Stop Order: A Trigger and a Market Order
A stop order rests inactive until price reaches a trigger level, at which point it becomes a market order and executes at whatever is available. It is two orders in one, and the second half carries every cost a market order carries.
How it works
A stop order sits dormant until price trades at or through a level you named. Nothing happens before that. It is not in the order book and it is not visible to anyone.
When the trigger is hit, it becomes a market order. From that moment it carries the spread, the slippage and the depth of the book — every cost on that page, arriving at once.
Thinking of it as two separate things is the fix for most confusion about stops. The trigger is a decision about where you are wrong. The execution is a market order placed at the least convenient moment available, and those two deserve separate consideration.
The two uses, which are different strategies
As an exit, it is a stop loss — and the crucial wording is that it caps the loss at a price, not at an amount. Where the fill lands is a separate question, and in fast conditions the answer is worse than the trigger.
Position size calculated as “risk divided by stop distance” therefore assumes something the order type does not provide. It is a good estimate in normal conditions and an underestimate in exactly the conditions that produce large losses.
As an entry, a buy stop above the market is a breakout order — buy if price gets strong enough to reach this level. That is the mirror of a buy limit, which buys weakness, and the two express opposite views about what a price reaching a level means.
In practice
Stops are held at the broker and released as orders when triggered. They are not in the depth display, which is why no chart of “where the stops are” can be anything but an inference — the liquidity pool page covers what that inference is worth.
The obvious level is obvious to everyone. A tick under the swing low is where a great many stops sit, which makes that price a concentration of orders — and concentrations get reached. That is not a conspiracy; it is where the liquidity is.
A cluster of stops triggering together shows up as a volume spike. That is the only observable evidence that a cluster existed, and it arrives after the fact.
On a longer timeframe a stop rests for weeks, through sessions you are not watching and events you did not anticipate.
A gap past the trigger fills at the open. The market was closed, price arrived beyond your level, and the order executed at the first available price. The loss is whatever the open decided — this is the failure mode that makes overnight risk categorically different from intraday risk.
The fill costs the same round trip as any other market order: 2% of a median bar’s range on this history.
And on a small bar that cost dominates. The same 0.0098 in price units is 2% of a median bar and 45% of the smallest bar in this 576-bar series, where ranges span 0.17 to 1.10 between the tenth and ninetieth percentiles.
What a stop order is not
It is not a promise about the exit price. It is a commitment to attempt an exit, at whatever the market offers.
It is not visible to the market. Nobody sees your stop, including the exchange, until it fires.
It is not a stop-limit order. That adds a price bound and, with it, the risk of not filling at all.
And it is not protection against a gap. No order type is. The only protection against overnight risk is position size or not holding overnight.
When it fails
In a range stops are taken at both ends. Price probes the low, triggers the longs’ stops, reverses, probes the high, triggers the shorts’. Both groups are removed at the worst available price, and the range continues.
The second failure is the gap. The loss is set by the open, not by the level, and no amount of careful placement changes that.
A third is the obvious placement. A tick beyond the visible extreme is where the crowd is, and it is reached more often than a slightly wider stop would be. Wider costs more when hit and is hit less often; which is better is an empirical question about your instrument, not a rule.
A fourth is the thin-book trigger. A stop fires when price is moving, which is when the book is emptiest, which is when the fill is worst.
And a fifth is treating stop distance as risk. It is an estimate of risk that holds in ordinary conditions and fails in the ones that matter, which is precisely backwards from what a risk control should do.
The original data
On this site’s shared 576-bar history, the round-trip cost of 0.0098 price units is 2% of the median
bar range of 0.493 and 45% of the smallest bar of 0.022; it exceeds 10% of the bar’s range on 15 of the
576 bars, and bar ranges span 0.17 to 1.10 between the tenth and ninetieth percentiles. The figures are
in research/series-measurements.json, produced by site/measure_series.py.
The 15-bar figure is the one worth carrying into stop placement. On 3% of bars in this series, the execution cost alone consumed more than a tenth of everything the bar had to offer — and those are the quiet bars, which is where stops sit for long stretches waiting. The measurement that would tell you what your stops really cost is your own fill data: for every stopped-out trade, the distance between the trigger price and the actual fill. Brokers provide both numbers, almost nobody compares them, and the average difference is the part of your risk model that is currently a guess.
Related
Stop loss covers the exit use and what it is for. Market order is what a stop becomes on trigger, with the full cost picture. And stop-limit order is the variant that bounds the price and risks no fill.
The lesson that cost me most was that a stop is a price, not a promise. I had sized several positions on the assumption that the stop distance was the loss, and the first time a gap taught me otherwise the arithmetic was not close.
— Michael Whitman
This page is educational, not financial advice. Test every idea on your own charts before risking money.