Is your stock portfolio voting for the bull or the bear?© Lumos sp/stock.adobe.com, © prakasitlalao/stock.adobe.com; Photo illustration Encyclopædia Britannica, IncImagine for a minute that every trading day in the stock market is an election that offers investors one of two choices when they step into the voting booth: “I’m with the bull” (buy) or “I’m with the bear” (sell). The advance/decline (A/D) line, an old-school technical indicator, is one way to get a sense of who’s winning or losing and which direction the “popular vote,” or overall market, has been heading.

　　The advance/decline line tracks the difference between the number of stocks rising in price on a given day and the number falling. It’s typically applied to stocks trading on a major exchange like the New York Stock Exchange (NYSE) or Nasdaq, or an equity benchmark such as the S&P 500 or Nasdaq Composite. The A/D line’s roots in the U.S. markets stretch back to the 1920s and 1930s; the measure was popularized by Dow theory proponents such as Richard Russell, longtime author of the Dow Theory Letters newsletter.

　　The A/D line can also give you an idea of the “breadth” of a market rally or sell-off (i.e., what percentage of stocks are participating in a market move up or down). For example, as of February 2025, the top 10 stocks in the market capitalization–weighted S&P 500 comprised about 37% of the index; the top 26 stocks account for half the index of 500 stocks. On any given day, if a handful of large-cap stocks registered big gains but the vast majority of stocks were down, the index might register a gain—but the A/D line would tell a different story.

　　Many professional investors and traders follow the A/D line because it can help provide a sense of the market’s strength or weakness and overall market sentiment. Over the past few decades, the A/D line has also in hindsight appeared to signal or precede a major move or trend change in stocks.

　　Calculating the advance/decline lineThe base calculation for the A/D line starts with a simple question: For a specific set of stocks, how many of them were higher and how many were lower versus the previous day? And second, how does that data compare with the advances and declines from the day before?

　　Although an A/D line can be calculated on any subset of stocks (e.g., a specific index or stock sector), perhaps the most widely followed is the one that tracks the 2,200 actively traded stocks on the NYSE.

　　The first step in the A/D line calculation determines the baseline by subtracting the current day’s declining stocks from the current day’s advancing stocks, yielding a straightforward figure called net advances. For example, a day that saw 1,400 of the 2,200 NYSE stocks advance in price and 800 stocks decline generates a net advance of 600; if 1,200 stocks fell and 1,000 rose, you’d get a net decline of 200 (expressed as a negative, or-200, net advances).

　　The next step is to add the current day’s A/D net advances (or declines) to the previous day’s A/D value. If today’s net advances value was 600 and yesterday’s net advances value was 400, the current day’s A/D line would be (600 + 400) = 1,000. If the previous day had instead produced a net decline of 400 (i.e., -400 net advances), the current day’s A/D would be (600 – 400) = 200. This step helps in comparing A/D trends over periods of weeks or months.

　　The A/D ratio. The A/D line can also be expressed as a ratio. To calculate the advance/decline ratio, simply divide the number of advances on any given day by the number of declines that day. For the two examples:

　　Example 1, with 1,400 advances and 800 declines, would produce an A/D ratio of (1,400 ÷ 800) = 1.75.Example 2, with 1,000 advances and 1,200 declines, would produce an A/D ratio of (1,000 ÷ 1,200) = 0.833.A ratio above 1.0 implies that more stocks advanced than declined that day; a ratio below 1.0 indicates the opposite.

　　Interpreting the advance/decline lineThe A/D line can help ascertain the “health”—strength or weakness—of a market trend and gauge whether a trend may be about to reverse. A rising market with a consistently strong or positive A/D line would suggest most stocks and industries are participating in the gains and contributing to overall market strength.

　　One limitation of the A/D line is that it gives equal weight to all stocks, whereas the S&P 500 is weighted based on market cap. These days a few mega-cap technology stocks such as Apple (AAPL), Microsoft (MSFT), NVIDIA (NVDA), and Amazon.com (AMZN) make up an outsize portion of the index’s value. So if the S&P 500 is rising but A/D values are weak or negative, it could indicate that a rally is being driven by a relatively small number of large companies while most others are lagging. In this regard, the A/D line can potentially identify two types of “divergence”:

　　Bearish divergence. If the S&P 500 or other benchmarks are rising while the A/D line is turning down, that could be a signal that buyers are losing conviction and the market may be about to reverse lower. Bullish divergence. If the S&P 500 has been dropping but the A/D line is trending up, that could signal sellers are stepping back and the broader market may be about to turn higher.Broadly speaking, if market benchmarks and the A/D line have been moving in the same direction over time, investors may feel confident that a trend will continue. As you might expect, the A/D line for NYSE stocks during the bull market of 2023–24 was typically positive, averaging a net daily advance just above 107.

　　But the A/D line can swing sharply from day to day and can give misleading signals, particularly during volatile markets (see figure 1). It also gives equal weight to all companies, which can lead to mismatches in comparisons to cap-weighted benchmarks like the S&P 500. That means it’s important to compare A/D values over longer periods and also weigh them against other indicators, such as trading volume, moving averages, and the Relative Strength Index, for context and confirmation.

　　Figure 1: ADVANCING RESEARCH. A divergence between a cap-weighted index such as the S&P 500 (SPX – candlestick chart) and the NYSE advance/decline issues (NYAD – histogram) may signal a reversal in the overall market trend. Source: StockCharts.com. For educational purposes only.Source: StockCharts.com. For educational purposes only.Divergence between the A/D line and the broader market has foreshadowed major reversals at key moments in market history, according to the Corporate Finance Institute. One divergence happened in March 2008, about six months before the global financial crisis sent stocks into a steep nosedive. Another divergence unfolded shortly before the dot-com bubble burst in 2000.

　　The bottom lineWidely followed stock benchmarks like the S&P 500 and the Dow Jones Industrial Average are one way to track the market, but they don’t always tell the whole story. The top indexes may be saying one thing, but what’s happening beneath the market surface could be quite different. Indicators such as the A/D line can offer insights and nuances to help investors gain a fuller sense of the market and signal possible opportunities.

　　In other words, as you follow the market, don’t limit yourself to a single piece of data. You can track the headlines, but before committing to a trade or investment decision, consider seeking confirmation from the A/D line along with the fundamental and technical analysis tools available through your brokerage account or market research services, such as Bloomberg Terminal, Morningstar, or Zacks. ​

　　Specific companies and services mentioned in this article are for educational purposes only and do not constitute an endorsement.

　　ReferencesAdvance-Decline (A/D) Line | corporatefinanceinstitute.comBreadth Statistics: What Do They Tell You? | traders.com