The simple version

Nvidia reported results after Wednesday's close, and by Thursday morning the percentage was in every headline before the market had opened.

That figure came from a session most investors never touch. It is produced by a comparatively small amount of trading routed through electronic systems rather than through the exchange, and the Financial Industry Regulatory Authority says directly that it does not change official closing prices or determine the next day's opening price.

The common assumption is that a premarket number is an inflated preview that fades once real trading starts. That is not reliably true either. It is simply a different measurement, taken in a different market, and it can land above or below where the session goes.

The numbers

• FINRA defines pre-market trading as typically taking place from 7 to 9:30 a.m. Eastern and after-hours trading from 4 to 8 p.m. Eastern, with overnight trading from 8 p.m. to 4 a.m. more recently available for certain stocks (FINRA)
• Rules among brokerage firms about when and how customers can participate may vary, including which venue is used, which products are eligible, and whether only limit orders are accepted (FINRA)
• The regular session has traditionally run from 9:30 a.m. to 4:00 p.m. Eastern, and access to extended hours grew with the rise of Electronic Communications Networks (U.S. Securities and Exchange Commission)
• The SEC lists lack of liquidity, wider spreads between bid and ask prices, price volatility, competition with professional traders, and computer delays among the risks of trading outside regular hours (SEC)
• Extended-hours trading does not change exchanges' official closing prices or determine the next day's opening prices; the prices recorded at 4 p.m. are the official closing prices regardless (FINRA)
• On August 27 Nvidia's premarket prices implied a range of about 5.7% to 8.0% above the prior close of $209.66, and the last premarket print was about 6.3% above it (market data, five-minute bars)
• It opened at $222.59, about 6.2% above the prior close. The regular session had not closed when this was written, so no closing figure is stated here (market data)
• Mutual funds and unit investment trusts generally must calculate net asset value at least once every business day, typically after the major U.S. exchanges close (SEC, Investor.gov)

Why a thin market moves further

A price is what one buyer and one seller agreed on. How much that price tells you depends on how many other buyers and sellers were available to disagree with it.

During regular hours an enormous number of participants are active, and the gap between the best offer to buy and the best offer to sell stays narrow. A single order barely moves the quote, because there is depth on both sides absorbing it.

Extended hours have a fraction of that participation, and the regulators describe the consequences rather than leaving them to inference. The SEC's own guidance names lack of liquidity, wider spreads between the bid and ask prices, and greater price fluctuations for stocks with limited trading activity. FINRA adds that stocks can be more volatile in these sessions because of the speed with which some investors react to earnings and similar announcements.

You can watch that thinness in the numbers themselves. Across Thursday morning, Nvidia's premarket prices implied moves ranging from roughly 5.7% to 8.0% above the previous close. That is a spread of more than two percentage points in the same session, on the same news, before the opening bell.

The official price is set somewhere else

This is the part that gets lost. Extended-hours prices are real trades at real prices, but they do not carry the official status that the regular session's prices do.

FINRA states it plainly: extended-hours trading does not change exchanges' official closing prices or determine the next day's opening prices, and the share prices recorded at 4 p.m. are considered the official closing prices for that day regardless of what happens afterward.

So a premarket quote is not an early version of the opening price. It is a separate market's opinion, formed by fewer participants, that carries no official weight and that the opening does not have to honor.

Thursday is a reasonable illustration of how loosely those two connect. The last premarket print sat about 6.3% above the previous close and the stock opened about 6.2% above it, which is close. But the premarket range had spanned from about 5.7% to about 8.0% across the morning, so which premarket moment you happened to read would have told you a noticeably different story.

The Real Cost lens on a number you cannot act on

For most households the premarket figure is not merely unofficial. It is unreachable, and for fund holders it is irrelevant by construction.

• Mutual funds and unit investment trusts generally calculate net asset value at least once every business day, typically after the exchanges close, so no premarket or after-hours print affects the price a fund holder receives
• Exchange-traded funds are a separate case, because they trade during the session like a stock, and the two should not be treated as the same product
• Extended-hours access varies by broker, and a firm may limit the venue, the eligible products, or accept only limit orders, so the session that produced a headline number may not have been open to you at all
• The SEC notes that many after-hours traders are professionals at large institutions who may have access to more information than individual investors
• None of that is a reason to seek access or to trade, and nothing here suggests acting on a premarket price is a good idea

The translation is narrow and worth keeping narrow. A premarket percentage is a real number from a small market that sets no official price. For anyone holding a mutual fund, it describes a moment that will never touch their account.

What this means

When a headline quotes a move before the opening bell, the accurate reading is that a smaller set of participants traded at that level, and that the exchange's official prices are determined elsewhere. It may prove close to the day's move, or the day may go further, or it may reverse. The premarket figure does not tell you which.

The broader habit is to ask how many participants produced a price before treating it as settled. That question applies well beyond stocks, to any market where the quoted number and the volume behind it get reported separately, or where the volume is not reported at all.

What this is NOT

This is not advice about trading, order types, timing, or extended-hours access, and it does not suggest anyone should or should not trade outside regular hours. This is not advice to buy, sell, or hold any security or fund, and Nvidia appears only as a dated factual example of price movement, with no view expressed on its results, its valuation, or whether any move was warranted. This is not a prediction of any price. The regular session had not closed when this was written, so no closing figure is asserted. Extended-hours availability, session times, and execution practices vary by broker and venue, and the published rules of each govern. This is not investment or financial advice of any kind.

Sources

• U.S. Securities and Exchange Commission, After-Hours Trading: Understanding the Risks: https://www.sec.gov/about/reports-publications/investorpubsafterhourshtm
• Financial Industry Regulatory Authority, Extended-Hours Trading: Know the Risks: https://www.finra.org/investors/insights/extended-hours-trading
• U.S. Securities and Exchange Commission, Investor.gov, net asset value: https://www.investor.gov/introduction-investing/investing-basics/glossary/net-asset-value
• U.S. Securities and Exchange Commission, Mutual Funds and ETFs, A Guide for Investors: https://www.sec.gov/investor/pubs/sec-guide-to-mutual-funds.pdf