What is a Gap Fill? Definition, Formula, and Example
A gap fill occurs when a stock's price retraces back through the price range it skipped over during an overnight or intraday gap, 'filling' the void left on the chart.
What is a Gap Fill?
A gap fill is the price move that closes a gap on a chart. A gap forms when a stock opens at a price with no overlap to the prior session's trading range — for example, closing at $100 and opening at $105 after earnings. The range between $100 and $105 contains zero traded volume. A gap fill happens when price later trades back down through that empty zone, touching the prior close at $100. A "full fill" reaches the pre-gap closing price; a "partial fill" enters the gap zone but reverses before completing it.
How a Gap Fill Is Identified
There is no formula — a gap fill is identified by two price levels:
- Gap boundary (top): the prior session's high (for gap-ups) or low (for gap-downs)
- Gap boundary (bottom / fill level): the prior session's close... more precisely, the prior session's low for gap-ups and high for gap-downs define the full gap zone
Gap size = |Open − Prior Close| / Prior Close × 100
A gap-up from a $100 close to a $105 open is a 5% gap. The fill is confirmed when price trades at or below $100 (the prior close) — the strictest definition — or when it trades below the prior session's high, which is the looser "gap zone" definition. Traders distinguish four gap types because fill probability differs: common gaps (low volume, fill quickly), breakaway gaps (new trend, fill rarely), runaway/continuation gaps (mid-trend), and exhaustion gaps (trend end, fill fast).
Worked Example: NVDA Earnings Gap
Suppose NVDA closes at $120.00 on earnings day and gaps up to open at $131.00 the next morning — a 9.2% gap. The gap zone is $120.00–$131.00. Over the following three sessions, profit-taking drags the stock down: $128, then $124, then it prints $119.80 intraday. The moment it trades at $120.00, the gap is officially filled. A trader who shorted the failed breakout at $129 with a target at the fill level captures the entire $9 move; a trader waiting to buy "the fill" gets their entry at $120 with a stop just below it.
When Traders Use Gap Fills
Gap fills anchor two common strategies. First, gap-and-fade: when a stock gaps on thin news or low pre-market volume, traders short the open expecting a fill, targeting the prior close. Second, gap-and-go failure: when a breakaway gap loses momentum intraday and re-enters the gap zone, the fill level becomes the stop or reversal trigger. Swing traders also use unfilled gaps as magnets — price frequently revisits old gap zones weeks later, making them natural support/resistance areas on the chart.
Limitations and Common Misconceptions
The phrase "all gaps eventually fill" is wrong. Breakaway gaps on genuine catalysts — a major earnings beat, an acquisition — routinely never fill; AAPL has unfilled gaps from years ago. Fill statistics also depend entirely on gap type and size, so quoting a single "fill probability" is meaningless. Another misconception: a fill is not automatically a reversal. Price can fill a gap and keep going straight through it. Finally, gap zones on illiquid small-caps are noise — spread width alone can manufacture apparent gaps.