An overnight gap represents a discontinuous price jump occurring between yesterday's closing price and today's official market opening price. Because no trading occurred within this price window during normal market hours, price jumped the space.
Gap % = (Today's Open − Yesterday's Close) / Yesterday's Close × 100
Worked example: INTC
Intel closed yesterday at $35.00 and opened today at $33.70:
Gap % = ($33.70 − $35.00) / $35.00 × 100 = −3.70%
Because INTC opened $1.30 below its prior settlement, the stock registered a 3.70% gap down.
Same-day fill versus held status
The engine evaluates a single core question for every gap session: did price completely close the gap before the market close?
Both days gapped down by the same amount. Only the close decides which one filled.
Gap filled: the session's closing price trades back through yesterday's closing price.
Gap held: the session's closing price fails to reach yesterday's closing price, leaving a residual price void at the bell.
Temporal constraint
This metric evaluates same-day behavior exclusively. It does not track whether a gap fills days or weeks later, only whether it closed by the bell on the session it opened in.
The headline fill rate quantifies the historical frequency with which gaps of comparable magnitude and direction have closed by the end of the session for that specific ticker.
Magnitude-ranked matching
To build an accurate historical sample, today's gap is ranked against every historical gap in the stock's own record:
Directional separation: gaps down are matched exclusively against past gaps down.
Size proximity: the engine selects past sessions with the closest percentage gap magnitude.
INTC's 3.70% gap down is among the largest it has printed, isolating 94 matched historical sessions, which yielded a 10% same-day fill rate.
Fill rate decay across gap sizes
A common trading myth posits that gaps always fill. On a same-day horizon, empirical data demonstrates the exact opposite: fill rates drop sharply as gap magnitude increases.
Gap size
SPY fill rate
INTC fill rate
Structural interpretation
Under 0.25%
40%
46%
Minimal displacement; highest fill frequency
0.25 to 0.5%
28%
39%
Minor displacement; moderate resistance to fill
0.5 to 1%
19%
30%
Structural displacement; fill rates decline below one third
Severe institutional gap; fills roughly 1 in 11 sessions
Same-day fill rates by gap size, measured on SPY and INTC.
Both names fill small gaps most often and large gaps least. INTC sits above SPY at every size until the largest gaps, where both settle near 9%.
Key takeaway
Once an overnight gap exceeds 3.00%, same-day fill probabilities fall to roughly 9%. Expecting a large gap to fill before the bell runs directly counter to the historical odds.
Chapter 3
Decoupling Fills from Open-to-Close Drift
Because a gap fill is a binary metric (all or nothing), it cannot describe intraday price movement on its own. Evaluating open-to-close performance alongside the fill rate reveals the true behavioral personality of the session.
Two intraday measures
To measure intraday behavior independently of the overnight gap, the platform pairs the fill rate with two open-to-close metrics:
Average open-to-close return: the mean percentage change from today's opening price to today's closing price across the matched set.
Open-to-close green odds: the percentage of matched historical sessions that closed higher than their opening price.
Combining the fill rate with open-to-close metrics separates a partial recovery from a trend continuation, even when both share the same low fill rate:
Both days share a 10% fill rate, so neither gets back to yesterday's close. The open-to-close numbers are what tell them apart: a slight drift up on the left, a hard push down on the right.
Although INTC rarely filled its 3.70% gap down (10% probability), the stock exhibited a mild positive intraday drift (+0.15%) from the open, with nearly half (47%) of those sessions finishing above the open. This profile describes an asset that stabilizes at the open and holds its ground without recovering the overnight loss.
Chapter 4
The Landing Table
The front page runs the same analysis across every ticker at once, so you can see the day's gaps ranked without opening them one at a time.
The columns
Today's Gap %, how far this morning's open sat from yesterday's close.
Historical Fill Rate, how often gaps of that size on that stock closed by the bell.
Avg Open to Close and the green/red odds, the pair from Chapter 3.
Each figure is calculated from that ticker's own matched gaps. The numbers are therefore not comparable side to side in the way they look. Two rows both showing 20% are each describing their own stock.
Every column sorts. Sorting on Today's Gap % puts the morning's biggest movers at the top, and sorting on the fill rate is the quickest way to see which of them history says are most likely to close the distance again.