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Reading the session · 4 min read
Gap up, gap down, and premarket change: which price are you comparing?
Learn how gap up and gap down are calculated, how opening gaps differ from premarket changes, and why the reference price and timestamp matter.
“This stock is up 20% today” sounds clear, but it leaves out a crucial question: compared with which price? Premarket change, the opening gap, and the latest move can all be correct while describing different things.
Gap: the open versus the previous regular-session close
When discussing a daily open, gap up usually means opening above the previous regular-session close; gap down means opening below it. A common opening-gap formula is (open ÷ previous close − 1) × 100%. Other chart contexts also use “gap” for spaces between bars, so establish the definition first.
U.S. regular trading generally runs from 9:30 a.m. to 4 p.m. Eastern, except early-close days. Nasdaq’s market page distinguishes premarket and regular sessions, and its opening and closing cross FAQ explains official opening prices. A provider’s first trade, first chart point, and official opening price need not always match.
One day, four different percentages
These prices are entirely fictional: previous close $10; one premarket observation $11; regular open $12; a later observation $11.50.
| Question | Comparison | Result |
|---|---|---|
| Premarket versus previous close | 11 ÷ 10 − 1 | +10% |
| Opening gap | 12 ÷ 10 − 1 | +20% |
| Latest versus previous close | 11.50 ÷ 10 − 1 | +15% |
| Latest versus open | 11.50 ÷ 12 − 1 | About −4.17% |
Why do two pages show different changes?
Check the quote timestamp, whether extended hours are included, whether the previous close reflects corporate-action adjustments, and where the opening reference comes from. A premarket percentage is a changing observation, not an already-established regular-session opening gap.
Review the session with FennEdge
Start with the Top Gappers Calendar to inspect historical samples by trading day. Then use Gapper HOD Timeline to see when those stocks reached their daily highs. A historical high is known afterward; it was not an advance signal available at that moment.
FennEdge currently calculates historical gaps using the provider’s first regular-session chart price versus the previous regular-session close. That is not independent verification of the official opening-auction price. Keep the reference and timestamp in your notes.
Align the reference before explaining the move
A gap describes a price difference. It does not promise continuation, a reversal, or a gap fill. Add price, market-cap, and float context, then look for evidence in news, disclosures, and trading data.
This article explains common concepts; examples are not trading recommendations.