Volume and on-balance volume (OBV)

Volume is the number of shares that change hands during a period, usually shown as bars beneath the price chart. Unlike most indicators, it comes from trading activity rather than from price, and it gives a sense of how many participants are behind a move. On-balance volume (OBV) goes one step further by turning daily volume into a running total that rises and falls with price direction.

What volume tells you

A price move on heavy volume involves many buyers and sellers, which is generally taken as stronger evidence than the same move on thin volume. A stock that gaps higher on several times its usual volume has attracted a lot of attention, whereas a drift upward on dull volume may reflect only a small number of trades.

Useful ways to judge volume:

• Compare to a baseline. Raw volume is hard to interpret on its own. Compare each day to the stock's average, such as its 50-day average volume. A ratio of today's volume to that average is called relative volume.
• Volume at key moments. Breakouts, breakdowns and reversals that come on volume well above average are generally taken more seriously. See breakouts and false breakouts.
• Trend versus volume. In a healthy advance, volume often rises on up days and eases on pullbacks. If a rally keeps rising while volume fades, fewer participants are supporting it, which is a caution rather than a signal.
• Spikes. Sudden volume spikes often accompany earnings reports, news, analyst actions and index changes. A spike by itself does not say whether buyers or sellers were in control, since every trade has both a buyer and a seller. The price action on the day gives the direction.

Volume needs care in some situations. It is measured in shares, so a stock split changes the share count and volume history should be adjusted to compare periods across the split. Index rebalancing days and options expiration days can also produce heavy volume that has little to do with a change in investor conviction.

The OBV formula

If today's close > yesterday's close:  OBV = yesterday's OBV + today's volume
If today's close < yesterday's close:  OBV = yesterday's OBV - today's volume
If today's close = yesterday's close:  OBV = yesterday's OBV

OBV was popularized by Joseph Granville. The idea is that volume flows into a stock on up days and out of it on down days, so the running total shows whether volume is accumulating on the buying side or the selling side.

Example (illustrative numbers)

Start OBV at 0 and add each day's volume with the sign of the price change.

OBV ends up at 3.5 million. The absolute number has no meaning, because it depends entirely on the day you started counting. What matters is the direction and shape of the line, and how it compares with price.

How to read OBV

• Confirmation. If price makes a new high and OBV also makes a new high, volume is supporting the advance.
• Divergence. If price makes a new high but OBV does not, volume has not kept pace with the price. That is a caution, not a sell signal, and it can persist for a long time. The same logic applies to lows. The idea resembles RSI divergence, though based on volume rather than momentum.
• OBV breaking out first. Sometimes OBV pushes above its own earlier highs while price is still in a range. Some chart readers treat this as a hint that buying is building, but it is only a hint.
• Trend lines on OBV. Because OBV is a line, readers sometimes draw trend lines on it or compare it to its own moving average.

Common mistakes and limits

• All-or-nothing counting. OBV assigns the entire day's volume to the up side or the down side, even if the stock closed up by only one cent. A tiny gain and a decisive rally on the same volume add exactly the same amount.
• Reading the level. The OBV value depends on the starting date and has no meaning on its own. Compare shapes, not numbers, across different stocks.
• Large spikes distort it. One enormous-volume day, from a block trade or index event, can dominate the line for months.
• Volume is not direction. High volume on its own is neither bullish nor bearish. Always read it together with price.
• Not a predictor. OBV reflects what has already traded. It can help show whether a trend is well supported, but it does not forecast what comes next.

For the wider framework, see technical analysis basics. Volume also helps in judging whether a move through a support or resistance level is worth trusting.

Where to see it on StocksXray

The scanner has a volume spike preset that surfaces stocks trading at unusual volume. Because StocksXray uses end-of-day data, the result reflects the completed session rather than intraday activity. Open the stock's chart to see which way price moved and whether the spike coincided with a breakout, an earnings reaction or something else.

Key takeaways

• Volume measures participation; compare it with the stock's own average rather than reading raw numbers.
• Moves on above-average volume are generally treated as more convincing than moves on thin volume.
• OBV adds volume on up days and subtracts it on down days; its level is arbitrary, and its direction and divergences are what matter.
• Volume spikes show attention, not direction, so always read them with price.
• Adjust for splits, and be aware that index and options events can inflate volume.