A stop loss can limit a planned loss, but it cannot guarantee the price of the fill when a market gaps. Position size decides how much a worse-than-planned fill can damage your account before the market opens.
The plan looked clean the night before: enter at $44, place the stop at $42, risk $2 per share. Then the market opened at $38. The stop order triggered into available liquidity, and the exit filled there. The trade lost $6 per share, three times the amount written into the plan.
That gap is not a failure of arithmetic. It is the risk the arithmetic has to leave room for.
A stop price is a trigger, not a promise
A stop order tells the broker when to seek an exit. It does not create buyers at the stop price. If news lands after the close, liquidity disappears, or the opening auction reprices the stock below your level, the next available price may be far away.
With 100 shares, a $2 planned loss was $200. A fill at $38 made it $600 before commissions or other trading costs. With 500 shares, the same gap turns into $3,000.
The important number was never only the distance from $44 to $42. It was the account-level amount that could be lost if the exit price came back worse than planned.
That distinction is easy to ignore when charts are moving smoothly. It becomes obvious at the open, when the market has already moved and the trade is no longer yours to negotiate.
The Swiss franc showed what discontinuity looks like
On January 15, 2015, the Swiss National Bank ended its minimum exchange rate policy for the euro against the Swiss franc. Thomas Jordan was chairman of the Swiss National Bank at the time. The franc moved sharply after the announcement, and the speed of the move caused severe losses across retail foreign exchange markets. Alpari UK later entered insolvency after client losses exceeded its available funds.
Reuters documented the event as it unfolded. The point is not that a stock opening at $38 is equivalent to a currency shock of that scale. It is that both expose the same mechanical limit: an exit instruction cannot fill at a price the market has already passed.
In Switzerland that day, the reference point traders had relied on disappeared. In an overnight stock gap, your $42 stop can become a historical reference before the opening print. The market does not owe the plan its original risk number.
Educational content, not financial advice.
Size the trade for the loss you did not plan
A trader cannot calculate every possible gap. You can decide how much uncertainty the position is allowed to carry.
Start with the planned risk per share, then add a gap allowance that reflects the instrument and the event risk. A liquid large-cap stock on an ordinary session has a different overnight profile from a small-cap stock, a cryptocurrency position held through the weekend, or any position held into earnings.
If the planned loss is $2 per share but you decide a $6 exit is plausible enough to respect, size the position from $6. That approach may produce a smaller share count. It also means the account can survive the version of the trade where the stop does not fill where you hoped.
A few practical checks help make this visible before an order is queued:
- Write down the planned stop price and a separate adverse-fill price.
- Calculate the loss at both prices, using the full position size.
- Reduce or skip the position when the adverse-fill loss exceeds your per-trade limit.
- Treat scheduled announcements, earnings, and thin trading periods as reasons to widen the uncertainty range, not as reasons to trust a tight stop.
This is the same discipline behind [max drawdown planning](\/blog\/max-drawdown-planning-what-four-losses-taught-maya-about-trading-discipline-9cba4da8\/). A risk limit that only works in the expected version of a trade does not protect much when the market behaves differently.
Approval creates a pause before exposure
An approval-gated workflow cannot remove gap risk. It can make the assumptions visible before a real order is exposed to it.
Before approving a queued trade, review the share count alongside two figures: the loss at the stated stop and the loss at a worse exit. If those numbers lead to different decisions, the smaller position or no trade is useful information. The setup may still be valid. The account-level risk may not be.
This matters most when a signal looks strongest near a known event. A chart can offer a clear entry and stop while the calendar contains the part that can break the plan. The queued order before earnings deserves the same scrutiny as the stop itself, as explored in [The Queued Order Ten Minutes Before Earnings, and What Pressure Can Override](\/blog\/the-queued-order-ten-minutes-before-earnings-and-what-pressure-can-override-12627848\/).
The trader who woke to a $38 fill did not need a better explanation of where the stop was. They needed a position size that assumed the market could open somewhere else. That is the pre-market decision worth making while the $42 stop still looks reassuring.
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