A gap happens when a stock opens at a meaningfully different price than where it closed the day before. The difference between yesterday's close and today's open is the gap. It forms in pre-market and after-hours trading, and when the regular session opens at 9:30 AM, the first trade starts at that new price level.
Gaps are one of the most reliable sources of intraday setups. The first 30-60 minutes after a gap open follow predictable patterns often enough that experienced traders build entire strategies around them. Not because gaps are magic, but because they create a specific psychological and structural situation that tends to resolve in predictable ways.
There are two things a gap can do: it can continue (gap-and-go), or it can reverse and fill (gap fill). Learning to distinguish between the two is where the real skill is.
Why gaps happen
Gaps form because information arrives outside market hours. The most common causes:
Earnings reports. A company beats estimates and the stock gaps up 8% before the open. The market is repricing based on new information.
News events. FDA drug approval, merger announcement, executive departure, macro data release (jobs report, CPI print). Any material news that arrives overnight creates a gap.
Sector or market momentum. Sometimes a stock gaps simply because the overall market or its sector is gapping. SPY up 1.5% pre-market will lift most stocks with it.
General pre-market activity. Some gaps form slowly through accumulation of buy or sell orders in thin pre-market volume, not from a single news event.
The cause of the gap matters for how you trade it. News-driven gaps with strong fundamental catalysts tend to be more sticky (gap-and-go). Sympathy gaps dragged up by the market tend to fill more often.
The 30-minute opening range rule
Before I place a single trade on a gapping stock, I wait for the first 30 minutes to develop. The high and low of that first 30-minute candle become the "opening range." This range is one of the most watched levels in day trading.
Breaking above the 30-minute high signals bullish continuation. Breaking below the 30-minute low signals reversal or further selling.
This matters because the first 30 minutes after the open is price discovery. Institutions are reacting, retail traders are chasing, algorithms are adjusting. The price swings hard in both directions. Entering during this window without the opening range defined is essentially guessing.
After 9:50-10:00 AM, the range has settled and a direction has usually established itself. That's when I start trading.
Gap-and-go strategy
Gap-and-go means the stock gaps up (or down), and then continues moving in the direction of the gap throughout the day.
The setup for a bullish gap-and-go:
Stock gaps up 3%+ on meaningful volume (at least 2x average daily volume in the pre-market alone is a good sign). The stock has a clear fundamental catalyst, not just riding the overall market. During the first 30 minutes, price holds above the gap (doesn't fill the gap). The 30-minute candle closes in the upper half of its range. Volume is expanding or at least maintaining pace.
Entry: a breakout above the 30-minute high after 10:00 AM.
Stop: below the 30-minute low. If the gap-and-go thesis is correct, the 30-minute low should not be breached.
Target: I don't use a fixed target on gap-and-go trades. I trail my stop using the 5-minute chart structure, exiting when price makes a lower low on the 5-minute. These can run for 1-3 hours on a strong day.
Why gap-and-go stocks continue: when a stock gaps up on earnings or major news, many traders who held the stock overnight see gains and might be tempted to sell. If the stock absorbs that selling pressure and stays above the gap level, it signals that demand is overwhelming supply. The buyers are in control. Latecomers who missed the pre-market move then chase, pushing the stock higher throughout the day.
Gap fill strategy
"Gaps always fill" is one of those trading maxims that sounds authoritative and is only sometimes true. Studies vary, but roughly 60-70% of gaps fill at some point, though the timeframe can range from the same day to months later.
The gap fill strategy is simpler: you bet the gap reverses and price returns to the previous close.
The setup for a bearish gap fill (stock gapped up, you expect it to fill):
Stock gaps up 2-4% but on average or below-average volume (weak conviction behind the gap). The gap is NOT explained by a strong fundamental catalyst. The sector or overall market is not supporting the move. The first 30 minutes shows price immediately starting to fade from the open.
Entry: short below the 30-minute low (or long below the prior close on a downside gap).
Stop: above the 30-minute high.
Target: the prior day's close (the full fill) or the 50% fill level (halfway between yesterday's close and today's open).
The logic: without a strong catalyst, the gap is likely driven by pre-market thin liquidity and will mean-revert when real volume enters at 9:30 AM. Institutional traders often sell into the open on muted-catalyst gaps because they have better information and know the gap was overdone.
How to tell gap-and-go from gap fill
This is the actual skill, and it's not always obvious in real time. Here are the filters I use:
Volume
This is the most reliable tell. A gap-and-go has huge volume. An earnings gap that's going to run will often trade 50-100% of its average daily volume in the first hour alone. The buying pressure is relentless.
A gap fill has weak volume. The stock gapped up, but the volume supporting the open is thin. Nobody with conviction is buying it at these elevated prices.
The nature of the catalyst
Strong catalyst (EPS beat of 20%+, FDA approval, major acquisition at a premium): gap-and-go is more likely.
Weak or no catalyst (sympathetic move with a sector, analyst price target increase, vague positive headline): gap fill is more likely.
Pre-market behavior
If the stock gapped up and then continued higher in pre-market right up to the open, buyers are aggressive. Gap-and-go probability is higher.
If the stock gapped up and then churned sideways or faded a bit in the final 30 minutes of pre-market, some of the initial enthusiasm has already unwound. Gap fill probability increases.
The first 5 minutes
When the stock opens at 9:30 AM, watch the initial candles on the 1-minute chart. Buying pressure (large green candles, high volume) within the first 5 minutes strongly suggests gap-and-go. Immediate selling (red candles within 2-3 minutes of open) on high volume often signals the gap is already under distribution and will fill.
I've seen experienced traders make their gap trade decision in the first 5 minutes with a high degree of accuracy, just from reading those initial candles. This takes practice to develop but it's a learnable skill.
Partial gaps vs full gaps
A full gap happens when today's open is completely outside yesterday's range. Yesterday closed at $50, today opens at $54. The entire gap ($4) is open space with no prior trading there.
A partial gap (also called a common gap) happens when today's open is above yesterday's close but still within yesterday's range. Yesterday traded between $48 and $54 and closed at $50. Today opens at $52. There's a gap from $50 to $52, but the stock is still inside yesterday's overall price range.
Full gaps are stronger signals than partial gaps for both gap-and-go and gap fill setups. The price territory between yesterday's close and today's open represents a vacuum: no one traded there, which creates a binary outcome (price either fills the vacuum by returning to yesterday's close, or the gap is accepted and price discovers new territory above it).
Common gap types and their tendencies
Not every gap is a trading opportunity. Some gap types fail more often than others.
Earnings gaps on major beats (EPS 15%+ above estimates, revenue beat, raised guidance): these tend to gap-and-go. The fundamental repricing is real and institutional money will chase them for days.
Earnings gaps on in-line results ("beats" of $0.01 per share): these often gap-and-go initially but reverse by midday. The initial reaction outpaces the quality of the result.
Sympathy gaps (stock gaps because a competitor or sector peer reported earnings): these fill more often than fundamental gaps. The connection to the sector leader is real but usually weaker than a direct catalyst.
Market-wide gaps (SPY gaps up, everything gaps up): these are tricky. Stocks with their own strong technical setup and relative strength tend to gap-and-go. Weak stocks being dragged up by the market often give you the first real selling opportunity when the market opens.
Practice reading gap opens
Common questions
Do gaps always fill? No, but most eventually do, somewhere between 60-70% depending on the dataset. The timing varies enormously: some fill the same day, some fill months later. Don't rely on "gaps always fill" as a rule for short-term trades without other confirmation.
What's the best gap size to trade? For gap-and-go strategies, I prefer 3-8% gaps. Under 3%, the move is small and the potential gain is limited. Over 10%, the stock is often overextended and more prone to intraday reversal. For gap fill strategies, 2-5% gaps tend to fill more reliably than larger gaps (larger gaps often have stronger catalysts behind them).
Can I trade gaps on ETFs? Yes. SPY and QQQ gaps are extremely tradeable because the liquidity is so deep. The opening range breakout strategy works well on index ETFs. The gap-and-go tends to be less explosive than individual stocks but more consistent.
Should I trade the gap open on earnings day for stocks I hold? Be careful. If you're long a stock going into earnings, and it gaps up 10% at the open, you face a decision: sell into the gap, hold for more, or sell part. The gap-and-go vs fill analysis matters here. If volume and catalyst support continuation, holding or trimming makes sense. If the reaction looks overextended, taking profit at or near the open can lock in gains before the fill.