Trading Volume
Trading volume is the number of shares (or the dollar value) traded in a stock over a period, usually a day. It is the simplest and most widely available liquidity proxy: high-volume stocks are generally easier to enter and exit, because a given order is small relative to the flow of other traders. Volume drives the denominator of many liquidity ratios, including the Amihud illiquidity measure and turnover. But raw volume is a crude gauge on its own. It can spike on bad news as everyone rushes for the exit, so a volume surge can signal stress rather than healthy liquidity. Volume must be read alongside the spread and price impact, not in isolation.
Why it matters
Volume is the crowd at the marketplace. The bigger the crowd of other buyers and sellers, the more easily your own order gets absorbed without anyone noticing. That is why traders compare their order size to average daily volume before trading. But a crowd can also be a panic. A huge volume day during a crash is not a sign of comfort, it is everyone trying to leave through the same door at once, and prices gap as they go. So volume tells you how busy the market is, not whether trading is cheap.
Formulas
Worked examples
Stock A trades 5,000,000 shares a day; Stock B trades 80,000. You must buy 100,000 shares of each in one day. What does volume tell you about the difficulty?
For Stock A the order is 2% of daily volume, a routine trade absorbed easily with little impact. For Stock B the order is 125% of an entire day’s volume, so buying it in one session would demand far more shares than normally change hands and push the price up sharply. Volume relative to order size, not the absolute share count, signals the liquidity challenge. Stock B will cost much more in price impact.
Common mistakes
- ✗High trading volume always means a stock is liquid and safe. Volume can spike during panics as everyone sells at once, so a surge may signal stress and gapping prices, not cheap trading.
- ✗Volume alone fully measures liquidity. Volume is one input. Two stocks with identical volume can have very different spreads and price impact, so it must be read with the spread and the Amihud ratio.
- ✗Share volume is comparable across stocks. A 1-dollar stock and a 200-dollar stock with the same share volume trade vastly different dollar amounts, which is why ratios use dollar volume.
Revision bullets
- •Volume = shares (or dollars) traded over a period
- •Simplest, most available liquidity proxy
- •Feeds the Amihud ratio and turnover denominators
- •Compare order size to average daily volume (participation rate)
- •A panic-driven volume spike signals stress, not healthy liquidity
Quick check
Why can a sudden surge in trading volume be a warning sign rather than a sign of good liquidity?
A trader compares a 200,000-share order to a stock’s average daily volume of 400,000 shares. This 50% figure is the
Connected topics
Sources
- Jorion, FRM Handbook (2011)Jorion, P. Financial Risk Manager Handbook. 6th ed. GARP / Wiley, 2011.Treats trading volume as a primary, if crude, indicator of market liquidity.
- Amihud, Y. "Illiquidity and Stock Returns: Cross-Section and Time-Series Effects." Journal of Financial Markets, 5(1), 31–56, 2002.Uses daily dollar volume as the denominator of the illiquidity ratio.