Core terms
Gap fill
A return of price to the prior session's close after an opening gap. A gap-fill count states whether it means a full or a partial fill (such as the gap midpoint) and whether an intraday touch or the session close is the basis.
"Gap fill" blurs separate measurements. A full fill reaches the prior close; a partial fill gives back part of the gap, often half (the gap midpoint: prior close plus half of open minus prior close). The basis can be an intraday touch (any regular-session trade at or below the prior close) or the close (the session closes below it). A stock can trade a cent under the prior close at 10:20 and close 25% green: an intraday fill and a close-basis hold on the same day (Illustrative).
On a close basis, of 671 US common stocks and ADRs that opened at least 20% above the prior close, 62 (9.2%) closed below the prior close and 208 (31.0%) closed below the gap midpoint (2026-07-01 to 2026-10-02; FennEdge archive; as of 2026-10-02). FennEdge has not yet counted intraday touches.
What traders should verify
Before counting fills, write down which prior close (split-adjusted), full or partial, touch or close, and whether premarket trades count. Keep day-level fill rates separate from the results of fade trades with stops.
Do not conflate
Trader usage; there is no official definition. Rates for different definitions differ widely and are often quoted without saying which was used. A close-basis rate describes days judged at the close; it is not the rate at which an intraday fade reaches a target, because some days run through a stop before fading.
Primary source
Trader usage: there is no official definition, and platforms and tools may calculate it differently.