What Is Pre-Market Trading?
Pre-market is the trading session that runs before the regular market open, conducted on electronic venues with far less participation than regular hours. Volume is a small fraction of the normal session, spreads are wider, and prices reached in it frequently do not hold when the regular session begins.
The pre-market session is a real market with very few people in it. That single fact explains the wide spreads, the dramatic-looking moves, and why the opening price so often lands somewhere else entirely.
How it works
It runs on electronic venues before the regular open. Brokers differ in how much of that window they give clients access to, and some offer none of it.
Participation is a small fraction of normal. Fewer participants means fewer resting orders, which means a thinner book at every price level.
Order types are usually restricted. Many brokers accept only limit orders in this window, which is a sensible restriction given what a market order would do in a book that thin.
Why the prices behave differently
A small order moves price a long way. With little resting size, a modest trade consumes several levels of the book and prints a price far from the previous one.
So the percentage move overstates the information. A stock shown up 8% pre-market may have moved on a few hundred shares, which is a fact about the book rather than about the company.
And the regular open is a separate event. It is an auction that collects overnight interest in one place, and it prices against far more participants than the session before it.
A worked example
Take a stock that reports earnings before the open. The news lands, pre-market trading begins, and a price appears well above the previous close.
That price was set by whoever was awake and willing. The size behind it is generally small, and it can move again several times before the regular session starts.
At the open the auction reprices everything. Institutions that were not trading pre-market participate, and the resulting price frequently sits some distance from the pre-market quote.
Anybody who bought into the pre-market move is now marked against that. The pre-market print was real - it simply was not a price the whole market had agreed to.
What it is actually useful for
Seeing that news exists. A stock quoted far from its close before the open is telling you something happened, which is worth knowing before you look at anything else.
Ranking the day’s candidates. A list of names moving pre-market is a reasonable starting shortlist for a session, on the same footing as any other screen.
Judging the scale of a reaction. A 2% pre-market move and a 20% one are different in kind, even if neither number survives the open exactly.
And nothing beyond that. It is an early indication with a thin sample behind it, and treating it as a level you can act on is where the usefulness ends and the trouble starts.
One more caution about the quote itself. Different data providers show different pre-market prices, because they draw from different venues and the venues are not consolidated the way they are in regular hours - so two screens can disagree about what a stock is doing before the open.
The original data
On this site’s shared series: median bar range 0.493, ninetieth percentile 1.101, largest bar 2.338. Direction runs average 2.01 bars with a longest of 11. A round trip costs 0.0098, about 2% of the median bar range.
The 0.0098 round trip is a liquid-market figure. In a book as thin as the pre-market session, the real cost of entering and leaving is a multiple of it, which is the single most important adjustment to make before trading that window.
And the largest bar at 2.338 against a median of 0.493 shows the scale of a fast move. Pre-market moves routinely exceed that proportion, on a fraction of the volume, which is why they are so often retraced.
Trading it, if you are going to
Use limit orders only. A market order in a thin book fills wherever the book runs out, and that is usually a long way from the quote.
Size down, substantially. The position that is reasonable at midday is not the position that is reasonable in a session with a tenth of the participants.
Check what volume produced the price. A move on a few hundred shares and a move on a hundred thousand are different events displayed identically.
And decide whether you are trading it or reading it. Watching the pre-market to plan the regular session is sound; trading it because a number looked large is a decision made on the thinnest data of the day.
When it fails
The characteristic failure is acting on the headline percentage. A stock quoted up 12% before the open looks like a move already underway, and the reasonable response feels like joining it.
Then the open reprices it. The auction brings in participants who were absent, the price settles somewhere else, and the position taken pre-market was entered at a level that existed for a handful of shares and a few minutes.
A second failure is placing market orders, which most brokers restrict for exactly this reason.
A third is assuming your broker offers the full window, when access varies and often starts late.
A fourth is reading a pre-market chart as a normal chart, where each bar may represent a handful of trades.
And a fifth is expecting the direction to persist. The open is a separate auction with a different population, and it is under no obligation to agree.
Related
After-hours covers the equivalent session on the other side of the day. Liquidity covers why a thin book prices badly. And volume covers the figure that tells you how much a move is worth.
Pre-market prices get quoted everywhere as if they were the market. They are a handful of participants trading in a thin book. It is worth watching for what it tells you about the news, and it is not worth treating as a price you could have had - because the size that produced it is usually tiny.
— Michael Whitman
This page is educational, not financial advice. Test every idea on your own charts before risking money.