A stop-loss order instructs your provider to close a position once the market reaches a specified level. It is a discipline mechanism and a risk cap, but it is an instruction about when to act, not a guarantee of the price you receive.
What it does reliably
Used consistently, a stop-loss enforces the invalidation level you decided on before entering — the point at which the reasoning behind the trade no longer holds. Combined with a position size derived from your risk budget, it turns "how much could I lose" from a guess into an arithmetic result.
Where the guarantee breaks
- Gaps. If the market jumps past your level without trading at it — around a weekend, a scheduled release or a shock — the order fills at the next available price, which can be worse.
- Liquidity. In thin conditions the executed price can differ materially from the requested level.
- Order type. A stop order becomes a market order when triggered; a stop-limit may not fill at all if price runs through the limit.
The practical rule
Guaranteed-stop products exist at some providers and typically carry a premium; their terms differ by provider and jurisdiction. Absent that, size every position on the assumption that a stop can slip, and keep per-trade risk small enough that a normal run of losses stays survivable.
This is educational reference material and is not financial, investment, or trading advice.