Gappers: gap percentage, gap and go, and gap fill
A gapper is a stock that opens well away from its previous close. Learn to compute the gap from the right two prices, tell a premarket move from the official opening gap, and use "gap and go" and "gap fill" as descriptions of what happened, not as predictions.
Start in plain English
Every morning, small-cap traders scan for gappers: stocks trading well above or below the previous day's close before the regular session opens. News, earnings and filings released overnight or before 09:30 ET show up as a price gap, and those stocks often carry most of the morning's volume. Knowing how a gap is measured, and what the slang around it means, keeps your notes and your reviews consistent.
The core idea: a gap is the distance between the regular-session open and the previous close, as a percentage of the previous close.
gap % = (open ÷ previous close − 1) × 100
Using a percentage lets you compare stocks at different prices: a $0.72 gap on a $2.40 stock is +30.0%, while the same $0.72 on a $24.00 stock is +3.0%. A stock that closed at $2.40 and opens at $1.92 has a gap of −20.0%, a gap down.
Two words describe what happens next. Gap and go is slang for a stock that keeps moving in the gap's direction early in the session. Gap fill means price trades back to the previous close, erasing the gap. Both are descriptions of a day after it happened. Neither is a pattern that tells you what the next gapper is going to do.
One stock, three "gap" numbers
All prices and volumes in this example are Illustrative. Ashgrove Diagnostics is fictional and Nasdaq-listed.
Ashgrove closed yesterday at $2.40, the official closing price at 16:00 ET. At 07:05 ET it announced trial results. Here is what a trader could see by the open:
| Time (ET) | Price | Shares traded at that print | Against the $2.40 previous close |
|---|---|---|---|
| 07:52 | $3.62, the premarket high | 3,200 | (3.62 ÷ 2.40 − 1) × 100 = +50.8% |
| 08:15 | $3.30, as shown on a premarket gapper list | 1,500 | (3.30 ÷ 2.40 − 1) × 100 = +37.5% |
| 09:30 | $3.12, the official opening price | 214,000 in the opening auction | (3.12 ÷ 2.40 − 1) × 100 = +30.0% |
All three numbers are "the gap" to someone. A premarket list at 08:15 shows +37.5%. A trader quoting the premarket high says "it gapped 50%". The opening gap, the one used in FennEdge's studies and most daily recaps, is +30.0%: the official open against the previous close. The official open comes from the listing exchange's opening auction, which The Nasdaq opening cross explains.
Why use the opening gap as the standard? Both of its prices are official, so every source that uses them gets the same answer. Premarket prices depend on which trades a screen includes, and they can rest on very few shares: the 07:52 high printed on 3,200 shares, while the opening auction matched 214,000.
Two reference levels follow from the opening gap:
- Gap-fill level: the previous close, $2.40. From the $3.12 open, that is a fall of (2.40 ÷ 3.12 − 1) × 100 = −23.1%. A gap does not need to fill on the same day, or at all.
- Gap midpoint: halfway between the previous close and the open, (3.12 + 2.40) ÷ 2 = $2.76. Traders call a move back to this level a "half fill".
For a gap down the arithmetic is the same, mirrored: a fill means price trades back up to the previous close.
The gap also leaves things out. It does not say whether the news is material, how many shares can trade, or how wide the spread is at the open. Those are separate checks, covered in Float for day traders and Bid, ask and spread.
The conclusion. For Ashgrove, "the gap" was +50.8%, +37.5% or +30.0% depending on which price you used, and the levels worth writing down before 09:30 were the previous close ($2.40), the premarket high ($3.62), the open ($3.12) and the midpoint ($2.76). When you compare gappers across days, or read a statistic about them, check which of these prices it uses. A list ranked by premarket change and a study ranked by opening gap describe different sets of stocks. Write each level with its time and session, and you can review the day later without guessing which "gap" you meant.
The rest of this lesson covers what a premarket gapper list shows, how "gap and go" and "gap fill" are used, what FennEdge's archive measured on more than 600 gappers, and what to record when you review a gap day.
Premarket gappers: what the list shows
A premarket gapper list ranks stocks by their latest extended-hours price against the previous close. Read it with three limits in mind:
- It moves until 09:30. The +37.5% at 08:15 became a +30.0% opening gap. Some names on an 07:00 list are gone by the open, and others appear late.
- Volume is thin. A few thousand shares can set a premarket price. A large percentage on small volume is a weaker observation than the same percentage on heavy volume. Volume and relative volume covers how to put volume in context.
- Spreads are wider. Extended-hours quotes are often wider and can move sharply between trades, as FINRA's extended-hours guide notes. The trading day has three rooms covers the sessions.
How to build a watchlist from such a list is a separate skill, covered in FennEdge's article on the premarket watchlist. Trading the premarket itself is covered in the scalping course's Premarket scalping.
Gap and go, gap fill, runner: words for outcomes
The same opening can lead to very different days. Here are three Illustrative versions of Ashgrove's session, all starting from the $3.12 open:
| Day | What happened by 11:00 ET | Label traders use |
|---|---|---|
| A | Price held above $3.12, broke the $3.62 premarket high at 09:48 and reached $4.05 | "Gap and go" |
| B | Price fell through the $2.76 midpoint by 10:05 and touched $2.40 at 10:41 | "Gap fill" (a full fill) |
| C | Price traded between $2.90 and $3.40 all morning | No label; most days need none |
A few notes on the words:
- Gap and go has no official definition. Some traders mean "broke the premarket high", others "held above the open for the first hour". If you use the term in your notes, write down your own definition and apply it the same way every time.
- Gap fill is more precise: price trades back to the previous close. Say whether you mean intraday (touched $2.40 at any time) or close-basis (closed below $2.40), because the two give different counts.
- Runner is slang for a stock making a large, sustained move, sometimes over several sessions. There is no threshold.
All three labels are applied after the fact. At 09:30, days A, B and C looked the same. The scalping course covers both setups in depth: Gap fills and Opening range breakouts on gappers.
What FennEdge's archive shows
FennEdge studied every stock in its Top Gappers archive that opened at least +20% above the previous close, from 2026-07-01 to 2026-10-02: common stocks and common-stock ADRs only, with both the open and the previous close at $0.10 or higher, and no volume minimum (FennEdge archive; as of 2026-10-02; see Small-cap gapper statistics (2026)). The published study is What time do small-cap gappers hit high of day?.
How to read these numbers:
- They are close-basis and hindsight groupings. "Closed below the open" compares two prices from the same day after it ended. It says nothing about the path in between, where a stop would have been hit, or what a trade would have earned after costs. None of these figures is a trade win rate.
- "Closed at or above the open" is not a gap-and-go rate. A stock can break its premarket high at 09:48 and still close below the open, or chop all day and close a cent above it.
- The close-basis fill rate is low; intraday fills are not measured here. 9.2% closed below the previous close. How often price touched the previous close during the session is a different question that this sample has not yet answered.
- Most highs came early, often before the open. In this sample, 53.1% of highs printed in the premarket. A gapper's best price of the day was often already behind it at 09:30.
- One window, one definition. Stocks that ran in premarket but opened below +20% are not in the sample, and other periods can give different numbers.
Gap up vs. gap down
Everything above applies in reverse to stocks that open below the previous close; FennEdge's article Gap up vs. gap down compares the two. Large opening moves can also run into LULD price bands once the session starts, which LULD price bands and trading pauses explains.
Observe vs. infer
Return to Ashgrove at 09:31 ET (fictional; all figures Illustrative).
| What the screen shows (observed) | What a trader might infer | Why it is not established |
|---|---|---|
| Open $3.12, previous close $2.40, gap +30.0% | "It's a strong gapper" | The gap is a fact; "strong" is an interpretation of what comes next |
| Premarket high $3.62 at 07:52 | "It needs to break $3.62 to go" | A level to watch, not a rule the stock follows |
| Archive: 438 of 671 (65.3%) of ≥20% gappers closed below their open (2026-07-01 to 2026-10-02, FennEdge archive, as of 2026-10-02) | "Ashgrove closes red today" | A base rate for a defined sample is not a forecast for one stock |
| News released at 07:05 | "The news explains the gap" | Plausible, but other buyers may have had other reasons |
Common mistakes
- Comparing gaps measured from different prices. A premarket change and an opening gap are different numbers.
- Using "gap and go" without a definition. Write yours down before you count examples.
- Treating the fill level as a target. The previous close is a reference level, not a place price has to return to.
- Reading close-basis statistics as trade results. "Closed below the open" is not "a short from the open made money".
Apply it in FennEdge
FennEdge's gapper tools are historical review tools: they show completed sessions, not live signals.
- Top Gappers Calendar ranks each trading day's opening-gap leaders by open versus previous close, with day change and volume. Guests, Free and Plus see the latest trading day at default filters; Pro and Max see the full history with filters (as of 2026-10-06).
- Gapper HOD Timeline shows when each gapper printed its high of day. The latest completed session is public; earlier sessions need Pro or Max.
- Top Gappers recaps list the 10 largest opening gaps each day, with the method stated.
- Live, the Scanner (paid: Plus, Pro or Max) has gap-up and gap-down browse panels and session-high panels that rank premarket, regular-session and after-hours highs against the previous close.
What this lesson does not claim
- It does not claim gappers tend to rise or fall. The archive figures describe one sample, one window and one definition.
- It does not define "gap and go" or "runner" officially; no official definition exists.
- Ashgrove Diagnostics and days A, B and C are fictional and Illustrative.
Go deeper
- FennEdge first. The study What time do small-cap gappers hit high of day? gives the full method and caveats for the archive figures. The Top Gappers Calendar and Gapper HOD Timeline let you review past gap days yourself.
- Setups, in the scalping course. Gap fills and Opening range breakouts on gappers.
- Glossary. Gap fill, opening range breakout, catalyst.
- Previous lesson: Float for day traders. Next: LULD price bands and trading pauses.
Exercises
Three exercises follow this lesson: a concept check on what "gap fill" means, a calculation of an opening gap from the right two prices, and a scenario that asks what an archive statistic does and does not show. Exercises, answers and progress are available after signing in.
Primary sources
- FINRA. Extended-Hours Trading: Know the Risks.
- Nasdaq. The Nasdaq Opening and Closing Crosses: frequently asked questions.
- FennEdge. What time do small-cap gappers hit high of day? (study method and sample; FennEdge archive, as of 2026-10-02).
Sources checked 2026-10-06. Ashgrove Diagnostics and all its prices are fictional and Illustrative. Archive figures come from the FennEdge archive (n = 671, sessions 2026-07-01 to 2026-10-02, as of 2026-10-02). This lesson is education, not a recommendation to buy or sell any security. Past patterns do not indicate future results.