Arms Index (TRIN)

A market breadth indicator developed in the 1960s by Richard Arms Jr. that measures whether market moves are broadly supported or narrowly driven. The Arms Index, also known as the TRIN, is calculated as the ratio of advancing-to-declining stocks divided by the ratio of advancing-to-declining volume. It reveals whether gains or losses have conviction. A reading below 1.0 indicates strong, broad-based buying (more stocks rising on higher volume), while a reading above 1.0 signals broad selling pressure. Extreme values—below 0.50 or above 2.00—often reflect emotional excess, such as greed or fear. Moving averages of the TRIN, such as the 10-day or 21-day average, help smooth noise and highlight trends, making it a useful complement to price charts for spotting when index moves may be misleadingly driven by a handful of large-cap stocks. While not a forecasting tool, the TRIN provides investors with real-time insight into the quality and depth of market participation, serving as a practical pulse check on market strength or fragility.