What is a Stop-Loss Order? Definition, Formula, and Example
A stop-loss order is a standing broker instruction that automatically sells (or buys, for shorts) a security once its price crosses a preset trigger, capping downside without requiring the trader to watch the tape.
What is a stop-loss order?
A stop-loss order is a standing instruction to a broker to sell an existing long position — or buy back a short — once the market trades through a preset trigger price. Until the trigger hits, the order is dormant and carries no standing bid or offer on the book. The moment the security prints at or through the stop, the order converts into a live market or limit order and competes for the next available fill. It is the core tool for enforcing a risk budget on an individual trade without human intervention.
How a stop-loss order is structured
Every stop-loss has three components: trigger price, order type on activation, and time-in-force. For a long position entered at P₀, a trader risking R percent sets:
Stop price = P₀ × (1 − R)
Three variants dominate:
- Stop-market: converts to a market order on trigger. Guaranteed to fill, not guaranteed at the stop price. Slippage is unbounded in fast markets.
- Stop-limit: converts to a limit order at a specified limit price. Price protected, fill not guaranteed — if the stock gaps through the limit, the order sits unfilled.
- Trailing stop: trigger ratchets in the direction of favorable price movement by a fixed dollar amount or percentage, locking in gains as the position moves.
Worked example
A trader buys 100 shares of AAPL at $215.00 on 2026-04-15 and wants to risk 5% on the trade. The stop goes at $204.25 (215 × 0.95). Position risk equals (215 − 204.25) × 100 = $1,075. On 2026-04-17 AAPL trades down to $203.80 intraday. The stop-market triggers at $204.24 and fills near $203.50 — realized loss $1,150, with $75 slippage versus the stop price. Without the stop, AAPL closing the week at $195 would have put the trader down $2,000, nearly double.
If the same trader used a stop-limit at $204.25 with a $203.00 limit, and AAPL gapped from $205 to $202 on earnings, the stop triggers but the limit never fills — the trader still holds 100 shares at $202 with no protection engaged.
When traders use stop-loss orders
- Position sizing: shares = (account risk $) ÷ (entry − stop). A $500 risk budget with a $2.15 stop distance produces 232 shares. The stop is the pivot the entire size calculation hangs on.
- Overnight and weekend protection when the trader cannot monitor positions.
- Systematic breakout strategies that pre-commit to an exit below prior support.
- Covering short positions before runaway squeezes — a buy-stop above resistance caps short exposure.
Limitations and common misconceptions
Stop-losses do not guarantee the exit price. In gap events — earnings, overnight news, halts — the stock can open below the stop, and a stop-market fills at the open. During the May 2010 flash crash, blue-chip stops filled at pennies before the market snapped back. Whipsaw is the second hazard: volatile ranges tag the stop, exit the trader, then reverse. Setting stops tighter than one ATR invites this. Finally, stop orders cluster at round numbers and prior swing lows, and are visible to market makers as resting liquidity pockets — "stop hunting" is a documented intraday phenomenon around options-expiration Fridays.
Related terms
- What is Average True Range? — the volatility measure most traders use to size stop distance
- What is Bollinger Bands? — dynamic bands often used as trailing-stop anchors
- What is the VIX? — regime filter for widening or tightening stops
- What is a Moving Average? — common trailing-stop reference line
- What is a Short Squeeze? — the exact scenario buy-stops are designed to cover