Why a stop loss
can fill lower.
A stop price triggers an order. The available execution price determines the fill. A small gap between the two can change the risk of the same position.
Iron Phoenix research team · Reviewed 2 November 2026Stop price versus execution price
For a conventional sell stop order, reaching the trigger turns the order into a market order. Available liquidity can produce an execution price below the trigger. The SEC explains that stop prices do not guarantee execution prices; brokers may also use different trigger standards. A stop-limit order controls its limit price but can remain unfilled.
Follow the same 23 shares through both exits
This hypothetical example uses a $100 entry, 23 shares and a $0.10 estimated round-trip cost allowance per share. The worse fill is kept separate from that allowance.
| Scenario | Exit | Loss calculation | Loss incl. costs |
|---|---|---|---|
| Planned exit | $98.00 | 23 × ($100 − $98 + $0.10) | $48.30 |
| Lower fill | $97.50 | 23 × ($100 − $97.50 + $0.10) | $59.80 |
The $0.50 difference adds $11.50 to this position's loss. The account and original share count have not changed.
Re-sizing for the alternative fill
With a $50 risk budget and $2.60 of risk per share at the alternative fill, the whole-share calculation gives 19 shares. Their scenario loss is 19 × $2.60 = $49.40. A still-worse fill or different costs can exceed that calculation.
Try the position size calculator ↗
Check the assumptions before interpreting the result
Separate the stop trigger, alternative execution price and estimated costs. If an existing cost allowance already includes the identical worse-fill assumption, counting that amount again would double-count it. This example intentionally holds its $0.10 cost allowance fixed and adds the worse exit separately.
Market gaps, halts and thin liquidity can create outcomes beyond a small slider range. The tool does not estimate the probability of a fill and does not choose an order type for you.
Source and scope
Source: SEC Investor Bulletin: Stop, Stop-Limit, and Trailing Stop Orders, checked 2 November 2026. Calculations are the Iron Phoenix research team's hypothetical worked example.
Continue with the position sizing tool or the reverse valuation worksheet. This guide is educational and is not personalized investment advice.