How a stop order actually fills

A stop order that fills at exactly the price written on it has passed through four distinct stages, and a stop order that fills somewhere else has passed through the same four. Understanding where the two paths diverge requires separating the stages, because the everyday description of a stop as an order at a price collapses all four into one and hides the only moment that matters.

Stage one: resting, and where it rests 

A stop order does not sit in the order book of a venue. It sits with whichever system is monitoring the price for it, and that system is either the broker's server or the trading application on the device. The distinction has a practical consequence that is easy to test and easy to overlook: an order held on the server is monitored whether or not the application is running, while one held locally stops being monitored the moment the application closes or the connection drops.

Because nothing about a resting stop is visible to the market, it has no effect on the price. It consumes no liquidity, appears in no depth display and cannot be traded against. It becomes a real order only at the moment its condition is satisfied, and until then it is a monitoring rule rather than an instruction anybody outside the broker can see.

Stage two: the trigger, and which price satisfies it 

The trigger is a comparison between the stop's level and a live price, and the identity of that live price is the single most misread detail of the whole mechanism. A market quotes two prices at once, a bid and an ask, and the two are never equal. A stop on a long position is closed by selling, so it is compared against the bid. A stop on a short position is closed by buying, so it is compared against the ask.

The consequence is arithmetic rather than editorial. A chart drawn from bid prices will show the price touching the level of a short position's stop before the ask has reached it, and a chart drawn from ask prices does the reverse. A stop that appears from the chart to have been triggered early or late is usually being read against the wrong side of the quote, and platforms let the chart's price source be inspected precisely so this can be checked.

Some venues also require the trigger to be confirmed rather than accepting the first tick that crosses it, most often by requiring the crossing price to persist for a stated interval or by requiring a completed trade rather than a quote. Confirmation reduces triggers on single erroneous ticks and delays legitimate ones by the same interval. It is disclosed per venue and it is a design choice with a genuine cost on both sides.

Stage three: conversion into a live order 

Once the condition is satisfied the stop ceases to exist and something else takes its place. A plain stop becomes a market order, which carries no price boundary and will transact at whatever the book offers. A stop limit becomes a limit order at a separately stated price, which will not transact beyond that price and may therefore not transact at all.

This is the moment the fill price is decided, and it is later than the trigger by however long conversion and transmission take. In a quiet market that interval is measured in milliseconds and the price is unchanged across it. In a fast one the price moves inside it, which is the ordinary and unavoidable source of a fill away from the trigger level.

Stage four: the fill 

The converted order arrives at the market and is matched against whatever is resting there. If the quantity available at the best price covers it, it fills at one price. If not, it fills across successive levels and its price is the quantity weighted average of them. Neither outcome is related to the trigger level, which has already done its only job.

Reading the three cases together isolates the variable. Nothing about the order changed between them. The distance between the trigger and the fill is a property of the market in the interval, and it is the reason a stop is described as an instruction to transact once a level is passed rather than an instruction to transact at it.

What happens instead with a stop limit 

A stop limit replaces the fourth stage's open ended matching with a boundary. In the quiet case it fills identically. In the fast case it fills only if the market is still inside its limit price on arrival. In the gapped case it does not fill at all, and the order continues to rest as a live limit order at a price the market has already left behind.

Partial fills and attached orders 

A triggered stop for a quantity larger than the market can absorb at once may fill in pieces, each at its own price, and each piece appears on the statement as its own transaction. The position is reduced progressively rather than in one step, and the effective exit price for the whole is the weighted average of the pieces.

Where the stop is one leg of a one cancels the other pair, the fill of the first leg initiates the cancellation of the second. A partially filled leg complicates that: platforms differ in whether the opposite leg is reduced proportionally, cancelled outright or left at full size, and the behaviour is documented per platform rather than being an industry standard.