Volume and dollar volume are different
Volume is the number of shares traded in a day, and dollar volume is that number multiplied by price. A low-priced stock shows a large volume figure even when the same amount of money changes hands, while a high-priced stock looks thinly traded. Comparing raw volume across different stocks is therefore misleading. Dollar volume works better for comparing interest between stocks, and volume works better for tracking day-to-day changes within one stock. A stock split increases the share count and makes volume look like it suddenly jumped, so when comparing before and after a split, check whether the volume is split-adjusted.
Volume only means something compared with normal
A single volume bar cannot tell you whether volume is high or low. What matters is how far it departs from the stock's average over the last few weeks. When price moves sharply on volume well above normal, many traders read it as broad participation behind the move, and a move on light volume is often seen as easier to reverse. That describes a tendency, not a rule. Sometimes the reason for a volume spike is not company news but the calendar, and the main examples are options expiration and scheduled index changes.
When monthly options expire
Standard monthly options listed in the US usually expire on the third Friday of each month. If that day is a market holiday the schedule can move earlier, so check the exchange's official calendar for exact dates. On the third Friday of March, June, September and December, stock index futures, index options and stock options expire together, a day often called triple witching or quadruple witching. Today there are also many weekly expirations, and some index products expire even more often. In Korea, monthly KOSPI 200 options expire on the second Thursday, so the date rule differs from the US.
What shows up on the chart around expiration
On expiration day, trades to close or roll options positions pile up, along with stock and futures trades made to hedge those options. Volume can therefore rise well above normal even without any company news. On intraday charts you may see volume surge in the last few minutes as orders crowd into the closing price process. Some explanations say price tends to stay near strikes with large open positions, but that does not always happen and is hard to verify. The point for chart reading is not to treat an expiration-day volume spike as fresh interest in the company.
- Check the calendar to see if the high-volume day is a third Friday
- In a quarter-end month, check whether index futures also expired
- Use intraday bars to see whether trading crowded into the final minutes
- See whether volume returns to normal the next session
Index rebalancing inflates volume too
Major indexes adjust their members and weights on a regular schedule, and those dates often line up with quarterly options expiration. Funds that track an index must trade at the closing price on the change date, so stocks being added or removed can see huge volume that day. That volume comes from index rules and may have little to do with the company's business outlook. Index providers announce changes and dates in advance, so when you see unusual volume, check whether such an announcement was made.
Compared with Korean stocks
The Korean market collects orders during the last ten minutes of the regular session and sets the close in a single auction, and the US market also uses a closing auction. In both, trading crowds around the close, so a large share of daily volume can come from the final minutes. In Korea, program trading clustering on options expiration days and shaking the index near the close has been discussed for a long time, and heavily weighted names such as Samsung Electronics and SK Hynix tend to see higher volume that day. Large US tech stocks with heavy index weights can show a similar pattern.
Checking with this site's live tools
The large-cap scanner for Korean and US stocks shows many names' moves and trading at once, making it easier to tell whether one stock or the whole market saw more trading. The global markets and FX overview shows major indexes together around expiration, and the monthly returns tool for stocks and indexes lets you look at past records for a given month. Past records only describe what happened in that period; they do not mean the next expiration will look the same.
Summary and caution
Compare volume with normal, use dollar volume to compare interest across stocks, and first check whether a volume spike lines up with calendar events such as options expiration or index changes. Price action around expiration has structural causes and does not reveal direction. Options are complex products, and the selling side can lose far more than it collected, so read the risk disclosures carefully before trading them. This article is not investment advice and does not recommend any trade.
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