Every morning, scanners fill up with pre-market gappers: stocks trading well above or below the previous close before the regular session opens. A list shows where prices have moved. It does not tell you which moves deserve your attention.
This guide covers the step between the list and any decision: how to evaluate a pre-market gap, what to check, and what can change once the bell rings. It contains no picks or predictions and does not cover entries. For a gap-based strategy, see Gap and Go.
What is a pre-market gapper, and how do you measure gap percentage?
A gap is the difference between the previous regular-session close and a current pre-market price:
Gap % = (pre-market price − previous close) ÷ previous close × 100
Hypothetical example: a stock closed at $2.00 yesterday and its last pre-market trade printed at $2.60. The calculation is ($2.60 − $2.00) ÷ $2.00 × 100 = 30%.
Now suppose the quote is $2.50 bid and $2.70 ask. Measured from the bid, the gap is 25%; measured from the ask, it is 35%. The same stock can show different gap figures depending on whether the scanner uses last trade, bid, ask or midpoint. Check the method and timestamp before comparing names. A stale print describes an earlier moment, not the current market.
A price gap is not a setup
The gap size tells you how far price has moved. It does not tell you why it moved, who is trading or whether the price can hold. Two stocks with the same 30% gap can have different conditions: one might have a specific, verifiable announcement and steady trading; another might have a vague headline, only a few prints and a wide quote.
Treat a gapper list as a starting set to investigate, not as a list of trades. Building a Scanner and Watchlist covers how to build the list. Greenwick’s Day Trade Dash review shows one example of scanner columns such as gap, float and relative volume. Those are inputs to judgment, not recommendations.
Volume, liquidity and spread before the open
Pre-market trading can be less liquid, have wider spreads and be more volatile than regular hours. The SEC’s Investor Bulletin on extended-hours trading also cautions that prices outside regular hours may not carry over to the open.
For each name, check:
- Prints: Is trading continuous, or are there isolated trades?
- Volume: How does pre-market activity compare with this stock’s typical activity?
- Spread: How wide is the bid-ask spread in cents and as a percentage of price?
- Depth: Is there visible size near the quote, or only small displayed amounts?
A large gap formed by a few small prints and a wide spread may tell you more about thin trading than broad interest.
Catalyst and news reliability
Ask what caused the move. Catalysts: Why This Stock, Why Today explains what counts as a catalyst. Then check whether the specific news is reliable:
- Source: Is it a company release or filing, or only an aggregator headline, social post or recycled story?
- Timestamp: Was it published today, or is older news circulating again?
- Substance: Does the original item name figures, parties or dates, or use vague language?
- Consistency: Does the original document actually support the headline?
If you cannot find the original source, treat the catalyst as unconfirmed.
Float and supply
Buying or selling pressure can move stocks with fewer tradable shares more sharply, in either direction. Supply can also change: registered shares, warrants or a recent offering may add potential sellers. Float estimates can differ across providers and lag corporate changes. When share structure matters, check recent filings. Float, Share Structure and Dilution explains what to look for.
What can change after the open?
Pre-market conditions are a preview, not a promise. When regular-session trading starts, liquidity, spread and price behaviour can all change. The SEC notes that extended-hours prices may not match the opening price. FINRA explains that order imbalances can delay an opening and that trading halts can interrupt quoting and trading. See Halts, Circuit Breakers and SSR for the mechanics.
Before the open, write down what would make a gap no longer worth watching—for example, discovering that the news is old, seeing a persistently wide spread or watching activity dry up. Recheck those conditions after the bell instead of assuming the pre-market picture still applies.
Pre-open evaluation checklist
This is an evaluation aid, not a trading signal:
- Measure the gap consistently and note the time of the last print.
- State the catalyst in one sentence specific to the company.
- Find the original release or filing and verify its date.
- Check continuous prints, volume relative to typical activity, spread and visible depth.
- Review float using more than one source and look for recent offerings, shelf registrations or warrants.
- Note possible opening delays or halts and how the spread might change.
- Write down in advance what would make the gap no longer interesting.
- Log the outcome, including names you skipped, so you can review your filters later.
FAQ
What are pre-market gappers? Stocks trading above (gap up) or below (gap down) the previous regular-session close before the market opens.
Where can I find a list? Many charting platforms and dedicated scanners provide one. Use it as a starting point for research, not as a recommendation.
Does a big gap mean a stock will keep moving? No. A gap describes a price change that already happened. It does not predict what happens next, and pre-market prices may not hold at the open.
Why do scanners show different gap percentages? They may use the last trade, bid, ask or midpoint and different snapshot times. Check the methodology before comparing.
Educational content only; not financial advice or a recommendation about any security. Examples are hypothetical. Day trading and extended-hours trading carry significant risk.
Sources: SEC: Extended-Hours Trading: Investor Bulletin · FINRA: Trading Halts, Delays and Suspensions