Key Takeaways
- A gap-up screen filters for securities that opened today's session notably above the prior session's close, with no trades printed in between.
- Gap-ups commonly follow news released while the market is closed, earnings, guidance changes, upgrades, or deal announcements.
- Screens typically pair the gap size with volume, since a gap on heavy volume implies broader participation than a gap on thin volume.
- Gap fill, price trading back down to touch the prior close during the session, is tracked separately from the gap itself, since it shows whether the move is holding or being rejected.
- A gap up describes only the opening print relative to the prior close; it says nothing on its own about how the rest of the session plays out.
- Most gap-up screens are a starting filter, not a complete trade setup, they narrow a universe of symbols for further review, not a standalone signal.
Direct Answer
A gap-up screen filters for securities that opened today's trading session at a price notably higher than the prior session's close, leaving a visible gap on the chart with no trading in between. These screens are commonly built to include volume and gap-fill behavior, whether the gap gets retraced during the session, as extra criteria to judge whether the move is likely to hold.
What Is a Gap-Up Screen?
On a price chart, a "gap" is a visible break between one bar's range and the next, created because no trades occurred at the prices in between. A gap up happens when today's opening price sits above yesterday's closing price by a meaningful amount, the exact size is a setting a trader chooses, not a fixed exchange rule. A gap-up screen scans a universe of securities and returns the ones whose opening price cleared that threshold, so a trader can review a shortlist instead of checking every symbol individually.
Because a screen is only as useful as its filters, most gap-up screens don't stop at the gap percentage alone. Volume is a common companion filter, a security trading well above its typical volume alongside the gap suggests the move has real participation behind it, rather than being the product of a handful of trades in a thin market.
What Causes a Gap Up?
Gaps form when information reaches the market outside of regular trading hours, so the first trade of the new session already reflects it instead of the price working its way up gradually during the prior day. Common triggers include an earnings report that beats expectations, raised forward guidance, an analyst upgrade, a regulatory approval, or a takeover or merger announcement. Because the news lands while trading is closed (overnight, before the open, or after the prior close), the opening print jumps to a new level rather than drifting there.
Not every gap is news-driven, thinly traded securities can gap on comparatively small order imbalances at the open, which is one reason volume is checked alongside the gap itself rather than treated as a formality.
Gap Fill: Reading Whether the Move Holds
A gap fill is when price trades back down and touches the prior session's closing price during the current session, effectively closing the gap. Gap-fill behavior is tracked as a separate signal from the gap size itself, because the two answer different questions: the gap size shows how large the initial move was, while gap-fill behavior shows whether the market is defending that new price level or rejecting it.
A gap that fills quickly is often read as evidence the move lacked follow-through, buyers who chased the open ended up underwater, and sellers absorbed the initial enthusiasm. A gap that holds through the session, by contrast, is read as evidence the new information was durable enough that the market isn't giving the move back. Neither outcome is guaranteed by the gap alone; both are outcomes observed after the fact, which is why gap-fill status is something a screen tracks rather than predicts.
Building a Gap-Up Screen
A typical gap-up screen combines a small set of criteria rather than relying on the gap alone:
- Gap size, today's open compared with yesterday's close, usually expressed as a percentage.
- Volume, current or premarket volume relative to the security's typical volume, to gauge participation.
- Liquidity filters, a minimum price or average volume, to screen out securities too thin to trade practically.
- Gap-fill status, whether the gap has already been retraced back to the prior close during the session.
Different platforms expose these filters differently, and none of them are standardized across the industry the way an exchange rule would be, a "significant" gap on one screener's default settings may not match another's. The screen's job is to narrow a broad universe down to a manageable list; deciding what to do with any individual name on that list still requires reviewing the underlying news and chart.
Limitations and Common Mistakes
- Treating a gap up as automatically bullish, the gap only describes the opening print; plenty of gap-ups fade or fully fill before the close.
- Ignoring volume, a large percentage gap on thin volume can be far less meaningful than a smaller gap on heavy volume.
- Chasing the open, buying immediately at the opening price offers no confirmation that the gap will hold, and it's often the point of maximum emotional participation.
- Skipping the news check, a screen surfaces that a gap happened, not why; reviewing the underlying catalyst is a separate step the screen doesn't do for you.
- Assuming screener thresholds are standardized, "gap up" criteria vary by platform and by the settings a trader chooses, so results aren't directly comparable across tools without checking the underlying filter values.
The Open Is the Least Informed Moment of the Day
The temptation with a gap-up list is to act at the opening print, and that is the moment with the least information available and the most participation driven by reaction. Nothing about the session has been established yet: not whether the gap holds, not whether volume follows through, not whether the initial range gets accepted or rejected. Buying there means buying the news and the emotion around it rather than any confirmation.
The gap itself only describes the opening print. Plenty of gap-ups fade through the morning and some fill entirely before the close, which is why gap-fill behaviour is tracked separately from the gap: it is the evidence about whether the repricing is being accepted.
Volume is what separates a serious gap from a thin one. A large percentage move on light trade means few participants set that price, and it takes correspondingly little to move it back. A smaller gap on heavy relative volume often describes a more genuine change of view.
And the screen tells you that a gap happened, never why. Checking the underlying catalyst is a separate step, and it matters because a gap on an earnings surprise, a deal announcement and an analyst upgrade have different implications for what the rest of the session and the following week tend to look like.
Gap-Up Screen FAQs
What counts as a gap up?
A gap up happens when a security's opening price sits notably above its prior session's close, with no trades printed in between. There's no single fixed percentage that defines it across every screener, the threshold is a setting you choose, not a rule set by an exchange.
What causes a stock to gap up?
Gap-ups commonly follow news that lands after the prior close or before the next open, earnings beats, raised guidance, an upgrade, a takeover announcement, or other material news. Because the news arrives while the market is closed, the first trade of the new session reflects it immediately instead of trading up gradually.
What does gap fill mean?
A gap fill is when price trades back down through the gap and touches the prior session's closing price. A gap that fills quickly during the session is often read as a sign the gap-up move is being rejected; a gap that holds is read as a sign buyers are defending the new price level.
Why do gap-up screens include volume?
A gap on unusually high or above-average volume suggests broad participation behind the move, while a gap on thin volume can be more easily reversed by a small number of trades. Pairing gap size with a volume filter is a common way to separate gaps likely to hold from gaps likely to fade.
Is a gap-up always bullish?
No. A gap up only describes where a security opened relative to its prior close, it says nothing on its own about what happens for the rest of the session. Many gap-ups fade or fully fill before the close, which is why gap-fill behavior is screened alongside the gap itself rather than read as a standalone bullish signal.
What is the difference between a breakaway gap and an exhaustion gap?
They are classified by where they occur in a move rather than by anything visible in the gap itself. A breakaway gap opens a new move out of a consolidation; an exhaustion gap occurs late in an extended advance and is followed by a stall. The classification is applied afterwards, once it is known what followed, so a screen cannot distinguish them and neither can a chart at the time.
Should the screen exclude names that also gapped up the previous day?
It is worth deciding deliberately. Consecutive gaps compound, and because each percentage is computed against a base that has already jumped, the second gap is measured from an elevated reference. A name appearing on the list several days running is one continuing event rather than several. Whether to keep it depends on whether the screen is looking for new occurrences or for names currently in motion.
Does a constituent gapping up move its index?
Only in proportion to its weight. A small constituent gapping substantially is invisible at index level, while a heavily weighted one moves the index measurably. This matters when a gap-up list is being read as evidence about the market: the same list can be consistent with an index that barely moved, depending entirely on where in the weight distribution the names sit.
Why do gap-up lists cluster at certain times of year?
Because scheduled catalysts are themselves clustered. Reporting seasons concentrate company announcements into a few weeks, so the number of gaps rises sharply during them and falls between. Any statistic computed from the screen output therefore has a calendar pattern built into it, which is worth accounting for before reading a rise in the count as a change in market behaviour.