On-balance volume (OBV), creates a running total of positive and negative trading volume for a stock or security. One of the original momentum oscillators, OBV spawns from Joe Granville's theory that volume precedes price in an instructive, measurable fashion. The formula's calculation is simple, rising whenever volume on up days is greater than volume on down days and vice versa.
To measure a security's OBV, you need to understand the relationship of closing prices between two successful trading days. When the second day's price closes above the prior day's close, OBV = previous OBV + current trading volume. If prices close lower on the second day, OBV = previous OBV - current trading volume.
Despite being plotted on a price chart and measured numerically, the actual individual quantitative value of OBV is not relevant. The indicator itself is cumulative, while the time interval remains fixed by a dedicated starting point, meaning the real number value of OBV arbitrarily depends on the start date. Instead, traders and analysts look to the nature of OBV movements over time; the slope of the OBV line carries all of the weight of analysis.
Analysts look to volume numbers on the OBV to track large, institutional investors. They treat divergences between volume and price as a synonym of the relationship between "smart money" and the disparate masses, hoping to showcase opportunities for buying against incorrect prevailing trends. For example, institutional money may drive up the price of an asset, then sell after other investors jump on the bandwagon. (For related reading, see "On-Balance Volume: The Way to Smart Money.")