differ by broker and market.
How PreTrAIde uses it
PreTrAIde's stated product facts do not say that it places, monitors or specifically models stop orders. Its pre-trade analysis lets a user specify execution conditions for each stock in a basket and assesses factors such as estimated spread, participation in daily volume, market impact and volatility. It returns a GO, NO-GO or MODIFY verdict with reasoning and a suggested execution strategy as decision support, not an order instruction. Users place orders themselves with their own broker; PreTrAIde never receives, transmits or executes them.
Common mistakes
- Treating the stop price as a floor or ceiling for the fill. A triggered market order can execute at a worse price, particularly after a gap.
- Confusing a stop order with a stop-limit order. A stop-limit order controls its limit price but may remain unfilled.
- Assuming every broker uses the same trigger. Confirm which price activates the order and whether it is eligible outside regular market hours.
- Ignoring partial fills and costs. A trigger does not ensure an immediate, complete fill at one price; spreads and any applicable fees also affect the outcome.